Samacheer Kalvi Class 10 Science Solutions Chapter 18 Heredity

Tamilnadu Samacheer Kalvi 10th Science Solutions Chapter 18 Heredity

Samacheer Kalvi 10th Science Heredity Text Book Back Questions and Answers

I. Choose the Correct Answer.

 

Question 1. According to Mendel alleles have the following character:
(b) Responsible for character
(c) Production of gametes
(d) Recessive factors
Answer: (b) Responsible for character
In simple words: Alleles are different forms of a gene, and they decide what character or trait an organism will show. For example, one allele might be for tallness, and another for dwarfness.

🎯 Exam Tip: Remember that alleles directly control the expression of traits. Understanding this is key to genetic crosses.

 

Question 2. 9:3:3:1 ratio is due to
(a) Segregation
(b) Crossing over
(c) Independent assortment
(d) Recessiveness.
Answer: (c) Independent assortment
In simple words: The 9:3:3:1 ratio is a special outcome from a dihybrid cross, where two different traits are inherited independently of each other. This means the genes for these traits are not linked.

🎯 Exam Tip: The 9:3:3:1 ratio is a hallmark of Mendel's Law of Independent Assortment in a dihybrid cross where both parents are heterozygous for both traits.

 

Question 3. The region where the spindle fibres get attached during cell division of the chromosome
(a) Chromomere
(b) Centrosome
(c) Centromere
(d) Chromonema
Answer: (c) Centromere
In simple words: During cell division, spindle fibers grab onto a specific spot on each chromosome called the centromere. This helps pull the chromosomes apart correctly.

🎯 Exam Tip: The centromere is essential for proper chromosome movement and segregation during mitosis and meiosis. Don't confuse it with centrosome (the organelle that organizes spindle fibers).

 

Question 4. The centromere is found at the centre of the chromosome.
(b) Metacentric
(c) Sub - metacentric
(d) Acrocentric.
Answer: (b) Metacentric
In simple words: When the centromere is located exactly in the middle of a chromosome, making both arms equal in length, it is called a metacentric chromosome. This position gives it a V-shape during cell division.

🎯 Exam Tip: The position of the centromere classifies chromosomes into different types, like metacentric (middle), submetacentric (slightly off-center), acrocentric (near one end), and telocentric (at the very end).

 

Question 5. The units form the backbone of the DNA.
(a) 5 carbon sugar
(b) Phosphate
(c) Nitrogenous bases
(d) Sugar phosphate
Answer: (d) Sugar phosphate
In simple words: The long chains of DNA are built from repeating units where a sugar molecule and a phosphate group are linked together. This creates the strong outer frame of the DNA helix.

🎯 Exam Tip: Remember that nitrogenous bases attach to the sugar, facing inward, forming the "steps" of the DNA ladder, while the sugar-phosphate forms the "handrails".

 

Question 6. Okazaki fragments are joined together by .
(a) Helicase
(b) DNA polymerase
(c) RNA primer
(d) DNA ligase
Answer: (d) DNA ligase.
In simple words: During DNA replication, DNA ligase acts like a glue, connecting the small DNA pieces (Okazaki fragments) on the lagging strand to form one continuous new strand.

🎯 Exam Tip: Each enzyme in DNA replication has a specific role; helicase unwinds, polymerase builds, and ligase seals. Knowing these functions is vital.

 

Question 7. The number of chromosomes found in human beings are:
(a) 22 pairs of autosomes and 1 pair of allosomes.
(b) 22 autosomes and 1 allosome
(c) 46 autosomes
(d) 46 pairs autosomes and 1 pair of allosomes.
Answer: (a) 22 pairs of autosomes and 1 pair of allosomes.
In simple words: Humans normally have 46 chromosomes in total. These are made up of 22 pairs of autosomes (which control body traits) and one pair of allosomes (sex chromosomes), making a full set.

🎯 Exam Tip: Remember the distinction between autosomes (body chromosomes) and allosomes (sex chromosomes) and their respective numbers in human cells.

 

Question 8. The loss of one or more chromosome in ploidy is called
(a) Tetraploidy
(b) Aneuploidy
(c) Euploidy
(d) Polyploidy.
Answer: (b) Aneuploidy
In simple words: Aneuploidy is a genetic condition where a cell has an abnormal number of chromosomes. This happens when there is either one too many or one too few chromosomes, not a complete set.

🎯 Exam Tip: Aneuploidy refers to an abnormal number of *individual* chromosomes, while euploidy refers to an abnormal number of *complete sets* of chromosomes.

II. Fill in the Blanks:

 

Question 1. The pairs of contrasting character (traits) of Mendel are called .
Answer: alleles or allelomorphs
In simple words: Mendel used different versions of a trait, like tall versus dwarf, which are called alleles or allelomorphs.

🎯 Exam Tip: Alleles are specific versions of a gene that determine a trait. For example, a gene for height might have a 'tall' allele and a 'dwarf' allele.

 

Question 2. Physical expression of a gene is called .
Answer: Phenotype
In simple words: What an organism looks like, or its observable traits, is called its phenotype. This is how the genes actually show up.

🎯 Exam Tip: Distinguish phenotype (what you see) from genotype (the genetic makeup). The phenotype is influenced by both genotype and environment.

 

Question 3. The thin thread like structures found in the nucleus of each cell are called .
Answer: Chromosomes
In simple words: Inside the cell's control center, the nucleus, there are tiny thread-like parts called chromosomes. These carry all the genetic information.

🎯 Exam Tip: Chromosomes are made of DNA and proteins and are visible during cell division; they are crucial for heredity.

 

Question 4. DNA consists of two ............ chains.
Answer: Polynucleotide chain
In simple words: DNA is made of two long strands, and each strand is a polynucleotide chain. These chains twist around each other to form a double helix.

🎯 Exam Tip: Remember that a polynucleotide chain is a polymer made up of many nucleotide monomers linked together.

 

Question 5. An inheritable change in the amount or the structure of a gene or a chromosome is called ..........
Answer: Mutation
In simple words: A mutation is a sudden, permanent change in the genetic material (DNA) that can be passed down to future generations. These changes can affect either a single gene or an entire chromosome.

🎯 Exam Tip: Mutations are the ultimate source of genetic variation, which is essential for evolution, but they can also cause diseases.

III. Identify Whether the Statements are True or False. Correct the False Statement.

 

Question 1. A typical Mendelian dihybrid ratio of F2 generation is 3 : 1.
Answer: False – A typical Mendelian dihybrid ratio of F2 generation is 9 : 3 : 3 : 1
In simple words: The 3:1 ratio is for a monohybrid cross, which looks at one trait. A dihybrid cross, which looks at two traits at the same time, has a different ratio of 9:3:3:1 in the second generation.

🎯 Exam Tip: Always remember the characteristic ratios: 3:1 for monohybrid cross and 9:3:3:1 for dihybrid cross. These are fundamental to Mendelian genetics.

 

Question 2. A recessive factor is altered by the presence of a dominant factor.
Answer: True
In simple words: Even when a dominant trait is present, the recessive factor (allele) is still there, just not expressed. It doesn't disappear or get changed.

🎯 Exam Tip: This statement highlights that dominant and recessive alleles coexist without altering each other, a core principle of Mendel's Law of Segregation.

 

Question 3. Each gamete has only one allele of a gene.
Answer: True
In simple words: When sex cells (gametes) are formed, they receive only one copy (one allele) for each gene from the parent. This ensures that offspring get one allele from each parent.

🎯 Exam Tip: This is a direct application of Mendel's Law of Segregation, stating that alleles separate during gamete formation so each gamete carries only one allele for each trait.

 

Question 4. Hybrid is an offspring from a cross between genetically different parent.
Answer: True
In simple words: A hybrid is simply a new individual created when two parents with different genetic traits are bred together. It means they got different versions of genes from each parent.

🎯 Exam Tip: Hybrids often show a mix of traits from both parents or may express only the dominant traits if the parents differed in those genes.

 

Question 5. Some of the chromosomes have an elongated knob-like appendages known as telomere.
Answer: False – Some of the chromosomes have an elongated knob-like appendages known as satellite
In simple words: The knob-like part on some chromosomes is called a satellite. Telomeres are the protective caps at the very ends of chromosomes, not elongated appendages.

🎯 Exam Tip: Distinguish between satellites (a secondary constriction with a knob-like body) and telomeres (protective ends of chromosomes) as they have different structures and functions.

 

Question 6. New nucleotides are added and new complementary strand of DNA is formed with the help of enzyme DNA polymerase.
Answer: True
In simple words: DNA polymerase is the main enzyme that builds new DNA strands. It adds the correct matching nucleotides to an existing DNA template during replication.

🎯 Exam Tip: DNA polymerase is crucial for synthesizing new DNA strands and also has proofreading activity to correct errors, ensuring genetic fidelity.

 

Question 7. Down's syndrome is the genetic condition with 45 chromosomes.
Answer: False – Down's syndrome is a genetic condition with 47 chromosomes (Trisomy 21).
In simple words: Down's syndrome happens when a person has an extra copy of chromosome 21, leading to a total of 47 chromosomes instead of the usual 46. It is not caused by having fewer chromosomes.

🎯 Exam Tip: Remember that Down's syndrome is an example of aneuploidy, specifically trisomy, where there is an extra copy of an autosomal chromosome.

IV. Match the Following:

 

Answer:
A. (iii) 22 pair of chromosome
B. (iv) 2n
C. (v) 23rd pair of chromosome
D. (i) Trisomy 21
E. (ii) 9:3:3:1
In simple words: Autosomes are the first 22 pairs of chromosomes, a diploid condition means having a full set of 2n chromosomes, allosomes are the 23rd pair that determine sex, Down's syndrome is caused by an extra copy of chromosome 21, and the dihybrid ratio shows trait distribution in a two-gene cross.

🎯 Exam Tip: Ensure you understand the definition of each term to accurately match them. Autosomes are body chromosomes, allosomes are sex chromosomes, and diploid (2n) represents the full set of chromosomes in somatic cells.

V. Answer in a Sentence.

 

Question 1. What is a cross in which inheritance of two pairs of contrasting characters are studied?
Answer: A dihybrid cross is a cross in which the inheritance of two pairs of contrasting characters is studied. It helps us understand how two different traits are passed down at the same time.
In simple words: It is called a dihybrid cross when we study how two different traits are inherited at once.

🎯 Exam Tip: Dihybrid crosses are more complex than monohybrid crosses but follow the same basic Mendelian principles of inheritance.

 

Question 2. Name the conditions when both the alleles are identical?
Answer: The condition where both alleles for a particular gene are identical is called homozygous. This means the individual has two copies of the same allele.
In simple words: When both copies of a gene are the same, it is called homozygous alleles.

🎯 Exam Tip: Homozygous can be either homozygous dominant (e.g., TT) or homozygous recessive (e.g., tt). The opposite is heterozygous (e.g., Tt).

 

Question 3. A garden pea plant produces axial white flowers. Another of the same species produced terminal violet flowers. Identify the dominant trait.
Answer: The dominant trait in this scenario is the axial white flower. Dominant traits are those that express themselves even when only one copy of the allele is present.
In simple words: The dominant trait is the axial white flower.

🎯 Exam Tip: In genetics, 'dominant' means the trait that shows up in a hybrid, while 'recessive' means the trait that is hidden by the dominant one.

 

Question 4. What is the name given to the segments of DNA, which are responsible for the inheritance of a particular character?
Answer: The segments of DNA responsible for the inheritance of a particular character are called genes. These genes carry the instructions for building and maintaining an organism.
In simple words: Genes are the parts of DNA that control what traits we inherit.

🎯 Exam Tip: Genes are the fundamental units of heredity; they encode information for proteins or functional RNA molecules.

 

Question 5. Name the bond which binds the nucleotides in a DNA.
Answer: Hydrogen bonds bind the nucleotides in a DNA molecule. These weak bonds are important for holding the two strands of the DNA double helix together.
In simple words: Hydrogen bonds connect the building blocks of DNA together.

🎯 Exam Tip: Hydrogen bonds are crucial for DNA structure and function; they are weak enough to be broken easily during replication and transcription but strong enough to maintain the double helix.

VI. Short Answer Questions.

 

Question 1. Why did Mendel select pea plant for his experiments?
Answer: Mendel chose pea plants for his experiments for several important reasons:
1. Pea plants naturally self-pollinate, making it very easy to create pure breeding individuals with specific traits.
2. They have a short life span and grow quickly, which allowed Mendel to observe many generations in a short time.
3. It is also simple to cross-pollinate pea plants artificially when needed, giving researchers control over breeding.
4. Pea plants show clear and distinct contrasting characters, like tall vs. dwarf or round vs. wrinkled seeds, which makes observing inheritance patterns easier.
5. Their flowers contain both male and female parts, meaning they are bisexual, further simplifying controlled breeding.
In simple words: Mendel picked pea plants because they grow fast, have clear traits, can self-pollinate easily, and are simple to cross-pollinate for experiments.

🎯 Exam Tip: Listing specific advantages of the pea plant is crucial. Mention its short life cycle, clear traits, self-pollinating nature, and ease of cross-pollination.

 

Question 2. What do you understand by the term phenotype and genotype?
Answer:
* The phenotype is the external expression of a particular trait, meaning it is what an organism looks like or what traits can be observed. This includes features like height, eye color, or disease presence.
* The genotype is the genetic expression of an organism, referring to the actual set of genes or alleles an individual possesses for a particular trait. It is the underlying genetic code.
In simple words: Phenotype is what you see (like tall or short), while genotype is the actual genetic code that causes it (like TT or Tt).

🎯 Exam Tip: Clearly differentiate between phenotype (observable characteristics) and genotype (genetic makeup). Phenotype is influenced by genotype and environment.

 

Question 3. What are allosomes?
Answer: Allosomes are special chromosomes that are responsible for determining the sex of an individual. They are also known as sex chromosomes or hetero-chromosomes. In humans, there are two types of sex chromosomes, which are X and Y chromosomes. For example, females typically have XX and males have XY.
In simple words: Allosomes are the chromosomes that decide if an individual will be male or female.

🎯 Exam Tip: Remember that humans have 22 pairs of autosomes and 1 pair of allosomes. The presence of a Y chromosome determines maleness.

 

Question 4. What are the Okazaki fragments?
Answer: Okazaki fragments are short segments of newly synthesized DNA. They are formed on the lagging strand during DNA replication. This happens because DNA synthesis on the lagging strand occurs in a discontinuous manner. These fragments are later joined together by DNA ligase to form a continuous strand.
In simple words: Okazaki fragments are small pieces of new DNA made during replication on one of the strands, which are then joined together.

🎯 Exam Tip: Clearly state that Okazaki fragments are short DNA pieces formed on the lagging strand during discontinuous replication.

 

Question 5. Why is euploidy considered to be advantageous to both plants and animals?
Answer: Euploidy is considered advantageous because it is a condition where an individual has one or more complete sets of chromosomes, which means more than the usual number of diploid (2n) chromosomes. This condition is often used in plant breeding and horticulture to produce plants with desirable traits. It has significant economic importance through the production of larger-sized flowers and fruits. For example, many cultivated strawberries are octoploid (8 sets of chromosomes). Euploidy also plays a crucial role in the evolution of new species by providing new genetic material.
In simple words: Euploidy means having extra complete sets of chromosomes, which helps in breeding larger fruits and flowers, and also helps in creating new species over time.

🎯 Exam Tip: Focus on the benefits of euploidy, especially in agriculture for enhanced traits like larger fruits/flowers, and its role in species evolution.

 

Question 6. A pure tall plant (TT) is crossed with the pure dwarf plant (tt), what would be the F1 and F2 generations? Explain.
Answer: When a pure tall plant (TT) is crossed with a pure dwarf plant (tt), here's what happens in the F1 and F2 generations:
* In the F₁ generation, all the plants produced will be tall. Their genotype will be Tt (heterozygous tall), and their phenotype will be tall. This is because the tall allele (T) is dominant over the dwarf allele (t).
* In the F2 generation, if the F₁ plants are self-pollinated, the offspring will show a mix of tall and dwarf plants. The genotypic ratio will be \( \text{TT} : \text{Tt} : \text{tt} = 1 : 2 : 1 \). The phenotypic ratio will be Tall : dwarf = 3 : 1. This means for every four plants, three will be tall (one pure tall TT and two hybrid tall Tt), and one will be dwarf (pure dwarf tt).
In simple words: When pure tall and pure dwarf pea plants cross, all F1 plants are tall. In the F2 generation, you get a mix of tall and dwarf plants in a 3:1 ratio, and the genes are 1:2:1 for pure tall, hybrid tall, and pure dwarf.

🎯 Exam Tip: Clearly state both genotypic and phenotypic ratios for F1 and F2 generations and explain the dominance of the tall trait in F1.

 

Question 7. Explain the structure of a chromosome.
Answer: Chromosomes are thin, long, and thread-like structures found inside the nucleus of eukaryotic cells. They are visible during cell division. Each chromosome consists of two identical strands, known as sister chromatids, which are joined together at a constriction called the centromere. Inside each chromatid is a spirally coiled thin structure called a chromonema. Along the length of the chromonema, there are numerous bead-like structures called chromomeres, which are believed to be compacted regions of DNA. Chromosomes carry the genetic information in the form of DNA.
In simple words: Chromosomes are long, thread-like structures in our cells that carry genes. Each one has two identical halves (chromatids) joined by a centromere, and they look like beads on a string when tightly coiled.

🎯 Exam Tip: When explaining chromosome structure, include key terms like chromatids, centromere, chromonema, and chromomeres, and mention their role in carrying genetic material.

 

Question 8. Label the parts of the DNA in the diagram given below. Explain the structure briefly.
Answer: The DNA molecule has a unique structure:
(i) A DNA molecule is made up of two long chains called polynucleotide chains. Each chain is a polymer of many small units called nucleotides.
(ii) These two chains are twisted around each other to form a double helix structure. They run in opposite directions to each other, which is described as an anti-parallel arrangement.
(iii) Nitrogenous bases, which are the 'rungs' of the DNA ladder, are located in the center. They are linked to the sugar-phosphate units, which form the strong backbone of the DNA molecule.
(iv) The pairing between the nitrogenous bases is very specific and always happens between a purine (Adenine or Guanine) and a pyrimidine (Cytosine or Thymine). These bases are held together by hydrogen bonds, forming the stable structure of the double helix.
In simple words: DNA has two twisted strands made of nucleotides. The bases in the middle connect these strands with hydrogen bonds, forming a specific ladder-like structure where A always pairs with T, and C with G.

🎯 Exam Tip: Remember the key features of DNA: double helix, anti-parallel strands, sugar-phosphate backbone, and specific base pairing (A-T, C-G) held by hydrogen bonds.

VII. Long Answer Questions

 

Question 1. Explain with an example of the inheritance of dihybrid cross. How is it different from a monohybrid cross?
Answer: A dihybrid cross involves studying the inheritance of two pairs of contrasting characteristics, or traits, at the same time. Mendel famously used pea plants to demonstrate this.
Mendel's Dihybrid Cross Example:
Mendel chose two contrasting traits for his experiment: the shape of seeds (round vs. wrinkled) and the color of seeds (yellow vs. green). He crossed pure breeding pea plants that had round-yellow seeds with pure breeding pea plants that had wrinkled-green seeds.
(i) In the first generation (F1), Mendel observed that all the offspring produced had round-yellow seeds. This showed that round shape and yellow color were dominant traits over wrinkled shape and green color. No wrinkled-green seeds appeared in the F1 generation.
(ii) When these F1 hybrid plants (round-yellow seeds) were self-pollinated, the second generation (F2) showed four different combinations of seed shape and color. These were: round yellow, round-green, wrinkled-yellow, and wrinkled-green seeds. The ratio of these phenotypes in the F2 generation was \( 9 : 3 : 3 : 1 \). This is known as the Dihybrid ratio.
Difference from Monohybrid Cross:
* A monohybrid cross is a genetic cross that focuses on the inheritance of only a single pair of genes or one trait. For example, crossing a tall pea plant with a dwarf pea plant to study height. In this case, parents differ by just one trait.
* A dihybrid cross, on the other hand, is a genetic cross that involves two pairs of genes, meaning two different independent traits are studied simultaneously. For example, Mendel studied both seed shape and seed color at the same time. This allows us to observe how different traits are inherited together or separately.
In simple words: A dihybrid cross studies two traits at once, like seed shape and color in Mendel's peas, showing a 9:3:3:1 ratio in the second generation. A monohybrid cross only studies one trait, like height, giving a 3:1 ratio.

🎯 Exam Tip: For the dihybrid cross, always explain Mendel's experiment with two contrasting traits, the F1 generation's dominance, and the characteristic 9:3:3:1 F2 ratio. Clearly define the difference by stating how many traits each cross studies.

 

Question 2. How is the structure of DNA organised? What is the biological significance of DNA?
Answer: DNA, or Deoxyribonucleic Acid, is the genetic material in almost all living organisms. Its structure and organization are fundamental to its function.
Organization of DNA Structure:
DNA is typically organized as a double helix, resembling a twisted ladder. Each side of the ladder, or strand, is a polynucleotide chain. These two strands run in opposite directions (anti-parallel). The 'rungs' of the ladder are formed by pairs of nitrogenous bases (Adenine (A) with Thymine (T), and Guanine (G) with Cytosine (C)) held together by hydrogen bonds. The 'side rails' are made of alternating sugar and phosphate groups.
During DNA replication, the double helix unwinds, and each strand acts as a template to create a new complementary strand. Enzymes like helicase unwind the DNA, and DNA polymerase adds new nucleotides. On one strand, synthesis is continuous (leading strand), while on the other, it's discontinuous, creating small Okazaki fragments that are later joined by ligase.
Biological Significance of DNA:
(i) DNA is primarily responsible for the transmission of hereditary information from one generation to the next. It carries all the genetic instructions needed for an organism's development, functioning, growth, and reproduction.
(ii) It contains all the information necessary for the formation of proteins, which are crucial for almost all biological processes. The sequence of bases in DNA dictates the sequence of amino acids in proteins.
(iii) DNA controls the developmental process and all life activities of an organism. It ensures that cells develop correctly, perform their specific functions, and respond to environmental changes.
In simple words: DNA is like a twisted ladder (double helix) that holds all our genetic instructions. It's important because it passes traits from parents to children, tells our bodies how to make proteins, and controls everything a living thing does.

🎯 Exam Tip: Describe the double helix structure, base pairing rules, and the anti-parallel nature of DNA. For significance, emphasize heredity, protein synthesis, and control of cellular activities.

 

Question 3. The sex of the new born child is a matter of chance and neither of the parents What would be the possible fusion of gametes to determine the sex of the child?
Answer: The sex of a newborn child is indeed a matter of chance and is determined solely by the father's sperm, not the mother. Here’s how it works through gamete fusion:
Human egg cells (from the mother) always carry an X chromosome, in addition to 22 autosomes (total 22+X).
Human sperm cells (from the father) can carry either an X chromosome or a Y chromosome, along with 22 autosomes (total 22+X or 22+Y).
Possible fusions of gametes:
* If an egg (22+X) is fertilized by an X-bearing sperm (22+X), the resulting individual will have a genotype of 44+XX. This individual will be biologically female.
* If an egg (22+X) is fertilized by a Y-bearing sperm (22+Y), the resulting individual will have a genotype of 44+XY. This individual will be biologically male.
Therefore, since the father produces two types of sperm (X or Y), it is the father's sperm that determines the sex of the child. The mother's egg always contributes an X chromosome.
In simple words: A baby's sex depends on the father's sperm. If an X sperm meets the egg, it's a girl (XX). If a Y sperm meets the egg, it's a boy (XY). The mother always gives an X chromosome.

🎯 Exam Tip: Clearly explain that eggs carry only X, while sperm carry either X or Y. Show the two possible fusions and their resulting genotypes (XX for female, XY for male) to illustrate paternal sex determination.

VIII. Higher Order Thinking Skills: (HOTS)

 

Question 1. Flowers of the garden pea are bisexual and self-pollinated. Therefore, it is difficult to perform hybridization experiment by crossing a particular pistil with the specific pollen grains. How Mendel made it possible in his monohybrid and dihybrid crosses?
Answer: Mendel overcame the challenge of pea plants being bisexual and self-pollinating by using a careful technique called artificial cross-pollination. This allowed him to control which plants bred together:
1. **Emasculation:** Mendel would carefully remove the stamens (male reproductive parts) from a pea plant flower before they matured and released pollen. This step, called emasculation, prevented self-pollination.
2. **Pollination:** After emasculation, he would then collect pollen from a different desired pea plant (the male parent) and dust it onto the stigma (female reproductive part) of the emasculated flower.
3. **Bagging:** To prevent unwanted pollen from other sources from landing on the stigma, Mendel would cover the pollinated flower with a bag.
This meticulous method ensured that the crosses were controlled, allowing him to perform both monohybrid and dihybrid crosses accurately and observe the inheritance patterns he discovered. He worked with nearly 10,000 pea plants and chose seven pairs of contrasting characters, which also made his experiments very robust.
In simple words: Mendel carefully removed the male parts of pea flowers to stop self-pollination. Then, he put pollen from a chosen plant onto the female part, and covered the flower. This way, he controlled which plants bred and could study traits accurately.

🎯 Exam Tip: Describe the steps Mendel took to perform artificial cross-pollination: emasculation, collection and transfer of pollen, and bagging. Emphasize the precision and control this method provided.

 

Question 2. Pure-bred tall pea plants are first crossed with pure-bred dwarf pea plants. The pea plants obtained in F₁ generation are then cross-bred to produce F2 generation of pea plants.
(a) What do the plants of F₁ generation look like?
(b) What is the ratio of tall plants to dwarf plants in F2 generation?
(c) Which type of plants were missing in F₁ generation but reappeared in F2 generation?
Answer:
(a) The plants of the F₁ generation will all look tall. This happens because the allele for tallness (T) is dominant over the allele for dwarfness (t). Their genetic makeup will be heterozygous tall (Tt).
(b) In the F2 generation, the ratio of tall plants to dwarf plants will be 3 : 1. For every three tall plants, there will be one dwarf plant.
(c) The dwarf plants were missing in the F₁ generation. They reappeared in the F2 generation. This illustrates Mendel's principle of segregation, where recessive traits hidden in F1 can be expressed again in F2.
In simple words: (a) F1 plants will all be tall. (b) In F2, there will be 3 tall plants for every 1 dwarf plant. (c) Dwarf plants were not seen in F1 but came back in F2.

🎯 Exam Tip: For F1 generation, explain dominance. For F2, state the phenotypic ratio (3:1). For the reappearing trait, identify the recessive trait and explain why it was hidden in F1 and reappeared in F2.

 

Question 3. Kavitha gave birth to a female baby. Her family members say that she can give birth to only female babies because of her family history. Is the statement given by her family members true? Justify your answer.
Answer: The statement made by Kavitha's family members is not true. The ability to give birth to only female babies is not hereditary or determined by family history. The sex of a child is determined randomly and depends entirely on the type of sperm that fertilizes the egg.
Here's the justification:
* The mother's egg always contributes an X chromosome (22 autosomes + X sex chromosome).
* The father's sperm can contribute either an X chromosome (22 autosomes + X sex chromosome) or a Y chromosome (22 autosomes + Y sex chromosome).
* If an X-sperm fertilizes the egg, the child will be female (XX).
* If a Y-sperm fertilizes the egg, the child will be male (XY).
Since the father produces both X and Y sperm in roughly equal numbers, there is an approximately 50% chance of having a boy and a 50% chance of having a girl with each pregnancy. Therefore, a family history of having only female children does not guarantee future female offspring.
In simple words: The family's statement is false. The father's sperm, not the mother's family history, decides the baby's sex. If an X-sperm fertilizes the egg, it's a girl; if a Y-sperm fertilizes, it's a boy.

🎯 Exam Tip: Emphasize that sex determination is a matter of probability linked to the father's sperm. Clearly explain the X and Y chromosome contributions from each parent.

IX. Value-Based Questions

 

Question 1. Under which conditions does the law of independent assortment hold good and why?
Answer: Mendel's Law of Independent Assortment states that during gamete formation, the alleles for different traits separate independently of one another. This law holds good under specific conditions:
1. **Genes are on different chromosomes:** The law applies when the genes controlling the two different traits are located on separate, non-homologous chromosomes. During meiosis, these chromosomes align and assort into gametes independently of each other.
2. **Genes are far apart on the same chromosome:** Even if genes are on the same chromosome, the law can still hold true if they are located far enough apart that crossing over occurs frequently between them. This effectively makes them behave as if they were on different chromosomes.
The reason for independent assortment is that the orientation of homologous chromosome pairs (each carrying two alleles) during metaphase I of meiosis is random. For instance, the pair carrying gene A and its alleles might orient independently of the pair carrying gene B and its alleles. This random orientation leads to all possible combinations of alleles in the resulting gametes.
In simple words: The Law of Independent Assortment works when genes for different traits are on different chromosomes or very far apart on the same chromosome. This happens because chromosomes randomly sort into new cells during reproduction, making sure different traits don't always stay together.

🎯 Exam Tip: Explain the two key conditions: genes on different chromosomes or genes far apart on the same chromosome. The biological "why" is due to the random alignment of homologous chromosomes during meiosis I.

Samacheer Kalvi 10th Science Heredity Additional Important Questions And Answers

I. Choose The Correct Answer.

 

Question 1. Exchange of genetic material take place in:
(a) vegetative reproduction
(b) asexual reproduction
(c) sexual reproduction
(d) budding
Answer: (c) sexual reproduction
In simple words: In sexual reproduction, living things swap their genetic information to make new life.

🎯 Exam Tip: Remember that "exchange of genetic material" implies the mixing of genes from two parents, which is characteristic of sexual reproduction.

 

Question 2. In human, the number of chromosomes in each cell is ______
(a) 22 pairs
(b) 21 pairs
(c) 23 pairs
(d) 20 pairs
Answer: (c) 23 pairs
In simple words: A normal human body cell has 23 pairs of chromosomes, which means 46 chromosomes in total.

🎯 Exam Tip: Distinguish between the total number of chromosomes (46) and the number of pairs (23), and remember that one of these pairs determines sex.

 

Question 3. If a round green seeded pea plant (RRYY) is crossed with wrinkled, yellow seeded pea plant. The seeds produced in F₁ generation are:
(a) round and yellow
(b) round and green
(c) wrinkled and green
(d) wrinkled and yellow
Answer: (a) round and yellow
In simple words: When you cross a plant with dominant round green seeds with one having recessive wrinkled yellow seeds, all the offspring in the first generation will have round yellow seeds.

🎯 Exam Tip: In genetics, dominant traits (like round shape and yellow color in peas) will always show up in the first generation if they are present, even if recessive traits are also inherited.

 

Question 4. In the new complementary strand of DNA, in one strand, the daughter strand is synthesized, as a continuous strand called ______
(a) Lagging strand
(b) Parent strand
(c) RNA primer
(d) Leading strand
Answer: (d) Leading strand
In simple words: During DNA copying, the leading strand is built in one long, unbroken piece.

🎯 Exam Tip: The leading strand is synthesized continuously towards the replication fork, while the lagging strand is synthesized discontinuously in short fragments.

 

Question 5. A zygote which has an X-chromosome inherited from the father will develop into a:
(a) boy
(b) girl
(c) x- chromosome does not determine the sex of a child
(d) either boy or girl
Answer: (b) girl
In simple words: If the father's sperm carries an X chromosome, the baby will be a girl because the mother always gives an X chromosome too, making it an XX combination.

🎯 Exam Tip: Remember that human females have XX sex chromosomes, and males have XY. The father's sperm determines the sex, as it can carry either an X or a Y chromosome.

 

Question 6. A pure tall plant (TT) is crossed to a short plant (tt). The ratio of pure tall plants to short plants in F2 is:
(a) 1:3
(b) 3:1
(c) 1:1
(d) 2:1
Answer: (b) 3:1
In simple words: When you cross a pure tall pea plant with a pure short one, and then cross their first generation offspring, you'll get three tall plants for every one short plant in the next generation.

🎯 Exam Tip: This 3:1 ratio is the classic phenotypic ratio for a monohybrid cross in the F2 generation, where one allele is dominant over the other.

 

Question 7. The number of pairs of sex chromosomes in the zygote of human is:
(a) one
(b) two
(c) three
(d) four
Answer: (a) one
In simple words: Humans have one pair of chromosomes that decides if they are male or female.

🎯 Exam Tip: Always specify "pairs" when referring to the sex chromosomes, as there is only one pair, but two individual chromosomes (X and Y, or two X's).

 

Question 8. Pure breeding varieties are otherwise called as:
(a) dominant
(b) recessive
(c) wild type
(d) mixed type
Answer: (c) wild type
In simple words: Pure breeding varieties consistently produce offspring with the same traits as themselves because they are homozygous. They are often called the "wild type" in genetics.

🎯 Exam Tip: "Pure breeding" or "true breeding" means the organism is homozygous for the trait and will always pass on that specific trait to its offspring.

 

Question 9. The genotype of a character is influenced by factors called:
(a) chromosome
(b) nucleus
(d) genes
Answer: (d) genes
In simple words: What a living thing's genes are made of (its genotype) depends on the specific genes it receives.

🎯 Exam Tip: The genotype is the genetic blueprint, and genes are the units of heredity that determine this blueprint.

 

Question 10. Monosomy is represented by:
(a) \( 2n + 1 \)
(b) \( 2n - 1 \)
(c) \( 2n + 2 \)
(d) \( 2n - 2 \)
Answer: (b) \( 2n - 1 \)
In simple words: Monosomy means a living thing is missing one chromosome from a pair, so it has one less chromosome than the normal diploid number.

🎯 Exam Tip: Remember that '2n' represents the normal diploid number of chromosomes. Monosomy is the absence of one chromosome from a pair.

 

Question 11. The term chromosome was introduced by:
(a) Bridges
(b) Waldeyer
(c) Balboni
(d) Flemming
Answer: (b) Waldeyer
In simple words: A scientist named Waldeyer was the first to use the word "chromosome" to describe the tiny, thread-like structures inside cells.

🎯 Exam Tip: Knowing the pioneers of genetics like Waldeyer and Mendel can help you understand the historical context of scientific discoveries.

 

Question 12. Diagrammatic representation of Karyotype of a species is:
(a) Idiogram
(b) Albinism
(c) Karyo tyning
(d) Heredity
Answer: (a) Idiogram
In simple words: An idiogram is like a map that shows all the chromosomes of a living thing, drawn out to see their sizes and shapes.

🎯 Exam Tip: An idiogram is a visual aid that represents the detailed structure of chromosomes within a karyotype, which is the complete set of chromosomes in a species.

 

II. Fill In The Blanks:

 

Question.
1. The Genotypic ratio of Monohybrid cross is .........
2. ......... is a graphical representation to calculate the probability of all possible genotype of off spring in a genetic cross.
3. The gene is present at a specific position on the chromosome called .........
4. The end of the chromosome is called .........
5. The chromosomes with satellites are called as .........
6. ......... act as a aging clock in every cell.
7. Nitrogen base + sugar = .........
8. The two strands of DNA open and separate at the point forming .........
9. Nullisomy is represented by .........
10. The gametes produced by the organisms contain a single set of chromosomes is .........
Answer:
1. The genotypic ratio of a monohybrid cross is \( 1:2:1 \).
2. The Punnett square is a graphical representation used to calculate the probability of all possible genotypes of offspring in a genetic cross. It's a simple way to predict inheritance patterns.
3. The gene is present at a specific position on the chromosome called the locus.
4. The end of the chromosome is called the telomere.
5. The chromosomes with satellites are called sat-chromosomes.
6. Telomeres act as an aging clock in every cell, protecting chromosome ends.
7. Nitrogen base + sugar = Nucleoside.
8. The two strands of DNA open and separate at the point forming the replication fork.
9. Nullisomy is represented by \( 2n - 2 \).
10. The gametes produced by the organisms contain a single set of chromosomes and are called haploid (n).
In simple words: These terms describe basic genetics: a monohybrid cross has a 1:2:1 gene ratio, Punnett squares help predict offspring traits, genes sit on a locus, chromosome ends are telomeres, and gametes have half the normal chromosomes (haploid).

🎯 Exam Tip: For fill-in-the-blanks, focus on understanding the key terminology and their definitions, especially for genetic concepts like ratios and chromosome parts.

 

III. Match The Following:

Answer:
A. (iii) 22 pair of chromosome
B. (iv) \( 2n \)
C. (v) 23rd pair of chromosome
D. (i) Trisomy 21
E. (ii) 9:3:3:1
In simple words: This matching exercise connects biological terms with their correct definitions or related concepts, helping to clarify different aspects of genetics.

🎯 Exam Tip: Carefully read both columns and ensure you understand the meaning of each term before attempting to match them, as incorrect terminology can lead to errors.

 

IV. State Whether True Or False, If False Write The Correct Statement:

 

Question.
1. The daughter strand synthesized in DNA is called logging strand.
2. The centromere is found near the centre of the chromosome in sub metacentric.
3. Primary construction in chromosome is called as nucleolar organizer.
4. T.H. Morgan was awarded Nobel prize for determining the role of chromosome in heredity.
5. Adenine links Thymine with three hydrogen bonds.
Answer:
1. **False.** The daughter strand synthesized continuously in DNA is called the leading strand. The lagging strand is made in pieces.
2. **True.**
3. **False.** The primary constriction in a chromosome is called the centromere, not the nucleolar organizer.
4. **True.** T.H. Morgan received the Nobel Prize for his discoveries concerning the role played by the chromosome in heredity.
5. **False.** Adenine and Thymine link together with two hydrogen bonds, not three. Guanine and Cytosine form three hydrogen bonds.
In simple words: This section checks if you know basic facts about DNA copying, chromosome parts, and genetic links. Some statements are true, while others are false and need correction.

🎯 Exam Tip: For true/false questions, pay close attention to every word. A single incorrect word can make a statement false. For false statements, always provide the correct information to show your understanding.

 

V. Answer In A Word Or Sentence:

 

Question 1. What is heredity?
Answer: Heredity is the process by which traits and characteristics are passed down from parents to their children across generations. This is how family resemblances occur.
In simple words: Heredity is how features like eye color or height are passed from parents to their offspring.

🎯 Exam Tip: When defining heredity, include both the idea of "transmission" and "from one generation to the next" for a complete answer.

 

Question 2. What is Alleles or Allelomorphs?
Answer: Alleles (also called allelomorphs) are different versions of the same gene that determine a specific trait. For example, a gene for flower color might have an allele for red and an allele for white.
In simple words: Alleles are different forms of a gene that control a specific trait.

🎯 Exam Tip: Emphasize that alleles are *versions* of a gene and determine *contrasting characters* to fully explain the concept.

 

Question 3. Define variation.
Answer: Variation refers to the observable differences that exist between individuals of the same species, even among offspring from the same parents. These differences are crucial for evolution.
In simple words: Variation means how living things of the same kind are different from each other, even brothers and sisters.

🎯 Exam Tip: When defining variation, remember to include both differences within a species and among offspring of the same parents.

 

Question 4. What is Terminus?
Answer: The terminus is the specific point on a DNA molecule where the two replication forks, moving in opposite directions during DNA replication, finally meet and merge. This marks the end of DNA synthesis.
In simple words: The terminus is where the two parts of DNA that are being copied meet up and finish the copying process.

🎯 Exam Tip: Clarify that the terminus is the *meeting point* of replication forks, not just any end of the DNA strand.

 

Question 5. Write the expanded form of DNA.
Answer: DNA stands for Deoxyribonucleic acid. This complex molecule carries all the genetic information of an organism.
In simple words: DNA means Deoxyribonucleic acid. It's the main building block of our genes.

🎯 Exam Tip: Always remember the full form of important abbreviations like DNA and RNA in biology.

 

Question 6. What is the satellite?
Answer: A satellite, also known as a chromosomal satellite, is a small, rounded part of a chromosome that is attached to the main body by a thin thread. This feature is seen in some chromosomes.
In simple words: A satellite is a small, extra piece at the end of some chromosomes, connected by a thin part.

🎯 Exam Tip: Accurately describe the satellite as a "knob-like appendage" or "rounded part" connected by a "secondary constriction" to differentiate it from the main body of the chromosome.

 

Question 7. How many types of nitrogenous bases are present in DNA? Name them.
Answer: There are two main groups of nitrogenous bases found in DNA: purines and pyrimidines. The purines are Adenine (A) and Guanine (G), while the pyrimidines are Cytosine (C) and Thymine (T). These bases form the genetic code.
In simple words: DNA has two kinds of nitrogen bases. One kind is Purines, which are Adenine and Guanine. The other kind is Pyrimidines, which are Cytosine and Thymine.

🎯 Exam Tip: Remember the two main categories (purines and pyrimidines) and the specific bases within each. Knowing their complementary pairing (A with T, G with C) is also key.

 

Question 8. Why is DNA called polynucleotide?
Answer: DNA is called a polynucleotide because it is a very large molecule made up of many smaller repeating units called nucleotides. Each nucleotide has a sugar, a phosphate, and a nitrogenous base.
In simple words: DNA is called a polynucleotide because it's built from many small units called nucleotides all linked together.

🎯 Exam Tip: To answer why DNA is a polynucleotide, explain that "poly" means many, and "nucleotide" is its basic building block, formed by a sugar, phosphate, and base.

 

Question 9. Name two purine nitrogenous bases found in a DNA molecule.
Answer: The two purine nitrogenous bases found in a DNA molecule are Adenine (A) and Guanine (G). Purines have a double-ring structure.
In simple words: Adenine and Guanine are the two purine bases you find in DNA.

🎯 Exam Tip: Clearly state the names of the two purines. Knowing they are characterized by a double-ring structure can also be helpful.

 

Question 10. What are the three chemically essential parts of nucleotides containing a DNA?
Answer: The three essential chemical components that make up a nucleotide in DNA are a nitrogenous base (Adenine, Guanine, Cytosine, or Thymine), a five-carbon sugar called deoxyribose, and a phosphate group. These parts are fundamental to genetic structure.
In simple words: A nucleotide, which is DNA's building block, has three main parts: a nitrogen base, a sugar (pentose), and a phosphate.

🎯 Exam Tip: List all three components (nitrogenous base, pentose sugar, phosphate group) accurately. Specifying deoxyribose as the pentose sugar is also good practice.

 

Question 11. What are autosomes?
Answer: Autosomes are non-sex chromosomes that carry genes determining all the body (somatic) characteristics of an organism, except for sex-linked traits. Humans have 22 pairs of autosomes.
In simple words: Autosomes are chromosomes that decide all your body features except for your gender.

🎯 Exam Tip: Define autosomes by clearly stating they are non-sex chromosomes and that they determine somatic (body) characters.

 

Question 12. How is the sex of a new born determines in humans?
Answer: In humans, the sex of a newborn child is determined by the sperm contributed by the father. If the sperm carries an X chromosome, the child will be female (XX); if it carries a Y chromosome, the child will be male (XY). The mother always contributes an X chromosome.
In simple words: The father's sperm decides if a baby will be a boy or a girl. The mother always gives an X chromosome, and the father can give either an X or a Y.

🎯 Exam Tip: Focus on the father's contribution (X or Y sperm) as the determinant of sex, as the mother always provides an X chromosome.

 

Question 13. Define genetics.
Answer: Genetics is the scientific field within biology that studies genes, genetic variation, and heredity in living organisms. It explains how traits are inherited and expressed.
In simple words: Genetics is the study of genes, how living things are different, and how traits are passed down from parents.

🎯 Exam Tip: Include all three key aspects in your definition of genetics: genes, variation, and heredity.

 

Question 14. Define mutation.
Answer: A mutation is a sudden, inheritable change that occurs in the genetic material (DNA or RNA) of an organism. These changes can be small, like a single base alteration, or large, like a chromosome rearrangement.
In simple words: A mutation is a sudden change in a living thing's DNA that can be passed on to its children.

🎯 Exam Tip: Highlight that a mutation is a "sudden" and "inheritable" change in "genetic material" for a comprehensive definition.

 

Question 15. Name the types of chromosomes based on the position of centromere.
Answer: Chromosomes are classified into four main types based on where their centromere is located: telocentric (at the very end), acrocentric (near the end), submetacentric (off-center), and metacentric (in the middle). The centromere is essential for chromosome movement during cell division.
In simple words: Chromosomes are grouped by where their middle part (centromere) is. They can be at the end, near the end, off-center, or right in the middle.

🎯 Exam Tip: List all four types correctly and briefly describe the centromere's position for each (e.g., telocentric-terminal, metacentric-middle).

 

VI. Short Answer Questions

 

Question 1. What are chromosomes made up of?
Answer: Chromosomes are complex structures primarily composed of DNA, RNA, and various proteins, including histones and non-histone proteins. Metallic ions are also present. The proteins give the chromosome its shape and help in packaging the DNA.
In simple words: Chromosomes are made of DNA, RNA, and special proteins that give them shape.

🎯 Exam Tip: List the main chemical components (DNA, RNA, proteins like histones) and briefly mention their structural role for a complete answer.

 

Question 2. What is the mechanism behind the expression of a particular trait? Explain.
Answer: The expression of a trait is governed by genes, which are specific segments of DNA. These genes determine what proteins are made, and proteins then carry out cellular functions that result in the trait being expressed. The basic mechanism involves genetic information flowing from DNA to RNA to protein.
In simple words: Traits show up because of genes. Genes tell the body what proteins to make, and these proteins control how a trait looks or acts.

🎯 Exam Tip: Explain the central dogma of molecular biology: DNA makes RNA, and RNA makes protein, as this is the fundamental mechanism for trait expression.

 

Question 3. If a pure tall pea plant is crossed with a pure dwarf plant, then in the first generation only tall plant appears. (a) What happens to the traits of the dwarf plant? (b) In the second generation, the dwarf trait reappears? Why?
Answer:
(a) When a pure tall pea plant is crossed with a pure dwarf plant, the tall trait is dominant, so it covers up or masks the dwarf trait in the first generation (F1). The dwarf trait is still present in the genes but is not visible.
(b) Yes, the dwarf trait reappears in the second generation (F2) because the F1 hybrid plants (which are all tall but carry both tall and dwarf genes) are self-crossed. This allows the recessive dwarf alleles to combine again in some offspring, making the dwarf trait visible. This shows that traits separate during gamete formation and combine randomly.
In simple words: (a) The tallness gene is stronger, so it hides the dwarf gene in the first plant generation. (b) The dwarf trait comes back in the second generation because the hidden dwarf genes can finally pair up again and show themselves when the first generation plants reproduce.

🎯 Exam Tip: Clearly define dominant and recessive traits and how they interact in F1 and F2 generations. The reappearance of a recessive trait in F2 is a key concept of Mendelian genetics.

 

Question 4. Explain the types of chromosome-based on function.
Answer: Chromosomes are divided into two types based on their function:
1. **Autosomes:** These chromosomes carry genes that control all the body characteristics and traits, such as height, eye color, and blood type. Both males and females have the same number of autosomes.
2. **Allosomes:** Also known as sex chromosomes or heterochromosomes, these chromosomes are responsible for determining the biological sex of an individual. There are two main types: X and Y. For instance, human males have one X and one Y chromosome, while human females have two X chromosomes. These different types ensure the proper development of an organism's body and sex-related features.
In simple words: Chromosomes have two main jobs: 1. **Autosomes** control all body features like height or eye color. Everyone has the same number of these. 2. **Allosomes** decide if someone is male or female. These are called sex chromosomes, like X and Y.

🎯 Exam Tip: Differentiate clearly between autosomes (body characteristics) and allosomes (sex determination) and give examples for human sex chromosomes.

 

VII. Long Question And Answer:

 

Question 1. How are Mutation classified? Explain.
Answer: Mutations are classified based on the type of genetic material affected into two main categories: chromosomal mutations and gene mutations.
1. **Chromosomal Mutations:** These are sudden changes that affect either the structure or the total number of chromosomes in a cell.
(a) **Changes in Chromosome Structure:** These often happen due to mistakes during cell division. They involve changes in how genes are arranged on a chromosome, such as deletion (loss of a segment), duplication (repeated segment), inversion (reversed segment), or translocation (segment moves to a different chromosome).
(b) **Changes in Chromosome Number (Ploidy):** These mutations involve having an extra chromosome or missing one or more chromosomes. This is broadly called ploidy and includes two main types: euploidy (changes in entire sets of chromosomes) and aneuploidy (changes in individual chromosome numbers). Chromosomal mutations can significantly alter an organism's traits.
2. **Gene Mutations (Point Mutations):** These are smaller changes that happen in the specific sequence of nucleotides within a single gene. These changes can be a substitution (one base replaces another), deletion (a base is removed), or insertion (an extra base is added). Such alterations can lead to the production of incorrect proteins, which may affect the organism's function.
In simple words: Mutations are changes in genetic material. They are of two types: 1. **Chromosomal Mutations** are big changes to whole chromosomes, either their shape or how many there are. 2. **Gene Mutations** are small changes in the DNA code of a single gene. Both types can change how an organism looks or works.

🎯 Exam Tip: When explaining mutation classification, ensure you define both chromosomal and gene mutations, and provide examples for each type to demonstrate a thorough understanding.

 

Question 2. What is a mutation? Explain the two types of mutation.
Answer: A mutation is an unexpected and inheritable alteration in the genetic material (DNA) of a living thing. These changes are important because they can lead to new traits or diseases. Mutations are mainly grouped into two categories:
1. **Chromosomal Mutations:** These involve large-scale changes to chromosomes.
(a) **Structural Changes:** These are modifications within the physical makeup of chromosomes. They can be caused by errors during cell division and include the removal (deletion), doubling (duplication), flipping (inversion), or shifting (translocation) of chromosome segments.
(b) **Numerical Changes (Ploidy):** These mutations alter the total number of chromosomes in a cell.
(i) **Euploidy:** This is when an organism has extra complete sets of chromosomes. For example, triploidy (\( 3n \)) means three sets, and tetraploidy (\( 4n \)) means four sets. Tetraploid plants often grow larger fruits and flowers.
(ii) **Aneuploidy:** This involves the loss or gain of one or a few individual chromosomes. Examples include monosomy (\( 2n - 1 \), missing one chromosome), trisomy (\( 2n + 1 \), having one extra chromosome), and nullisomy (\( 2n - 2 \), missing a pair of chromosomes).
2. **Gene Mutations (Point Mutations):** These are smaller changes that occur at the level of individual genes, affecting the sequence of DNA building blocks (nucleotides). They involve the replacement, removal, or addition of one or more nitrogenous bases, which can change the genetic code and result in faulty proteins.
In simple words: A mutation is a sudden, inherited change in an organism's DNA. There are two main kinds: 1. **Chromosomal mutations** are big changes to whole chromosomes, either their structure or their number. 2. **Gene mutations** are small changes in the DNA code of a single gene, like one letter being wrong, added, or removed. Both can cause big changes in a living thing.

🎯 Exam Tip: For explaining mutation, start with a clear definition and then systematically describe the two main types, providing specific examples for each to illustrate the changes involved.

 

Question 3. Write a note on down's syndrome.
Answer: Down's syndrome is a genetic disorder first described by Dr. Langdon Down in 1866. It is characterized by the presence of an extra copy of chromosome 21, a condition also known as Trisomy 21. Children with Down's syndrome often experience intellectual disability, delays in physical development, and may have characteristic facial features. Other common issues include weak muscle tone, as well as vision and hearing impairments.
In simple words: Down's syndrome is a genetic condition where a person has an extra copy of chromosome 21. It can cause delays in learning and development, and sometimes affects how a child looks or hears.

🎯 Exam Tip: When writing about Down's syndrome, clearly mention it as "Trisomy 21" (an extra copy of chromosome 21) and list the common associated characteristics.

 

VIII. Higher Order Thinking Skills: (HOTS)

 

Question 2. In a population of 1000 individuals 360 belong to genotype AA, 480 to Aa and the remaining 160 to aa. Based on this data, the frequency of allele A in the population is:
Answer: To find the frequency of allele A, we count the total number of A alleles from all individuals and divide by the total number of alleles in the population. In 1000 individuals, there are 2000 alleles in total. Individuals with genotype AA contribute two A alleles each (360 x 2 = 720), and individuals with genotype Aa contribute one A allele each (480 x 1 = 480). So, the total number of A alleles is \( 720 + 480 = 1200 \). The frequency of A is then \( \frac{1200}{2000} = 0.6 \).
In simple words: To find the frequency of allele A, count all the 'A' genes in the entire population and divide by the total number of all genes present.

🎯 Exam Tip: For allele frequency calculations, remember to count both alleles from homozygous individuals (e.g., AA) and one allele from heterozygous individuals (e.g., Aa).

 

Question 3. A tall true breeding garden pea plant is crossed with a dwarf true breeding garden Pea plant. When the F1 plants are selfed, what is the resulting genotypic ratio?
Answer: When a pure tall pea plant (TT) is crossed with a pure dwarf pea plant (tt), all the plants in the first generation (F1) are tall (Tt). If these F1 hybrid plants are self-pollinated, the genotypes in the second generation (F2) will be in the ratio of \( 1 : 2 : 1 \). This means there will be 1 homozygous tall (TT), 2 heterozygous tall (Tt), and 1 homozygous dwarf (tt) offspring. This is a classic Mendelian genotypic ratio for a monohybrid cross.
In simple words: When you cross the tall offspring from pure tall and pure dwarf parents, the next generation will have a mix of 1 pure tall, 2 mixed tall, and 1 pure dwarf plant in terms of their genes.

🎯 Exam Tip: Clearly state the genotypic ratio \( 1:2:1 \) and explain what each number represents (homozygous dominant, heterozygous, homozygous recessive) in the context of the traits.

Given the explicit directive: "Process and map ONLY the questions located between page 29 and page 30 of this PDF." Upon reviewing the OCR content for pages 29 and 30, it is clear that these pages contain only navigation links, footer elements, and copyright information, and *no actual questions*. According to the "CONTENT PROCESSING RULES", specifically the "IGNORE AND SKIP — FOOTER / NAVIGATION" section, such content must be completely ignored. Therefore, strictly adhering to the instruction to process "ONLY the questions located between page 29 and page 30", and given that no questions exist in this specified range and the content present falls under the ignore rules, the resulting HTML output is empty.