Maharashtra Board Class 12 Biology Chapter 3 Inheritance and Variation Solutions

Multiple Choice Questions

 

Question 1. Phenotypic ratio of incomplete dominance in Mirabilis jalapa.
(a) 2 : 1 : 1
(b) 1 : 2 : 1
(c) 3 : 1
(d) 2 : 2
Answer: (b) 1 : 2 : 1
In simple words: In incomplete dominance, neither gene is completely dominant, so the hybrid offspring show a mixed trait. Crossing these hybrids results in 1 red, 2 pink, and 1 white flower, giving a 1:2:1 ratio.

🎯 Exam Tip: Remember that in incomplete dominance, both the genotypic and phenotypic ratios are identical (1:2:1). This is a very common board exam question!

 

Question 2. In dihybrid cross, \( \text{F}_2 \) generation offspring show four different phenotypes while the genotypes are ……………….
(a) six
(b) nine
(c) eight
(d) sixteen
Answer: (b) nine
In simple words: When tracking two traits at once, the offspring can look 4 different ways on the outside, but their actual genetic combinations can be grouped into 9 distinct patterns.

🎯 Exam Tip: Memorize the dihybrid \( \text{F}_2 \) phenotypic ratio (9:3:3:1) and genotypic ratio (1:2:1:2:4:2:1:2:1) to easily count the number of phenotypes (4) and genotypes (9).

 

Question 3. A cross between an individual with unknown genotype for a trait with recessive plant for that trait is ……………….
(a) back cross
(b) reciprocal cross
(c) test cross
(d) monohybrid cross
Answer: (c) test cross
In simple words: If you have a plant with a dominant trait but do not know its exact genes, you cross it with a pure recessive plant. The traits of the resulting offspring will reveal the unknown parent's genotype.

🎯 Exam Tip: Always remember that a test cross is a specific type of back cross used exclusively to identify the unknown genotype of a dominant phenotype.

Question 4. When phenotypic and genotypic ratios are the same, then it is an example of ................
(a) incomplete dominance
(b) complete dominance
(c) multiple alleles
(d) cytoplasmic inheritance
Answer: (a) incomplete dominance
In simple words: In incomplete dominance, the physical appearance directly shows the genetic mix, so both the physical ratio and genetic ratio end up being exactly the same (1:2:1).

🎯 Exam Tip: Remember that Mirabilis jalapa (four o'clock plant) is a classic textbook example of incomplete dominance where both ratios are 1:2:1.

 

Question 5. If the centromere is situated near the end of the chromosome, the chromosome is called ................
(a) Metacentric
(b) Acrocentric
(c) Sub-Metacentric
(d) Telocentric
Answer: (d) Telocentric
In simple words: A telocentric chromosome has its centromere located at the very end, which makes the chromosome look like a single straight rod.

🎯 Exam Tip: Use memory hooks for chromosome shapes: 'T' for Telocentric (Terminal/Tip) and 'M' for Metacentric (Middle).

 

Question 6. Chromosomal theory of inheritance was proposed by ................
(a) Sutton and Boveri
(b) Watson and Crick
(c) Miller and Urey
(d) Oparin and Halden
Answer: (a) Sutton and Boveri
In simple words: Sutton and Boveri proposed that chromosomes are the carriers of genetic material and that they separate and pair up just like Mendel's factors.

🎯 Exam Tip: Always associate Sutton and Boveri with the theoretical proposal of this theory, while Thomas Hunt Morgan provided its experimental proof.

 

Question 8. Find the mismatched pair:
(a) Down’s syndrome = 44 + XY
(b) Turner’s syndrome = 44 + XO
(c) Klinefelter’s syndrome = 44 + XXY
(d) Super female = 44 + XXX
Answer: (a) Down’s syndrome = 44 + XY
In simple words: Down's syndrome is caused by having an extra chromosome 21, making the total count 47 instead of the normal 46. The formula 44 + XY represents a normal male, not someone with Down's syndrome.

🎯 Exam Tip: Remember that Down's syndrome is an autosomal disorder (trisomy 21), so the abnormality lies in the autosomes (45 instead of 44), not the sex chromosomes.

 

Question 9. A colourblind man marries a woman, who is homozygous for normal colour vision, the probability of their son being colour blind is ……………….
(a) 0%
(b) 25%
(c) 50%
(d) 100%
Answer: (a) 0%
In simple words: A son inherits his X chromosome from his mother and his Y chromosome from his father. Since the mother has normal vision genes on both of her X chromosomes, the son will definitely receive a normal gene and cannot be colourblind.

🎯 Exam Tip: Remember that sex-linked recessive traits like colour blindness are passed from a father to his daughters (who become carriers), but never directly from a father to his son.

 

Very Short Answer Questions

 

Question 1. Explain the statements
a. Test cross is back cross but back cross is not necessarily a test cross.
b. Law of dominance is not universal.
Answer:
a. (1) Test cross is the cross between \( F_1 \) hybrid and its homozygous recessive parent. This helps in determining the unknown genotype of an individual.
(2) Back cross is the cross of offspring with any one of the parents, either dominant or recessive.
(3) Therefore, test cross can be a back cross – but back cross cannot be a test cross.
In simple words: A test cross is a specific type of back cross where the offspring is crossed only with the recessive parent. Since a back cross can involve either the dominant or recessive parent, not all back crosses are test crosses.

🎯 Exam Tip: Clearly define both terms (test cross and back cross) first, then use a simple logical statement to show how one is a subset of the other to secure full marks.

Question 2. Define the following terms:
a. Dihybrid cross
b. Homozygous
c. Heterozygous
d. Test cross
Answer:
a. A cross between parents differing in two heritable traits is called dihybrid cross. This type of cross helps in studying the inheritance pattern of two different characters simultaneously.
b. An individual possessing identical alleles for a particular trait is called homozygous or pure for that trait. E.g. TT for tallness and tt for dwarfness.
c. An individual possessing contrasting allele for a particular trait is called heterozygous. E.g. Tt showing tallness.
d. The cross of \( F_1 \) progeny with homozygous recessive parent is called a test cross.
In simple words: These terms describe genetic traits: a dihybrid cross studies two traits at once, homozygous means having identical genes for a trait, heterozygous means having different genes, and a test cross helps identify an unknown genotype by breeding it with a recessive parent.

🎯 Exam Tip: Always include genetic representations like 'TT', 'tt', or 'Tt' as examples when defining homozygous and heterozygous to secure full marks.

 

Question 3. What are allosomes?
Answer: Allosomes are the chromosomes which decide the sex of an organism. They are also commonly referred to as sex chromosomes, distinguishing them from autosomes.
In simple words: Allosomes are the specific chromosomes (like X and Y in humans) that determine whether an offspring will be male or female.

🎯 Exam Tip: Clearly state that allosomes are also known as sex chromosomes to demonstrate a complete understanding of the term.

 

Question 4. What is crossing over?
Answer: Crossing over is the process of forming new recombinations by interchanging and exchanging non-sister chromatid arms of the homologous chromosomes. This crucial genetic exchange occurs during the pachytene stage of prophase I in meiosis.
In simple words: Crossing over is when matching chromosomes swap pieces of their DNA during cell division, which helps create unique genetic combinations in offspring.

🎯 Exam Tip: Be sure to use precise terms like 'non-sister chromatids' and 'homologous chromosomes' as these are key terms examiners look for.

 

Question 5. Give one example of autosomal recessive disorder.
Answer: Thalassemia is a well-known example of an autosomal recessive disorder. Another common example is sickle cell anemia, which affects the shape of red blood cells.
In simple words: An autosomal recessive disorder is a genetic condition, like Thalassemia, that a child can only inherit if both parents pass down the faulty gene.

🎯 Exam Tip: Memorize Thalassemia and Sickle cell anemia as standard examples of autosomal recessive disorders for quick recall in multiple-choice or short-answer questions.

 

Question 6. What are X-linked genes?
Answer: Genes located on the non-homologous region of X chromosome are called X-linked genes. These genes do not have corresponding alleles on the Y chromosome.
In simple words: X-linked genes are special genes found only on the X chromosome that help determine certain traits.

🎯 Exam Tip: Remember that X-linked genes are located specifically on the non-homologous region, meaning they do not pair with the Y chromosome.

 

Question 7. What are holandric traits?
Answer: Genes located on the non-homologous region of Y chromosome are called Y-linked genes. The traits due to such genes are called holandric traits which are seen only in male sex. These traits are directly transmitted from father to son.
In simple words: Holandric traits are characteristics passed down only from fathers to sons because the genes for them are located on the Y chromosome.

🎯 Exam Tip: Clearly state that holandric traits are only expressed in males because females do not possess a Y chromosome.

 

Question 8. Give an example of chromosomal disorder caused due to non-disjunction of autosomes.
Answer: Down’s syndrome is an example of chromosomal disorder caused due to non-disjunction of autosomes. This condition is characterized by the presence of an extra copy of chromosome 21.
In simple words: Down's syndrome happens when chromosomes do not separate properly during cell division, leading to an extra chromosome in the cells.

🎯 Exam Tip: Mention "Trisomy 21" as a key term when discussing Down's syndrome to secure maximum marks.

 

Question 9. Give one example of complete sex linkage.
Answer: Sex linkage can be complete X linkage and complete Y linkage. X linkage is haemophilia and Y linkage is hypertrichosis. These traits are inherited together because their genes do not undergo crossing over.
In simple words: Complete sex linkage refers to traits that are always passed down together because their genes are located very close to each other on the sex chromosomes.

🎯 Exam Tip: Use haemophilia (X-linked) and hypertrichosis (Y-linked) as classic examples to demonstrate complete sex linkage clearly.

 

3. Short Answer Questions

 

Question 1. Enlist seven traits of pea plant selected / studied by Mendel.
Answer: Seven traits in pea selected by Mendel:
1. Tall habit versus dwarf habit (Height of the plant).
2. Purple flowers versus white flowers (Colour of flowers).
3. Axial flowers versus terminal flowers (Position of flowers).
4. Inflated pods versus constricted pods (Shape of pods).
5. Green pods versus yellow pods (Colour of pods).
6. Round seeds versus wrinkled seeds (Shape of seeds).
7. Yellow seeds versus green seeds (Colour of seeds). These contrasting characters allowed Mendel to easily observe inheritance patterns across generations.
In simple words: Mendel chose seven clear, opposite features of pea plants—like tall versus short height and round versus wrinkled seeds—to study how traits are passed down.

🎯 Exam Tip: List both the dominant and recessive forms for each of the seven traits to show a complete understanding of Mendel's experiments.

3. Yellow seeds versus green seeds. (Colour of seeds)
4. Round seeds versus wrinkled seeds. (Shape of seeds)
5. Green pods versus yellow pods. (Colour of pods)
6. Inflated pods versus constricted pods. (Shape of pods)
7. Axial flower versus terminal flower. (Position of a flower)

 

Question 2. Why law of segregation is also called the law of purity of gametes?
Answer: (1) Mendel’s law of segregation is also called Law of purity of gametes because, during formation of gametes, the alleles separate/ segregate from each other and only one allele enters a gamete. This ensures that each gamete receives only one clean copy of the gene. (2) The separation of one allele does not affect other. Since single allele enters a gamete means gametes will be pure for a trait. E.g. The contrasting characters such as tall (T) and dwarf (t) present in F1 hybrid (Tt) segregate during the formation of gametes. (3) Owing to this, two types of gametes i.e. T and t are formed which are pure for the characters which they carry. (4) Thus for example:
Tt (F1 hybrid)
↳ T
↳ t
In simple words: When plants make reproductive cells (gametes), the pair of genes for a trait splits up so that each cell gets only one gene. This means the gamete is completely pure and carries only one specific characteristic.

🎯 Exam Tip: Clearly state that gametes are always haploid and pure for a trait, and use the Tt hybrid diagram to illustrate the segregation clearly to score full marks.

 

Question 3. Pleiotropy.
Answer: 1. When a single gene controls two or more different traits, it is called a pleiotropic gene and the phenomenon is known as pleiotropy or pleiotropism. This single gene can influence multiple seemingly unrelated physical features. 2. The pleiotropic ratio is always 1 : 2 instead of normal 3 : 1. 3. Sickle-cell anaemia is caused by the gene HbS. The healthy or normal gene which is dominant is HbA. The heterozygotes or carriers i.e., HbA/HbS show anaemia as there is deficiency of haemoglobin due to sickling of RBCs. Abnormally low concentration of oxygen can cause sickling of RBCs.
In simple words: Pleiotropy is when one single gene decides more than one physical trait in the body. For example, the gene that causes sickle-cell anaemia also affects how much oxygen the blood can carry.

🎯 Exam Tip: Remember that the phenotypic ratio for pleiotropy is 1:2 instead of the standard 3:1 Mendelian ratio because the homozygous recessive condition is often lethal.

 

Question 4. What are the reasons of Mendel’s success?
Answer: Reasons for Mendel’s success:
1. Mendel planned his experiments carefully and these experiments consisted of large sample. He also made sure to focus on one trait at a time to avoid confusion.
2. He always recorded the results of number of plants of each type and their ratios.
3. The contrasting characters that he chose were easily recognizable.
4. The seven pairs of contrasting characters that he selected were under control of a single factor each. They were present on separate chromosomes and were transmitted from one generation to the next.
5. Mendel studied and introduced concept of dominance and recessiveness.
In simple words: Mendel succeeded because he planned his experiments very carefully using large groups of plants. He chose clear, easy-to-see traits and kept very accurate records of all his results.

🎯 Exam Tip: Clearly list at least three to four distinct points, such as his selection of contrasting characters and meticulous record-keeping, to secure full marks.

 

Question 5. “Father is responsible for determination of sex of child and not the mother”. Justify.
Answer:
1. Human male is heterogametic, i.e. he produces two different types of sperms. One is bearing X chromosome along with 22 autosomes and the other is Y bearing sperm with 22 autosomes.
2. Mother, on the other hand, is homogametic, producing all similar types of ova, i.e 22 + X chromosomal combination.
3. If 22+X bearing sperm fertilise an egg, female child is formed while if Y bearing sperm fertilizes an egg, male child is formed.
4. Thus the sex of the child is dependent upon type of sperm that father gives, therefore, it is said that father is responsible for determination of sex of a child. This biological mechanism shows that the mother's egg plays a passive role in determining the gender.
In simple words: The mother always gives an X chromosome to the baby, but the father can give either an X or a Y chromosome. If the father's sperm carries an X, it's a girl, and if it carries a Y, it's a boy.

🎯 Exam Tip: Use a simple genetic cross diagram showing XX (mother) and XY (father) gametes to make your justification visually clear and highly scoring.

 

Question 6. What is linkage? How many linkage groups do occur in human being and maize?
Answer:
1. Linkage is defined as the tendency of the genes to be inherited together because they are present in the same chromosome. Linkage group is group of genes situated on a chromosome.
2. Humans have 23 linkage groups because they have 23 pairs of chromosomes.
3. Maize plant has 10 linkage groups because they have 10 pairs of chromosomes. These linkage groups correspond directly to the haploid number of chromosomes in an organism.
In simple words: Linkage means genes located close together on the same chromosome tend to travel together when passed from parents to children. The number of linkage groups is always equal to the number of chromosome pairs.

🎯 Exam Tip: Always remember that the number of linkage groups in an organism is equal to its haploid number of chromosomes (n).

 

Question 7. PKU.
Answer:
1. PKU means phenylketonuria which is an autosomal recessive inborn error.
2. In this disorder the metabolism of phenylalanine does not occur due to deficiency of phenylalanine hydroxylase (PAH) enzyme.
3. This enzyme is necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine.
4. When PAH activity is reduced, phenylalanine accumulates in blood and cerebrospinal fluid and is converted into phenylpyruvate or phenyl-ketone which is a toxic compound. This may cause mental retardation. Excess phenylalanine is excreted in urine, hence this disease is called phenylketonuria.
5. PKU is caused by mutations in the PAH gene on chromosome no. 12.
6. Untreated PKU causes abnormal phenotype which includes growth failure, poor skin pigmentation, microcephaly, seizures, global developmental delay and severe intellectual impairment. However, at birth if an infant is checked for PKU, the further abnormalities can be avoided. A simple heel-prick blood test done shortly after birth can easily detect this condition early.
In simple words: PKU is a genetic disorder where the body cannot break down a substance called phenylalanine. This substance builds up in the body and can damage the brain if not treated early with a special diet.

🎯 Exam Tip: Mention "autosomal recessive", "phenylalanine hydroxylase enzyme", and "chromosome 12" to secure full marks in this short note.

 

Question 8. Compare X-chromosome and Y-chromosome.
Answer:

Feature	X-Chromosome	Y-Chromosome
Size and Shape	Larger and metacentric	Smaller and acrocentric
Chromatin Content	Contains more euchromatin and less heterochromatin	Contains less euchromatin and more heterochromatin
DNA and Genes	Has more DNA and a larger number of active genes	Has less DNA and a smaller number of active genes
Type of Linkage	Carries X-linked genes	Carries Y-linked (holandric) genes

Both chromosomes contain homologous regions that pair during meiosis, as well as non-homologous regions.
In simple words: The X chromosome is larger and carries many essential genes, while the Y chromosome is much smaller and mainly carries genes that determine male characteristics.

🎯 Exam Tip: Drawing a simple comparison table with key points like size, centromere position (metacentric vs acrocentric), and amount of euchromatin/heterochromatin is the best way to score full marks.

p>Question. Distinguish between X-chromosome and Y-chromosome.
Answer:

X-chromosome	Y-chromosome
1. X-chromosome is straight, rod-like and longer than Y-chromosome. It is metacentric.	1. Y-chromosome is shorter chromosome which is acrocentric.
2. X-chromosome has large amount of euchromatin and small amount of heterochromatin.	2. Y-chromosome has small amount of euchromatin and large amount of heterochromatin.
3. X-chromosome has large amount of DNA, hence it is genetically active due to more genes.	3. Y-chromosome has less amount of DNA, hence it is genetically less active or inert due to lesser genes.
4. Non-homologous region of X-chromosome is longer and contains more genes.	4. Non-homologous region of Y-chromosome is shorter and contains lesser genes.
5. Contains X-linked genes on non-homologous region.	5. Contains Y-linked genes on non-homologous region.
6. X-chromosome is present in men as well as women.	6. Y-chromosome is present only in men.

In simple words: X-chromosomes are longer, active, and found in both males and females, while Y-chromosomes are shorter, less active, and only found in males.

🎯 Exam Tip: Remember that X is metacentric (active, more DNA) and Y is acrocentric (less active, less DNA) to easily recall their differences.

 

Question 9. Explain the chromosomal theory of inheritance.
Answer: Chromosomal theory of inheritance was put forth by Sutton and Boveri after studying parallel behaviour of genes and chromosomes during meiotic division. This theory states the following points:
1. Chromosomal theory identifies chromosomes as the carrier of genetic material.
2. All the hereditary characters are transmitted by gametes. Nucleus of gametes, i.e. sperms and ova of the parents contain chromosomes which transmit the heredity to offspring.
3. Chromosomes are found in pairs in somatic or diploid cells.
4. During gamete formation, homologous chromosomes pair and segregate independently at meiosis. The diploid condition is converted into haploid condition. Thus each gamete contains only one chromosome of a pair. This elegant mechanism ensures that offspring receive a balanced set of chromosomes from both parents.
In simple words: This theory explains that chromosomes carry our genes and are passed from parents to children through sperm and egg cells. During this process, the double set of chromosomes splits so each child gets half from each parent.

🎯 Exam Tip: Clearly state the names of the scientists, Sutton and Boveri, and list all four key postulates to secure full marks.

 

Question 10. Observe the given pedigree chart and answer the following questions:
(a) Identify whether the trait is sex-linked or autosomal.
(b) Give an example of a trait in human beings which shows such a pattern of inheritance.

Pedigree Chart Structure:

• First Generation: Carrier female married to an affected male.
• Second Generation: Normal son, carrier daughter, and affected daughter (who marries a normal male).
• Third Generation: Normal daughter and affected son.

Answer:
Pedigree given above shows:
1. First Generation: Carrier woman marrying a sufferer man. Their three children are in following birth order.
2. Second generation: First son is normal, second daughter is carrier and third daughter is sufferer.
3. Third generation: The sufferer daughter marries a normal man. Her children are normal daughter and sufferer son.

(a) The above pedigree shows a sex-linked (X-linked) trait. Since criss-cross inheritance is seen in the trait, it must be sex-linked inheritance.
(b) Such a trait and its inheritance pattern can be seen in color blindness. This pattern of inheritance typically involves transmission from an affected father to his daughters, who then pass it to their sons.
In simple words: This pedigree chart shows a genetic trait passed down through sex chromosomes, specifically showing a criss-cross pattern from mothers to sons. A common human example of this type of inheritance is color blindness.

🎯 Exam Tip: When analyzing pedigree charts, look for "criss-cross" inheritance (affected father to carrier daughter to affected grandson) to quickly identify X-linked recessive traits.

 

Question 4. Match the Columns and rewrite the matching pairs.

Column I	Column II
(1) 21 trisomy	(a) Turner’s syndrome
(2) X-monosomy	(b) Klinefelter’s syndrome
(3) Holandric traits	(c) Down’s syndrome

Answer:
(1) 21 trisomy — (c) Down’s syndrome
(2) X-monosomy — (a) Turner’s syndrome
(3) Holandric traits — (b) Klinefelter’s syndrome
Matching these genetic conditions correctly helps in understanding chromosomal abnormalities and their phenotypic expressions.
In simple words: This matching pairs genetic disorders with their chromosomal causes, such as Down's syndrome being caused by an extra chromosome 21, and Turner's syndrome by a missing X chromosome.

🎯 Exam Tip: Memorize the specific chromosomal counts for each syndrome (e.g., Trisomy 21 for Down's, 45 (XO) for Turner's) to score full marks in matching questions.

Long Answer Questions

 

Question 1. What is dihybrid cross? Explain with suitable example and checker board method.
Answer:
1. A cross which involves two pairs of alleles is called a dihybrid cross. A phenotypic ratio of \( 9 : 3 : 3 : 1 \) obtained in the \( F_2 \) generation of a dihybrid cross is called a dihybrid ratio.
(2) Thus for example, when we cross a true breeding pea plant bearing round and yellow seeds with a true breeding pea plant bearing wrinkled and green seeds we get pea plants bearing round and yellow seeds in the \( F_1 \) generation. This classic experiment by Gregor Mendel laid the foundation for the law of independent assortment.
(3) When \( F_1 \) plants are selfed, we get a ratio of \( 9 : 3 : 3 : 1 \) in the \( F_2 \) generation, where 9 plants bear yellow round seeds, 3 plants bear yellow wrinkled seeds, 3 plants bear green round seeds and 1 plant bears green wrinkled seeds.
(4) Parents (\( P_1 \)) : RRYY × rryy
Gametes of \( P_1 \): RY and ry
\( F_1 \) generation : RrYy (Yellow round)
On selfing \( F_1 \) : RrYy × RrYy
Gametes of \( F_1 \) : RY, Ry, rY, ry
In simple words: A dihybrid cross is a genetic experiment where we study how two different traits, like seed shape and seed color, are passed down together. It helps us see how these traits mix and match independently in the offspring.

🎯 Exam Tip: Always write down the phenotypic ratio \( 9:3:3:1 \) clearly and label the parental genotypes (\( P_1 \)) and gametes to secure full marks.

P₂ Generation:

	RY	Ry	rY	ry
RY	RRYY	RRYy	RrYY	RrYy
Ry	RRYy	RRyy	RrYy	Rryy
rY	RrYY	RrYy	rrYY	rrYy
Ry	RrYy	Rryy	rrYy	rryy

Round Yellow : 9 Round green : 3 Wrinkled yellow : 3 Wrinkled green : 1
Phenotypic ratio : 9 : 3 : 3 : 1
Genotypic ratio : 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1

 

Question 2. Explain with suitable example an independent assortment.
Answer: (1) The law of independent assortment states that when hybrid possessing two or more pairs of contrasting characters bearing alleles form gametes, the alleles in each pair segregate independently of the other pair. Therefore, the inheritance of one pair of characters is independent of that of the other pair of characters. This fundamental principle ensures genetic diversity in sexually reproducing organisms.
(2) For example, when we cross a pea plant which is tall and having purple flowers with dwarf plant having white flowers we obtain all tall plants with purple flowers in F₁ generation. When F₁ generation are selfed, 9 : 3 : 3 : 1 ratio was obtained in F₂ generation with 9 tall and purple flower, 3 tall with white flowers, 3 dwarf with purple flowers and 1 which was dwarf and white. Tallness and purple colour are dominant traits while dwarfness and white colour are recessive traits.

(i) Homozygous tall purple – TTPP
(ii) Homozygous dwarf white – ttpp
In simple words: This law means that different traits (like height and flower color) are passed down to the next generation completely independently of each other. Inheriting one trait doesn't affect or force the inheritance of another.

🎯 Exam Tip: Always state the phenotypic ratio (9:3:3:1) clearly when explaining a dihybrid cross, and define which traits are dominant and recessive to secure full marks.

Dihybrid Cross: F2 Generation

• Parents (P): TTPP (Tall purple) × ttpp (Dwarf white)
• Gametes of P: TP × tp
• \( F_1 \) generation: TtPp (Tall purple)
• Gametes of \( F_1 \): TP, Tp, tP, tp

♀ \ ♂	TP	Tp	tP	tp
TP	TTPP Tall purple	TTPp Tall purple	TtPP Tall purple	TtPp Tall purple
Tp	TTPp Tall purple	TTpp Tall white	TtPp Tall purple	Ttpp Tall white
tP	TtPP Tall purple	TtPp Tall purple	ttPP Dwarf purple	ttPp Dwarf purple
tp	TtPp Tall purple	Ttpp Tall white	ttPp Dwarf purple	ttpp Dwarf white

Results:
Tall purple = 9
Tall white = 3
Dwarf purple = 3
Dwarf white = 1
Phenotypic ratio = \( 9 : 3 : 3 : 1 \)
The offspring of \( F_2 \) generation will be in the proportion of \( 9 : 3 : 3 : 1 \), where 9 are tall purple, 3 are tall white, 3 are dwarf purple and 1 is dwarf white.

 

Question 3. Define test cross and explain its significance.
Answer:
1. Definition of test cross: A cross between \( F_1 \) offspring and its homozygous recessive parent is called a test cross. It is a crucial tool in genetics to determine zygosity.
2. Significance of test cross:
• Test cross can be used to find out the genotype of any plant which shows dominant characters.
In simple words: A test cross is when you breed an organism showing a dominant trait with one that is homozygous recessive to find out its hidden genetic makeup.

🎯 Exam Tip: Always define the test cross clearly by mentioning the 'homozygous recessive parent' as this is the key term examiners look for.

 

Question 4. What is parthenogenesis? Explain the haplodiploid method of sex determination in honey bee.
Answer: I. Parthenogenesis is a natural form of asexual reproduction in which growth and development of embryos occur without fertilization by sperm. In some insects like honey bees, parthenogenesis means development of an embryo from an unfertilized egg cell.

II. In honey bee:
1. Sex determination is by haplodiploid system.
2. Sex is determined by the number of sets of chromosomes received by an individual.
3. The egg which is fertilized by sperm, becomes diploid and develops into female.
4. The egg which is not fertilized develops by parthenogenesis and develops into a male.
5. The queen and worker bee therefore contain 32 chromosomes. The drone, i.e. male bears 16 chromosomes.
6. The sperms are produced by mitosis while eggs are produced by meiosis. This unique mechanism ensures a balanced division of labor and reproductive roles within the hive.
In simple words: Parthenogenesis is when an egg develops into a new baby without needing to be fertilized by a sperm. In honey bees, fertilized eggs become females (queens or workers) with a double set of chromosomes, while unfertilized eggs become males (drones) with only a single set.

🎯 Exam Tip: Remember that male honey bees (drones) are haploid (16 chromosomes) and produced by parthenogenesis, while females are diploid (32 chromosomes). Highlighting this difference in chromosome numbers will help you secure full marks.

Sex Determination in Honey Bees (Haplo-diploid System)

• Female (Queen): Diploid (2n = 32) → Undergoes Meiosis → Eggs (n = 16)
• Male (Drone): Haploid (n = 16) → Undergoes Mitosis → Sperm (n = 16)
• Parthenogenesis: Unfertilized Egg (n = 16) develops directly into a Haploid male (Drone, n = 16)
• Fertilization: Egg (n = 16) fuses with Sperm (n = 16) → Diploid female (Queen/Worker, 2n = 32)

 

Question 5. In the answer for inheritance of X-linked. genes, Madhav had shown carrier male. His answer was marked incorrect. Madhav was wondering why his marks were cut. Explain the reason.
Answer: Males can never be carriers. They have single X and other Y chromosome. In X linked inheritance, the genes are present on the non-homologous region of X chromosome. Males do not have other X and hence if the genes are present on his X chromosome, they will not be suppressed in them. The Y chromosome does not have dominant gene to hide this expression as there is no homolorous region too. But in case of females, there are double X chromosomes and hence if X-linked gene is recessive, the other X can hide the expression of such X-linked gene. Thus she becomes a carrier without showing any physical characters. She is physically normal and does not suffer from such X-linked recessive disorder. Thus, Madhav will get his answer wrong due to incorrect concept.
In simple words: Males have only one X chromosome, so if they inherit a faulty X-linked gene, they will definitely show the disease and cannot be carriers. Females have two X chromosomes, so a healthy X chromosome can hide the effect of a faulty one, making them carriers.

🎯 Exam Tip: Always mention that males are hemizygous for X-linked traits because they have only one X chromosome, which prevents them from being carriers.

 

Question 6. With the help of neat labelled diagram, describe the structure of chromosome.
Answer: A typical eukaryotic chromosome shows the following structural parts:
1. Chromatids: Each metaphase chromosome consists of two identical sister chromatids joined together at the centromere.
2. Centromere (Primary Constriction): It is the narrow, non-staining region that holds the two chromatids together and contains a disc-shaped protein structure called the kinetochore for spindle attachment.
3. Secondary Constriction: Some chromosomes have additional constrictions called secondary constrictions, which act as nucleolar organizers.
4. Telomeres: These are the specialized terminal ends of chromosomes that prevent them from sticking to other chromosomes.
5. Chromonema: The thread-like coiled structure running through the length of each chromatid.
6. Chromomeres: Bead-like accumulations of chromatin material along the chromonema.
7. Satellite (Trabant): A small chromosomal segment separated from the main body by a secondary constriction.
In simple words: A chromosome looks like an 'X' shape made of two arms (chromatids) joined at a central button (centromere). The tips are called telomeres, which protect the ends from damage.

🎯 Exam Tip: When describing chromosome structure, always list and define key parts like the centromere, chromatids, secondary constriction, and telomeres to score full marks.

Structure of Chromosome

• Chromonemata
• Centromere
• Matrix
• Chromomeres
• Telomere
• Secondary constriction (ii)
• Primary constriction (centromere)
• Nucleolus
• Secondary constriction (i) or nucleolar organizer
• Satellite

(1) A chromosome is best visible during metaphase, when it is highly condensed.

(2) Chromosome shows two identical halves, called sister chromatids. Chromatids are held together at centromere which is also called primary constriction.

(3) Primary constriction has disc shaped plate called kinetochore. This plate is useful for attachment of spindle fibres at the time of cell division.

(4) Additional narrow areas called secondary constrictions are seen in some chromosomes which are known as nucleolar organizers. They help in the formation of nucleolus. At secondary constriction (i) there is nucleolar organising region. Secondary constriction (ii) shows attachment of satellite body or SAT body.

(5) Each chromatid is made up of sub-chromatids called chromonemata. Each chromonema consists of a long, unbranched, slender, highly coiled DNA thread. This double stranded DNA molecule extends throughout the length of the chromosome.

(6) The ends of the chromatid arms are called telomeres.

 

Question 7. What is criss-cross inheritance? Explain with suitable example.
Answer: Criss-cross inheritance is the type of inheritance in which the genes are passed on from father to daughter and then to her son, i.e. from male to female and from female to male (grandson). In other words, it is also said that the transmission is from the grandfather to his grandson through his daughter. This pattern of inheritance is commonly observed in sex-linked traits like hemophilia and color blindness.
In simple words: Criss-cross inheritance is when a genetic trait skips a generation by passing from a father to his daughter, and then from that daughter to her son (the grandfather's grandson).

🎯 Exam Tip: Clearly define the pathway (Father -> Daughter -> Grandson) and mention a classic example like color blindness or hemophilia to secure full marks.

Inheritance Of Colour Blindness Shows Criss-Cross Pattern

(1) Colour blindness is a sex-linked disorder in which the person concerned cannot distinguish between red and green colours.

(2) It is a recessively X-linked disorder, which is expressed in males. It is rarely seen in females.

(3) The genes for normal vision are dominant whereas those for colour blindness are recessive.

(4)

• Gene for normal vision: \( X^C \)
• Gene for colour blindness: \( X^c \)
• Normal female: \( X^C X^C \)
• Normal male: \( X^C Y \)
• Colour blind female: \( X^c X^c \)
• Carrier female: \( X^C X^c \)
• Colour blind male: \( X^c Y \)

 

Crosses Showing The Inheritance Of Colour Blindness

(i) A cross between normal female and colour-blind male:

• Parents: Colour blind male (\( X^c Y \)) \( \times \) Normal female (\( X^C X^C \))
• Gametes:
  • From male: \( X^c \), \( Y \)
  • From female: \( X^C \), \( X^C \)
• Offspring (F1 Generation):
  • \( X^C X^c \) (Carrier female)
  • \( X^C Y \) (Normal male)
  • \( X^C X^c \) (Carrier female)
  • \( X^C Y \) (Normal male)

🎯 Exam Tip: Always write down the genotypes of the parents and gametes clearly before drawing the genetic cross. Remember that a male inherits his X chromosome only from his mother, which explains the criss-cross inheritance pattern.

Inheritance of Colour Blindness

(ii) A cross of carrier female with normal male:

• Parents: Carrier female (\( X^C X^c \)) × Normal male (\( X^C Y \))
• Gametes: \( X^C \), \( X^c \) and \( X^C \), \( Y \)
• Progeny:
  • \( X^C X^C \) (Normal female)
  • \( X^C Y \) (Normal male)
  • \( X^C X^c \) (Carrier female)
  • \( X^c Y \) (Colour blind male)

(1) Normal female with Colour blind male. Such cross produces 50% carrier daughters and 50% normal sons.
(2) Carrier female with normal male. Such a cross produces 25% normal daughters, 25% normal sons, 25% carrier daughters and 25% colour blind sons.
(3) Colour blind father transmits the disorder to his grandson through his carrier daughter. The inheritance of characters from the father to his grandson through his daughter is called criss-cross inheritance.

 

Question 8. Describe the different types of chromosomes.
Answer: Chromosomes are classified into the following four types according to the position of the centromere in them. These structural variations play a crucial role during cell division as they affect how chromosomes move towards opposite poles:
(1) Metacentric: In metacentric chromosome, the centromere is situated in the middle of the chromosome. The two arms of the chromosome are nearly equal. It appears ‘V’-shaped during anaphase.
(2) Sub-metacentric: In sub-metacentric chromosome, the centromere is situated some distance away from the middle. Due to this, one arm of the chromosome is shorter than the other. It appears ‘L’-shaped during anaphase.
(3) Acrocentric: In acrocentric chromosome, the centromere is situated near the end of the chromosome. One arm of the acrocentric chromosome is very short while the other is long making it appear like ‘J’-shaped during anaphase.
(4) Telocentric: In telocentric chromosome, the centromere is situated at the proximal end of the chromosome. It appears ‘I’-shaped during anaphase.
In simple words: Chromosomes are grouped into four types based on where their center point (centromere) is located. This position gives them distinct shapes like V, L, J, or I when they pull apart during cell division.

🎯 Exam Tip: Remember to mention both the position of the centromere and the specific letter shape (V, L, J, I) formed during anaphase for each chromosome type to secure full marks.

(4) Telocentric : In telocentric chromosome, the centromere is situated at the tip of the chromosome. Telocentric chromosome has only one arm thus it appears rod-shaped.

 

II. Based on the functions, chromosomes are divided into autosomes and allosomes. Autosomes are somatic chromosomes which decide the body characters. Allosomes are sex chromosomes which decide the sex of the individual.