CBSE Class 10 Science Heredity Sure Shot Questions Set 01

CBSE Class 10 Science Chapter 9 Heredity and evolution MCQs

Question. Which of the following is a vestigial structure in python ?
(A) Teeth
(B) Scales
(C) Hind limbs
(D) Poison glands
Answer : C

Question. Which of the following can you call atavism ?
(A) Wings of bat
(B) Fins of fish
(C) Cervical fistula of man
(D) Flippers of whale
Answer : C

Question. Which of the following is used for dating the bones ?
(A) C14
(B) I121
(C) K32
(D) All of these
Answer : A

Question. Evolution is the best defined by
(A) inheritance of acquired characters
(B) descent with modifications
(C) spontaneous generation
(D) struggle for existence
Answer : B

Question. Which of the following is a pair of homologous organs ?
(A) Lungs of rabbit and gills of rohu
(B) Wings of bat and wings of butterfly
(C) Pectoral fin of whale and fore limb of horse
(D) Wings of grasshopper and wings of crow
Answer : C

Question. The character that proves that frogs have evolved from fishes is :
(A) The ability to swim in water
(B) The tadpole larva in frogs which resembles the fishes in many characters
(C) Similarity in the shape of the head
(D) None of these
Answer : B

Question. Analogous organs have a :
(A) Common embryonic origin but perform different functions
(B) Different embryonic origin and perform different functions
(C) Common embryonic origin and perform similar functions
(D) Different embryonic origin but perform similar functions
Answer : D

Question. The term for similarity in organ structure seen in great diversity is :
(A) Homology
(B) Identical
(C) Analogy
(D) Symmetrical
Answer : A

Question. The evidence of evolution is based on
(A) paleontology
(B) embryology
(C) anatomically
(D) all of these
Answer : D

Question. Which one is not a vestigial organ in man ?
(A) Vermiform appendix
(B) Plica semilunaris
(C) Ear muscles
(D) Epiglottis
Answer : D

Question. Which one of these is likely to have been absent in free form at the time of origin of life ?
(A) Oxygen
(B) Hydrogen
(C) Ammonia
(D) Methane
Answer : A

Question. Analogous structures are :
(A) Structurally similar
(B) Functionally similar
(C) Structurally and functionally similar
(D) Normally non-functional
Answer : B

Question. The term evolution in biology means that
(A) fossils are old
(B) life began in Sea
(C) living things constantly change
(D) none of the above
Answer : C

Question. Homologous structures have
(A) similar origin & dissimilar functions
(B) dissimilar origin but similar functions
(C) similar origin & similar functions
(D) dissimilar origin and dissimilar structures
Answer : A

Question. Who gave the term genetics ?
(A) W.Bateson
(B) Andrew Benson
(C) T. Boveri
(D) G.J. Mendel
Answer : A

Question. XX-XO type of sex determination and XX-XY type of sex determination are the examples of
(a) male heterogamety
(b) female heterogamety
(c) male homogamety
(d) both (b) and (c).
Answer. A

Question. Fossils are
(A) fovea in the retina of vertebrate eye
(B) remains of organisms presents in the rocks
(C) the fossa present in the bones
(D) foramens through which nerves pass
Answer : B

Question. Connecting link between Reptiles and Birds is
(A) Dimetrodon
(B) Dodo
(C) Archaeopteryx
(D) Sphenodon
Answer : C

Question. Fossils are dated now by
(A) stratigraphic position
(B) amount of calcium residue
(C) association with other animals
(D) radioactive carbon contents
Answer : D

Question. Select the incorrect statement.
(a) In male grasshoppers, 50% of sperms have\ no sex chromosome.
(b) Female fruitfly is heterogametic.
(c) Human male produces two types of sperms 50% having X chromosome and 50% having Y chromosomes.
(d) In turtle, sex determination is regulated by environmental factors.
Answer. B

Question. A cross between F₁ generation and the recessive parent is called-
(A) Monohybrid cross
(B) Test cross
(C) Back cross
(D) Both (B) and (C)
Answer : B

Question. Somatic variation are due to
(A) Light
(B) Habitat
(C) Nutrition
(D) All of these
Answer : D

Question. Which of these is an inheritable variation?
(A) Somatic variation
(B) Acquired variation
(C) Germinal variation
(D) Both (A) and (B)
Answer : C

Question. Mutation is a
(a) change that causes evolution when inherited
(b) change which affects the parents only but never inherited
(c) change which affects the offspring of F₂ generation only
(d) factor responsible for plant growth.
Answer. A

Question. Allele that cannot express itself in presence of another is
(a) codominant
(b) dominant
(c) recessive
(d) complementary.
Answer. C

Question. Some of the dominant traits studied by Mendel were
(a) round seed shape, green seed colour and axial flower position
(b) terminal flower position, green pod colour and inflated pod shape
(c) violet flower colour, green pod colour and round seed shape
(d) wrinkled seed shape, yellow pod colour and axial flower position.
Answer. C

Question. In plant, tall phenotype is dominant over dwarf phenotype, and the alleles are designated as T and t, respectively. Upon crossing one tall and one dwarf plant, total 250 plants
were obtained, out of which 124 displayed tall phenotype and rest were dwarf. Thus, the genotype of the parent plants were
(a) TT × TT
(b) TT × tt
(c) Tt × Tt
(d) Tt × tt.
Answer. D

Question. The percentage of yr gamete produced by YyRr parent will be
(a) 25%
(b) 50%
(c) 75%
(d) 12.5%.
Answer. A

Question. If a genotype consists of different types of alleles, it is called
(a) homozygous
(b) heterozygous
(c) monoallelic
(d) uniallelic.
Answer. B
 

CBSE Class 10 Science Chapter 9 Heredity and evolution Assertion & Reasoning Based MCQs

For question numbers 41-50, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(a) Both assertion and reason are true, and reason is correct explanation of the assertion.
(b) Both assertion and reason are true, but reason is not the correct explanation of the assertion.
(c) Assertion is true, but reason is false.
(d) Assertion is false, but reason is true.

Question. Assertion : Mendel successfully postulated laws of heredity.
Reason : He recorded and analysed results of breeding experiments quantitatively.
Answer. A

Question. Assertion : The principle of segregation given by Mendel is the principle of purity of gametes.
Reason : Gametes are pure for a character and do not mix up.
Answer. A

Question. Assertion : Test cross is a back cross.
Reason : In test cross, individual is crossed with recessive parent.
Answer. B

Question. Assertion : Pure lines are called true breeds.
Reason : True breeds are used for cross breeding.
Answer. B

Question. Assertion : In a monohybrid cross, offspring of F1 generation express dominant character.
Reason : Dominance occurs only in heterozygous state.
Answer. C

Question. Assertion : The traits that are obtained from parents are inherited traits.
Reason : These traits were developed in the parents during their lifetime.
Answer. C

Question. Assertion : If blood group of both mother and father is ‘O’ then the blood group of children will also be O.
Reason : Blood group in humans is determined by many alleles of a gene viz. IA, IB, IO.
Answer. B

Question. Assertion : In grasshoppers, females are heterogametic and males are homogametic.
Reason : In grasshoppers, male has only one sex chromosome (XO) whereas the female has sex chromosomes (XX).
Answer. D

Question. Assertion : If mother is homozygous for black hair and father has red hair then their child can inherit black hair.
Reason : Gene for black hair is recessive to gene for red hair in humans.
Answer. C

Question. Assertion : A child which has inherited X chromosome from father will develop into a girl child.
Reason : Girl child inherits X chromosome from father and Y chromosome from mother
Answer. C

CBSE Class 10 Science Chapter 9 Heredity and evolution Very Short Answer Type Questions

Question. Why is the progeny always tall when a tall pea plant is crossed with a short pea plant?
Answer. When a tall pea plant is crossed with a short pea plant, the resultant progeny is always tall because tallness is a dominant trait while shortness is a recessive trait. Hence, dominant trait expresses itself in the progeny.

Question. How many pairs of allelic characters did Mendel study in pea plant?
Answer. Mendel studied seven pairs of allelic characters in pea plant.

Question. What do you mean by a true breeding plant?
Answer. A true breeding plant is the one that when self-fertilised, produces offspring with the same traits. They will be either homozygous dominant or homozygous recessive.

CBSE Class 10 Science Chapter 9 Heredity and evolution Short Answer Type Questions

Question. (a) State one advantage of variation to a species.
(b) What are sex chromosomes? How many sex chromosomes are there in humans? Name them.
Answer. a) Variation increases the chance of the species survival in a changing environment.
(b) A sex chromosome is a type of chromosome that participates in sex determination. Humans have two sex chromosomes, X and Y.

Question. Explain in brief how Mendel interpreted his results to show that the traits may be dominant or recessive.
Answer. When Mendel crossed two pea plants with a pair of contrasting characters only one character appeared in all the members of F1 progeny, the other was not expressed.
On selfing F1, the hidden characters reappeared in just 25% of the offsprings and the other 75% shared the characters expressed in F1.
Mendel concluded that the character which expresses itself in F1 and in 75% of the individuals of F2 is dominating while the other is recessive.

Question. Explain determination of sex among human beings, with the help of a diagram.
Answer. Diploid organisms like human beings have separate sexes. In organisms, where sex is determined genetically, a pair of chromosomes called sex chromosomes determines the sex of the individual. All other chromosomes are termed autosomes. In human beings, there are 46 chromosomes. Of these, one pair is of sex chromosomes which are of two types– X chromosome and Y chromosome.
(i) A male individual contains one X chromosome and one Y chromosome i.e., XY.
(ii) A female contains two X chromosomes i.e., XX.
The sex of the child is determined at the time of fertilisation when male and female gametes fuse to form zygote. It can be shown as follows:

Question. A blue colour flower plant denoted by BB is cross-bred with that of white colour flower plant denoted by bb.
(i) State the colour of flower you would expect in their F1 generation plants.
(ii) What must be the percentage of white flower plants in F2 generation if flowers of F1 plants are self-pollinated?
(iii) State the expected ratio of the genotypes BB and Bb in the F2 progeny.
Answer. (i) : The colour of the flower in F1 generation will be blue with Bb genotype.
(ii) If the flowers of F1 generations are self pollinated, then the percentage of white flowers in F2 generation must be 25%.
(iii) The expected ratio of the genotypes BB and Bb in the F2 progeny is 1 : 2.
The above results could be depicted by the given cross:

Question. Write the basic features of mechanism of inheritance. How do Mendel’s experiment show that traits are inherited independently?
Answer. The basic features of mechanism of inheritance are as follows:
(i) Characters are controlled by genes and each gene controls one character.
(ii) Chromosomes are gene carrier and genes are basic unit of heredity.
(iii) One form of gene may be dominant on other, i.e., genes are allelic in nature.
(iv) The two forms of alleles separate at the time of gamete formation, i.e., they do not mix with each other.
(v) Two allelic forms of a gene are brought together in zygote.
Traits are inherited independently can be explained by dihybrid cross. A cross is made between a pure round yellow seeded pea plant (RRYY) with wrinkled green seeded pea plant
(rryy). Yellow colour is dominant over green and rounded seed shape over wrinkled seed shape. F1 plants are all round
and yellow seeded. F1 plants are self breed and produce F2 generation. F2 generation has four types of plants: rounded yellow, rounded green, wrinkled yellow and wrinkled green in the ratio of 9:3:3:1 respectively.
Each of the characters if considered separately shows a ratio of 3:1 as found in monohybrid cross. The F2 ratio of 9:3:3:1 shows two types of recombinants, wrinkled yellow and rounded green. They can be produced only if the alleles of the two different characters are free to recombine, i.e., separate and combine independent to each other.

Question. Plant with full green pod is said to be homozygous dominant. Plant with constricted yellow pod is said to be homozygous recessive.
Crossing of these two plants can also give rise to plants with full yellow pod. Plants with constricted green pod are also produced.
(i) What conclusion could Mendel draw from this observation?
(ii) Work out a cross upto F2 generation for such type.
Answer. (i) The plants obtained on crossing the given plants can have different combination as gametic fusion can take place in any manner (both parental combination and recombination) are possible, which proved that both the traits of the two characters are assorted independently, and hence, plants with different kind of pods are produced.

ASSIGNMENT QUESTIONS SET - 1
CHAPTER - 9
HEREDITY AND EVOLUTION

1. What is heredity? [1 Mark]

Answer:
1. Heredity is the transmission of characters or traits from parents to their offspring.
2. It is responsible for the resemblances seen between parents and children.

Teacher's Note:
a) Heredity ensures the continuity of genetic information across generations.
b) The study of heredity forms the core of genetics.

2. Name the plant on which Mendel performed his experiments? [1 Mark]

Answer:
1. Gregor Mendel performed his hybridization experiments on the garden pea plant.
2. Its scientific name is Pisum sativum.

Teacher's Note:
a) Garden pea was chosen due to its short life cycle and easily distinguishable contrasting traits.
b) Always mention both common and scientific names for completeness.

3. Define variation? [1 Mark]

Answer:
1. Variation refers to the differences in characters or traits among individuals of the same species.
2. It can be caused by crossing over during meiosis or mutations.

Teacher's Note:
a) Variations are the raw material for evolution.
b) They can be either inherited or acquired during a lifetime.

4. Define a gene? [1 Mark]

Answer:
1. A gene is the functional unit of heredity located on a chromosome.
2. It consists of a specific sequence of DNA that codes for a particular protein or functional RNA molecule.

Teacher's Note:
a) Genes determine specific traits in an organism.
b) They are passed from parents to offspring during reproduction.

5. Who proposed the theory of inheritance of acquired characters. [1 Mark]

Answer:
1. Jean-Baptiste Lamarck proposed the theory of inheritance of acquired characters.
2. This theory is also known as Lamarckism.

Teacher's Note:
a) Lamarck suggested that characters acquired during an organism's lifetime are passed to progeny.
b) This theory was later disproven by modern genetics.

6. State one of the evolutionary forces leading to the origin of a new species according to the synthetic theory of evolution. [1 Mark]

Answer:
1. Natural selection is a primary evolutionary force leading to the origin of a new species.
2. Other forces include genetic drift, mutation, and gene flow.

Teacher's Note:
a) The synthetic theory combines Mendelian genetics with Darwinian evolution.
b) Mention any one valid force as requested by the question.

7. Give an example of a vestigial organ present in human body. [1 Mark]

Answer:
1. The vermiform appendix is a well-known vestigial organ in the human body.
2. Other examples include the nictitating membrane and wisdom teeth.

Teacher's Note:
a) Vestigial organs are remnants of structures that were functional in ancestral species.
b) They provide anatomical evidence for evolution.

8. What is the evolutionary significance of the fossil Archaeopteryx? [1 Mark]

Answer:
1. Archaeopteryx serves as a connecting link between reptiles and birds.
2. It possesses reptilian features like teeth and a tail, alongside avian features like feathers and wings.

Teacher's Note:
a) Connecting links provide direct evidence of organic evolution.
b) They show how one major group of animals evolved from another.

9. Who proposed the double helical model of DNA? [1 Mark]

Answer:
1. James Watson and Francis Crick proposed the double helical model of DNA.
2. Their model was published in 1953 based on X-ray diffraction data.

Teacher's Note:
a) Rosalind Franklin and Maurice Wilkins also contributed crucial X-ray crystallography data.
b) The model explains how genetic information is stored and replicated.

10. Who proposed the theory of natural selection? [1 Mark]

Answer:
1. Charles Darwin and Alfred Russel Wallace independently proposed the theory of natural selection.
2. Charles Darwin is primarily credited for formulating it extensively in his book On the Origin of Species.

Teacher's Note:
a) Natural selection drives adaptation and evolution of species.
b) Organisms with advantageous traits survive and reproduce more successfully.

11. What is retro virus? [1 Mark]

Answer:
1. A retrovirus is a type of virus that contains RNA as its genetic material.
2. It uses the enzyme reverse transcriptase to convert its RNA into DNA inside a host cell.

Teacher's Note:
a) HIV is a classic example of a retrovirus.
b) They insert their viral DNA into the host genome.

12. What is a genetically modified organism (GMO)? [1 Mark]

Answer:
1. A genetically modified organism is one whose genetic material has been altered using genetic engineering techniques.
2. These modifications are done to introduce desirable traits such as pest resistance or enhanced nutritional value.

Teacher's Note:
a) Bt cotton is a common example of a plant GMO.
b) Recombinant DNA technology is typically used to create GMOs.

13. Name any two genetic diseases. [1 Mark]

Answer:
1. Haemophilia and cystic fibrosis are two examples of genetic diseases.
2. Other examples include sickle cell anaemia and phenylketonuria.

Teacher's Note:
a) Genetic diseases are caused by abnormalities in an individual's genome.
b) They can be inherited in various patterns such as autosomal recessive or X-linked.

14. Write the expanded form of DNA? [1 Mark]

Answer:
1. The expanded form of DNA is Deoxyribonucleic acid.

Teacher's Note:
a) It is the hereditary material in humans and almost all other organisms.
b) Spelling must be precise to secure full marks.

15. What are the components of chromosome? [1 Mark]

Answer:
1. Chromosomes are primarily composed of DNA and histone proteins.
2. A small amount of RNA is also present in them.

Teacher's Note:
a) DNA molecules wrap around histone proteins to form nucleosomes.
b) This packaging allows long DNA molecules to fit inside the cell nucleus.

16. What is a retrovirus? [1 Mark]

Answer:
1. A retrovirus is an RNA virus that replicates in a host cell through the process of reverse transcription.
2. It synthesizes DNA from its RNA template inside the host cell.

Teacher's Note:
a) This is a repeated question reinforcing key viral replication mechanisms.
b) Reverse transcriptase is the diagnostic enzyme for retroviruses.

17. What is sex chromosome? [1 Mark]

Answer:
1. A sex chromosome is a chromosome that determines the biological sex of an organism.
2. In humans, they are designated as X and Y chromosomes.

Teacher's Note:
a) Humans have one pair of sex chromosomes out of twenty-three total pairs.
b) They contrast with autosomes, which determine general body characteristics.

18. How is sex determination in human beings? [2 Marks]

Answer:
1. Sex determination in humans is genetic and depends on the type of sperm that fertilizes the egg.
2. Females have two X chromosomes (XX) and produce eggs with a single X chromosome.
3. Males have one X and one Y chromosome (XY) and produce two types of sperms, containing either an X or a Y chromosome.
4. If an egg is fertilized by an X-bearing sperm, a female child (XX) is born; if fertilized by a Y-bearing sperm, a male child (XY) is born.

Teacher's Note:
a) Human sex determination follows the XX-XY mechanism.
b) The father's sperm determines the sex of the child, contrary to traditional societal misconceptions.

19. What do you understand by evolution? [2 Marks]

Answer:
1. Evolution is the cumulative gradual change in living organisms over successive generations.
2. It leads to the development of new species from pre-existing ones.
3. It results in biodiversity and adaptation to changing environments.
4. Key driving mechanisms include natural selection, genetic drift, and mutation.

Teacher's Note:
a) Evolution is a slow, continuous process taking place over millions of years.
b) It explains both the unity and diversity of life on Earth.

20. Define homologous organs? [2 Marks]

Answer:
1. Homologous organs are those organs that have the same basic structural design and developmental origin.
2. However, they perform different functions in different organisms.
3. They provide evidence of divergent evolution and common ancestry.
4. Example: Forelimbs of a human, a bat, and a whale.

Teacher's Note:
a) Remember the rule: same structure, different function.
b) Divergent evolution leads to homologous structures.

21. Explain Darwin’s theory of evolution? [3 Marks]

Answer:
1. Darwin's theory of evolution by natural selection states that organisms produce more offspring than can survive due to limited resources.
2. There is continuous variation among individuals, and those with favorable variations are better adapted to survive environmental challenges.
3. The survivors reproduce and pass these advantageous traits to their offspring, a process termed survival of the fittest.
4. Over generations, these accumulated variations lead to the formation of new species.

Teacher's Note:
a) Overproduction, struggle for existence, and natural selection are core tenets.
b) Darwin could not explain the exact physical mechanism of inheritance during his time.

22. Define genetics. What is the contribution of Mendel in this branch of biology? [3 Marks]

Answer:
1. Genetics is the branch of biology that deals with the study of heredity and variations.
2. Gregor Johann Mendel is known as the father of genetics for his pioneering hybridization experiments on garden peas.
3. He formulated fundamental laws of inheritance, namely the Law of Segregation and the Law of Independent Assortment.
4. He introduced the concept of discrete units of inheritance, later termed genes.

Teacher's Note:
a) Mendel used statistical analysis and large sample sizes for his pea plant experiments.
b) His work laid the mathematical and empirical foundation for modern genetics.

23. Where are the genes located? What is the chemical nature of gene? [2 Marks]

Answer:
1. Genes are located on chromosomes inside the nucleus of a cell.
2. Chemically, genes are segments of deoxyribonucleic acid (DNA) that code for specific proteins.

Teacher's Note:
a) Chromosomes act as vehicles carrying genes from parents to offspring.
b) The nucleotide sequence determines the specific functional output of a gene.

24. During which stage of cell division can chromosome be seen? Write the features of prokaryotic and eukaryotic chromosome. [3 Marks]

Answer:
1. Chromosomes can be best seen during the metaphase stage of cell division.
2. Prokaryotic chromosomes consist of a single circular DNA molecule without histones, not enclosed within a true nucleus.
3. Eukaryotic chromosomes are multiple linear structures composed of DNA tightly wound around histone proteins inside a membrane-bound nucleus.
4. Eukaryotic chromosomes undergo condensation prior to cell division.

Teacher's Note:
a) Metaphase chromosomes are thickest and most condensed, making them ideal for karyotyping.
b) The absence of a nuclear membrane in prokaryotes distinguishes their nucleoid region.

25. Who provided the evidence of DNA as a genetic material? Write the names of components of DNA? [3 Marks]

Answer:
1. Alfred Hershey and Martha Chase provided unequivocal evidence that DNA is the genetic material through their famous bacteriophage experiment in 1952.
2. A DNA molecule is composed of three main chemical components: a pentose sugar (deoxyribose), a phosphate group, and nitrogenous bases.
3. The nitrogenous bases are divided into purines (adenine and guanine) and pyrimidines (cytosine and thymine).

Teacher's Note:
a) Hershey-Chase used radioactive isotopes of sulfur and phosphorus to track proteins and DNA.
b) Nucleotides form the repeating monomeric units of the DNA polymer.

26. What do you understand by the double helical structure of DNA? Who proposed this structure? [3 Marks]

Answer:
1. The double helical structure of DNA consists of two polynucleotide chains wound around a central axis in a right-handed double helix.
2. The two strands are antiparallel, with the sugar-phosphate backbones on the outside and nitrogenous bases paired inside via hydrogen bonds.
3. Adenine pairs exclusively with thymine, and guanine pairs with cytosine.
4. James Watson and Francis Crick proposed this iconic structure in 1953.

Teacher's Note:
a) Base pairing rules (A=T and G\(\equiv\)C) ensure structural stability and accurate replication.
b) The dimensions of the double helix are uniform along its length.

27. Describe the different types of chromosomes. [3 Marks]

Answer:
1. Chromosomes are classified based on the position of the centromere into four main types.
2. Metacentric chromosomes have a centrally located centromere, forming two equal arms.
3. Sub-metacentric chromosomes have the centromere slightly away from the center, resulting in one shorter and one longer arm.
4. Acrocentric chromosomes have the centromere positioned close to one end, while telocentric chromosomes have the centromere at the terminal end.

Teacher's Note:
a) The centromere is the constricted region where spindle fibers attach during cell division.
b) Karyotype analysis utilizes these morphological distinctions to identify chromosomal anomalies.

28. How many types of nitrogenous bases are present in DNA? Name them. [2 Marks]

Answer:
1. There are four types of nitrogenous bases present in DNA.
2. They are Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).
3. Adenine pairs with thymine, while cytosine pairs with guanine.
4. They belong to two chemical classes: purines (adenine, guanine) and pyrimidines (cytosine, thymine).

Teacher's Note:
a) RNA contains uracil in place of thymine.
b) Chargaff's rule states that the amount of adenine equals thymine, and guanine equals cytosine.

29. How do embryological studies provide evidence for evolution? [2 Marks]

Answer:
1. Comparison of embryos of different vertebrates shows striking similarities in early stages of development.
2. These developmental similarities among diverse groups indicate a common ancestry and evolutionary relationship.

Teacher's Note:
a) Ernst Haeckel proposed that ontogeny recapitulates phylogeny.
b) vertebrate embryos share common features like gill slits in early stages.

30. Define evolution. And Describe the contribution of Lamarck? [3 Marks]

Answer:
1. Evolution is the gradual process of development of complex organisms from simpler pre-existing forms over millions of years.
2. Lamarck proposed the theory of inheritance of acquired characters, suggesting that physical changes in organisms during their lifetime can be passed to offspring.
3. He used the example of giraffes stretching their necks to reach high foliage, passing longer necks to the next generation.

Teacher's Note:
a) Evolution involves heritable changes in populations over generations.
b) Lamarckism was later disproved because somatic variations are not inherited.
c) Mentioning the exact mechanism proposed by Lamarck secures full marks.

31. What are homologous organs? How do they provide evidence in support of evolution? [3 Marks]

Answer:
1. Homologous organs are those organs which have the same basic structural design and origin, but different functions.
2. They indicate common ancestry, showing that different species evolved from a common ancestor.
3. Example includes the forelimbs of humans, lizards, and birds, which share skeletal structure adapted for different uses.

Teacher's Note:
a) Remember the rule of thumb: same structure, different function means homologous.
b) Students often confuse homologous organs with analogous organs; focus on embryonic origin.
c) Divergent evolution is the underlying evolutionary process here.

32. Define the following terms? (i) Vestigial organs (ii) Analogous organs. [2 Marks]

Answer:
1. (i) Vestigial organs are non-functional, reduced or rudimentary organs that were functional in ancestral forms.
2. (ii) Analogous organs are organs that have different basic structures and origins, but perform similar functions due to similar habitat.

Teacher's Note:
a) Human appendix and wisdom teeth are common examples of vestigial organs.
b) Analogous organs represent convergent evolution.

33. What are transgenic organisms? Which property of DNA is used as a tool in genetic engineering? [2 Marks]

Answer:
1. Transgenic organisms are those organisms that contain foreign genes integrated into their genome from another species.
2. The property of DNA being universal, cuttable by restriction enzymes, and able to replicate is used in genetic engineering.

Teacher's Note:
a) Recombinant DNA technology relies on plasmids and restriction endonucleases.
b) Bt cotton is a well-known example of a transgenic plant.

34. Explain how the sex of the child is determined at the time of conception in human beings. [3 Marks]

Answer:
1. Human beings have 23 pairs of chromosomes, including one pair of sex chromosomes (XX in females, XY in males).
2. Females produce eggs with only X chromosomes, while males produce two types of sperms, fifty percent with X and fifty percent with Y chromosomes.
3. If a sperm carrying an X chromosome fertilizes the egg, the child is female (XX); if a sperm carrying a Y chromosome fertilizes the egg, the child is male (XY).

Teacher's Note:
a) Chromosomal sex determination is strictly determined by the male gamete.
b) Students must clearly state the role of X and Y chromosomes.
c) Punnett square explanation helps visualize the 50:50 probability.

35. One of the examples of two analogous organs can be the wing of parrot and [1 Mark]
(A) Flippers of whale.
(c) Foreleg of horse
(b) Front leg of frog
(d) Wing of housefly

Answer: (d) Wing of housefly

1. Analogous organs have different structures but perform the same function.
2. Wings of a parrot are modified forelimbs, while wings of a housefly are outgrowths of the body wall.
3. Hence, the correct option is (d).

Teacher's Note:
a) Both bird and insect wings serve the common function of flight despite different origins.

36. Mention the ways by which variant genotypes are produced in organism? [2 Marks]

Answer:
1. Mutations in DNA sequences introduce new genetic variants.
2. Crossing over and independent assortment during meiosis produce genetic recombination in sexually reproducing organisms.

Teacher's Note:
a) Sexual reproduction is the primary driver of variation in populations.
b) Mutation provides the raw material for evolution.

37. In human beings blue eye colour is recessive to brown eye colour. If a brown eyed man has a blue eyed mother then find
a) What are the possible genotypes of his father?
b) What is the genotype of the man and his mother? [2 Marks]

Answer:
1. a) The possible genotypes of his father are homozygous brown (BB) or heterozygous brown (Bb).
2. b) The genotype of the mother with blue eyes is homozygous recessive (bb), and the man must be heterozygous brown (Bb) because he received one recessive b allele from his mother.

Teacher's Note:
a) Assign alleles carefully: let B be brown and b be blue.
b) A person showing recessive phenotype must be homozygous (bb).

38. What are fossils? Of what interest are fossils to the evolutionary biologists? [2 Marks]

Answer:
1. Fossils are the preserved remains or impressions of prehistoric organisms found in sedimentary rocks.
2. They provide direct evidence of evolution by revealing intermediate stages between species and showing how life forms changed over time.

Teacher's Note:
a) Carbon dating helps determine the age of fossils.
b) Archaeopteryx is a classic fossil linking reptiles and birds.

39. Who isolated DNA for the first time from pus cells? [1 Mark]

Answer:
Friedrich Miescher isolated DNA for the first time, originally calling it nuclein.

Teacher's Note:
a) Remember the historical milestone of DNA discovery from leukocyte nuclei in pus.

40. Why is DNA called polynucleotide? [1 Mark]

Answer:
DNA is a long polymer made up of many repeating units called nucleotides linked together.

Teacher's Note:
a) The prefix poly means many, referring to the chain of nucleotide monomers.

41. Name two purine nitrogenous bases found in a DNA molecule. [1 Mark]

Answer:
Adenine and guanine are the two purine bases found in DNA.

Teacher's Note:
a) Purines are double-ringed nitrogenous bases, unlike pyrimidines which are single-ringed.

42. Who put forward the double helical model of DNA? [1 Mark]

Answer:
James Watson and Francis Crick put forward the double helical model of DNA.

Teacher's Note:
a) Their model was based on X-ray diffraction data obtained by Rosalind Franklin and Maurice Wilkins.

43. What are the three chemically essential parts of nucleotides constituting a DNA? [1 Mark]

Answer:
A nitrogenous base, a pentose sugar (deoxyribose), and a phosphate group.

Teacher's Note:
a) Nucleoside plus a phosphate group forms a complete nucleotide.

44. Guinea pig having black colour when crossed with guinea pig having same colour produced 80 offspring, out of which 60 were black and 20 were white. Now, find out: (a) What is the possible genotype of the guinea pigs?
(b) Which trait is dominant and which trait is recessive?
(c) What is this cross called as and what is its phenotypic ratio? [3 Marks]

Answer:
1. (a) The genotype of both parent guinea pigs is heterozygous (Bb).
2. (b) Black coat color is the dominant trait, and white coat color is the recessive trait.
3. (c) This cross is called a monohybrid cross, and its phenotypic ratio is 3 black to 1 white (3:1).

Teacher's Note:
a) A 3:1 phenotypic ratio in offspring indicates both parents are heterozygous.
b) Recessive traits only appear in the homozygous condition.
c) Check numbers: 60 black and 20 white simplify to 3:1.

45. Distinguish between acquired and inherited traits giving one example of each. [2 Marks]

Answer:
1. Acquired traits are non-heritable characteristics developed during an organism's lifetime (e.g., muscle mass built through exercise).
2. Inherited traits are genetic characteristics passed down from parents to offspring through DNA (e.g., eye color or blood group).

Teacher's Note:
a) Acquired traits cannot be passed to the next generation because they do not affect germ cells.
b) Inherited traits involve changes in DNA.

46. Why did Mendel chosen pea plant for his experiments? [3 Marks]

Answer:
1. Pea plants have several distinct, easily identifiable contrasting characters.
2. They have a short life cycle, producing many offspring in a single generation, which makes statistical analysis easy.
3. They normally reproduce by self-pollination, but can also be easily cross-pollinated manually.

Teacher's Note:
a) Mendel selected Pisum sativum due to clear contrasting traits like tall and dwarf.
b) Controlled pollination allowed accurate hybridization experiments.
c) Mentioning clear contrasting traits is vital for scoring full marks.

47. Cat's paw, human hand and horse's legs-are these organs homologous or analogous? Give reason [2 Marks]

Answer:
1. These are homologous organs.
2. Reason: They have similar basic skeletal architecture and common embryonic origin, but are adapted to perform different functions.

Teacher's Note:
a) Mammalian forelimbs share pentadactyl limb structure.
b) Emphasize common ancestry and divergent evolution.

48. Wings of bird and wings of insect-are these organs homologous or analogous? Give one suitable season to support your answer. [2 Marks]

Answer:
1. These are analogous organs.
2. Reason: Although both are used for flight, they have different structural origins and embryonic designs, representing convergent evolution.

Teacher's Note:
a) Bird wings are modified forelimbs with bones, while insect wings are membranous cuticular extensions.
b) Similar function with different origin defines analogous structures.

49. Give one difference between eyes and eye spot. Which animal possesses eye spots? [2 Marks]

Answer:
1. An eye is a complex optical organ capable of forming a proper visual image, whereas an eyespot is a simple photoreceptor that can only detect the presence or absence of light.
2. Planaria (flatworms) possess simple eyespots.

Teacher's Note:
a) Eyespots represent an evolutionary precursor to complex eyes.
b) Planaria use eyespots for negative phototaxis to avoid light.

50. Give one difference between artificial selection and natural selection. [1 Mark]

Answer:
1. In artificial selection, humans choose the desired traits for breeding, whereas in natural selection, nature selects the traits that help an organism survive best in its environment.
2. Artificial selection is directed by human interference, while natural selection is driven by environmental pressures and adaptation.

Teacher's Note:
a) Both selection types drive evolutionary changes in populations over generations.
b) Emphasize the role of human agency versus environmental survival.

51. What is true - humans have evolved from chimpanzees or humans and chimpanzees both have evolved from a common ancestor? [1 Mark]

Answer:
1. It is true that humans and chimpanzees both have evolved from a common ancestor.
2. Chimpanzees did not evolve into humans; rather, both species diverged from a shared ancestral primate lineage.

Teacher's Note:
a) Corrects a common misconception about linear human evolution from modern apes.
b) Highlight that branching evolution creates sister species from ancestral forms.

52. What is the mechanism behind the expression of a particular trait? Explain briefly. [2 Marks]

Answer:
1. DNA acts as the information source for making proteins in the cell.
2. A specific segment of DNA (gene) codes for a particular protein, which in turn directs the synthesis of specific enzymes responsible for expressing that trait.

Teacher's Note:
a) Focuses on the central dogma concept linking DNA, proteins, and physical traits.
b) Watch for incomplete explanations that omit the role of cellular enzymes and proteins.

53. What will happen to the expression of a particular trait if a gene gets altered? [1 Mark]

Answer:
1. If a gene gets altered, the protein synthesized by it will be modified or non-functional.
2. Consequently, the expression of the particular trait will be affected, altered, or completely suppressed.

Teacher's Note:
a) Connects gene mutation directly to protein structure and functional trait changes.
b) Reinforce how genetic changes manifest as altered phenotypic characteristics.

54. What are various ways by which genes can enter a population? [2 Marks]

Answer:
1. Genes can enter a population through gene flow or migration, where individuals from another population breed with members of the local population.
2. Genes can also enter through natural selection favoring advantageous mutations or through genetic drift introducing new allele frequencies in small populations.

Teacher's Note:
a) Highlights migration and gene flow as primary vectors for introducing new alleles.
b) Remind students that mutation and genetic drift also alter population gene pools.

55. How will new species arise in case:
(a) Two sub-populations are separated due to a huge mountain in between them?
(b) A small population of individuals gets drifted away from the main land due to sea? [2 Marks]

Answer:
1. (a) Geographical isolation prevents interbreeding between the two sub-populations, leading to independent genetic drift and natural selection on each side until they become reproductively isolated new species.
2. (b) The small isolated group undergoes genetic drift due to the founder effect, accumulating unique variations separate from the main population until speciation occurs.

Teacher's Note:
a) Explains speciation driven by physical barriers and reproductive isolation.
b) Distinguish between geographical isolation and genetic drift mechanisms.

56. Only advantageous variations help in the evolution of an organism giving rise to a new species. Explain with the help of an example. [2 Marks]

Answer:
1. Advantageous variations increase an organism's chances of survival and reproduction in a changing environment, allowing those traits to be passed on to successive generations.
2. For example, red beetles living in green bushes are easily eaten by crows (disadvantageous variation), whereas a rare green color variation provides camouflage from predators, helping them survive, reproduce, and eventually dominate or form a new adapted population.

Teacher's Note:
a) Demonstrates natural selection through the classic beetle coloration example.
b) Ensure students explicitly mention survival advantage and reproductive success.

57. A trait may be inherited, but may not be expressed." Justify this statement with the help of a suitable example. [2 Marks]

Answer:
1. Inheritance means the gene is passed from parents to offspring, but expression depends on whether the allele is dominant or recessive and interacting factors.
2. For example, in Mendel's pea experiments, the F1 generation inherited the recessive allele for dwarfness from one parent, but it was not expressed because the dominant tall allele masked it.

Teacher's Note:
a) Clarifies the crucial distinction between genotype (inheritance) and phenotype (expression).
b) Use Mendel's monohybrid cross results as the standard explanatory model.

58. (a) What is genetics?
(b) Give the common name of plant on which Mendel performed his experiments.
(c) What for did Mendel use the term factors and what are these factors called now?
(d) What are genes? Where are the genes located? [4 Marks]

Answer:
1. (a) Genetics is the branch of science that deals with the study of heredity and variations.
2. (b) Garden pea plant (Pisum sativum).
3. (c) Mendel used the term factors to represent units of inheritance that transmit traits; these factors are called genes today.
4. (d) Genes are functional segments of DNA responsible for heredity and protein synthesis, located on chromosomes inside the nucleus of a cell.

Teacher's Note:
a) Covers four fundamental biological definitions systematically.
b) Ensure accurate scientific terminology for chromosome and gene location.

59. 'It is a matter of chance whether a couple will give birth to a male child or a female child." Justify this statement with the help of a flow chart showing the fusion of sex chromosomes. [3 Marks]

Answer:
1. Human females produce eggs with XX chromosomes (all eggs carry an X chromosome), while males produce sperm with XY chromosomes (half carry X and half carry Y).
2. If a sperm carrying an X chromosome fertilizes the egg, the zygote becomes XX (female child); if a sperm carrying a Y chromosome fertilizes the egg, the zygote becomes XY (male child).
3. Since fertilization of an egg by either an X-carrying or Y-carrying sperm occurs with equal probability, it is entirely a matter of chance.

Parents: Father (XY) × Mother (XX)
Gametes: (X), (Y) × (X), (X)
Offspring Fusions:
- Sperm X + Egg X = XX (Female)
- Sperm Y + Egg X = XY (Male)

Teacher's Note:
a) Explains chromosomal sex determination mechanism clearly with equal probabilities.
b) A flowchart or cross diagram is mandatory for full marks.
c) Emphasize that the father's sperm determines the sex of the child.

60. What are homologous organs? How do they provide evidence in support of evolution? [2 Marks]

Answer:
1. Homologous organs are those organs that have the same basic structural design and origin, but perform different functions in different species.
2. They provide evidence for evolution by indicating common ancestry, showing how divergent evolution adapted similar ancestral structures to different environmental needs.

Teacher's Note:
a) Define homology clearly by contrasting structure (same) and function (different).
b) Link homologous structures directly to divergent evolution from a common ancestor.

61. Who provided the evidence of DNA as a genetic material? Write the names of components of DNA? How many types of nitrogenous bases are present in DNA? Name them. [3 Marks]

Answer:
1. Alfred Hershey and Martha Chase provided conclusive evidence of DNA as the genetic material.
2. The components of DNA are deoxyribose sugar, a phosphate group, and nitrogenous bases.
3. There are four types of nitrogenous bases present in DNA: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).

Teacher's Note:
a) Credits the correct classic experiment (Hershey-Chase bacteriophage experiment).
b) List all three structural components of nucleotides clearly.
c) Name all four specific nitrogenous bases without omission.

62. Name the two homologous structures in vertebrates. Why are they so called? How do such organs help in understanding an evolutionary relationship? [3 Marks]

Answer:
1. Forelimbs of a frog and a bird (or forelimbs of humans and whales) are homologous structures.
2. They are called homologous because they share a similar basic anatomical plan and embryonic origin despite being adapted for different uses.
3. They help understand evolutionary relationships by pointing to a common ancestral vertebrate from which these different species evolved along divergent pathways.

Teacher's Note:
a) Provides concrete vertebrate examples illustrating homologous anatomy.
b) Reinforces the link between anatomical similarity and genealogical relatedness.

63. Will geographical isolation be a major factor in the speciation of a self-pollination plant species? Why or why not? [2 Marks]

Answer:
1. No, geographical isolation will not be a major factor in the speciation of a self-pollinating plant species.
2. Since self-pollinating plants do not depend on external agents or cross-pollination with other populations for reproduction, physical barriers do not significantly restrict their gene flow or reproductive isolation.

Teacher's Note:
a) Tests understanding of reproductive isolation mechanisms in self-pollinators.
b) Highlight that geographical barriers only impact species that interbreed across populations.

64. What are vestigial organs? Name any two vestigial organs in man and name organ which is vestigial in man but not in birds. [2 Marks]

Answer:
1. Vestigial organs are non-functional, reduced, or degenerate structures that were functional and fully developed in ancestral species.
2. Two vestigial organs in man are the vermiform appendix and wisdom teeth.
3. The tail vertebra (coccyx) is vestigial in man, whereas true tail vertebrae are present and functional in birds.

Teacher's Note:
a) Define vestigial structures accurately in terms of evolutionary regression.
b) Verify human examples like appendix and nictitating membrane.

65. All dead organisms do not leave their fossil records, but in some cases their fossils are formed. How do these fossils records form a direct evidence of past happenings? [2 Marks]

Answer:
1. Fossils are preserved traces or impressions of dead organisms trapped in sedimentary rocks over geological time.
2. They provide direct evidence of past happenings by revealing the chronological sequence of organisms, showing transitional forms, and proving how complex life forms evolved gradually from simpler ancestral forms.

Teacher's Note:
a) Explains fossilization and its role in establishing evolutionary timelines.
b) Emphasize the direct stratigraphic evidence provided by rock layers.

66. Evolution is a process in which simple life forms change into complex life forms by gradual changes. But, there is a difference between chemical and organic evolution. Differentiate by giving three points. [3 Marks]

Answer:

Teacher's Note:
a) Structured as a three-point difference table covering definition, stage, and mechanism.
b) Distinguish clearly between prebiotic chemical synthesis and biological speciation.

67. There are a number of ways by which the genes enter a population. Explain briefly the three ways. [3 Marks]

Answer:
1. Gene Flow / Migration: Movement of individuals from one population to another introduces new alleles into the recipient population's gene pool.
2. Mutation: Spontaneous changes in DNA sequences create entirely new alleles within the population.
3. Genetic Drift / Natural Selection: Random chance events or selective survival can alter allele frequencies, introducing or fixing new genes within small populations.

Teacher's Note:
a) Explains gene pool additions through migration, mutation, and selection mechanisms.
b) Ensure each of the three ways is distinct and properly elaborated.

68. Why can the wings of a bird and the wings of a bat not be considered analogous? (Imp.) [2 Marks]

Answer:
1. Wings of a bird and wings of a bat have a common basic structural design of forelimbs.
2. Therefore, they are homologous organs, not analogous organs, despite having a similar function of flying.

Teacher's Note:
a) Homologous organs have similar origin and basic structure but perform different or similar functions.
b) Analogous organs have different origin and structure but perform similar functions.

69. How did the Mendelian 'factors' acquire a change in the terminology? Who changed it? [2 Marks]

Answer:
1. Mendelian factors were later termed as genes, which are units of heredity.
2. This new terminology was given by Wilhelm Johannsen in 1909.

Teacher's Note:
a) Mendel used the term 'factor' for units controlling traits.
b) Johannsen coined 'gene' to describe these physical and functional units of heredity.

70. What is palaeontology? What is its importance? (Imp.) [2 Marks]

Answer:
1. Palaeontology is the branch of science that deals with the study of fossils.
2. It is important because fossils provide direct evidence of evolution and help establish evolutionary relationships between current and extinct species.

Teacher's Note:
a) Fossils are preserved remains or traces of ancient organisms found in sedimentary rocks.
b) Carbon dating of fossils helps determine their exact geological age.

71. The genotype of green stemmed tomato plants is denoted as GG and that of purple stemmed tomato plants is denoted as gg. When these two are crossed with each other: (a) What colour of stem would you expect in the F1 progeny?
(b) Give the percentage of purple stemmed plants if F1 plants are self pollinated.
(c) In what ratio would you find the genotypes GG and gg in the progeny?
Draw flow chart in support of your answer. [5 Marks]

Answer:
1. (a) Green stemmed because the green stem trait (G) is dominant over the purple stem trait (g).
2. (b) 25% of the plants will be purple stemmed in the F2 progeny.
3. (c) The ratio of genotypes GG to gg in the F2 progeny is 1:1.
4. Cross Flow Chart:
Parents: Green stem (GG) × Purple stem (gg)
Gametes: G and g
F1 Progeny: Gg (All green stemmed)
Self-pollination of F1 (Gg × Gg) gives F2 genotypes: 1 GG : 2 Gg : 1 gg.

Teacher's Note:
a) Apply Mendel's law of dominance and monohybrid cross principles.
b) Remember that F1 generation only shows the dominant phenotype.

72. How has the method of artificial selection by humans helped in the evolution of different vegetables? Explain in brief giving an example. [3 Marks]

Answer:
1. Humans have cultivated wild cabbage for over two thousand years and selected different traits to breed distinct vegetables.
2. For example, selecting for sterile flowers gave rise to cauliflower, and selecting for swollen stems resulted in kohlrabi.
3. This process of selecting desired variations artificially mimics natural selection but leads to human-preferred varieties.

Teacher's Note:
a) Artificial selection accelerates divergence from a common ancestor.
b) Other variations from wild cabbage include broccoli and kale.

73. (a) Write two factors which could lead to the rise of a new species.
(b) (i) What is the scientific term of the organs shown below? (ii) How do these organs provide evidence in support of evolution? [3 Marks]

Answer:
1. (a) Genetic drift and natural selection, or geographical isolation.
2. (b) (i) Homologous organs.
3. (b) (ii) They indicate common ancestry by having similar basic structural designs despite performing different functions in various species.

Teacher's Note:
a) Speciation occurs due to accumulation of genetic variations over generations.
b) Forelimbs of humans and lizards are classic examples of homologous structures.

74. (a) Name the type of sex chromosome present in human male and human female.
(b) With the help of a flow chart determine genetically in human beings the sex of the offspring if a sperm carrying X chromosome fertilizes the egg? [3 Marks]

Answer:
1. (a) Human male has XY chromosomes and human female has XX chromosomes.
2. (b) Flow chart representation:
Father (XY) produces sperm: X or Y
Mother (XX) produces egg: X only
Fertilization of egg (X) with sperm (X) results in XX (Female offspring).

Teacher's Note:
a) Human sex determination is strictly based on paternal chromosomes.
b) An X-carrying sperm fertilizing an egg always produces a female child.

75. In pea plant round seed is dominant over the wrinkled. If a cross is carried between these two plants, give answer to the following questions.
(a) Mention the genes for the traits of parents.
(b) State the trait of F1 hybrids.
(c) Write the ratio of F2 progeny obtained from this cross. What is the name of the cross? [2011] [3 Marks]

Answer:
1. (a) Genes for parents are RR (round) and rr (wrinkled).
2. (b) The trait of F1 hybrids is round seeds (Rr).
3. (c) Phenotypic ratio of F2 progeny is 3:1 (Round : Wrinkled); the cross is a monohybrid cross.

Teacher's Note:
a) Monohybrid cross deals with a single pair of contrasting characters.
b) Genotypic ratio in F2 is 1:2:1 (RR : Rr : rr).

76. Give appropriate terms for the following :
(a) The trait which can express itself in next generation.
(b) The trait an organism have due to inheritance.
(c) Origin of a new species from pre-existing one. [3 Marks]

Answer:
1. (a) Dominant trait.
2. (b) Inherited trait.
3. (c) Speciation.

Teacher's Note:
a) Dominant traits mask recessive traits in heterozygous conditions.
b) Speciation is a key driver of biodiversity.

77. If a pure tall pea plant is crossed with a pure dwarf plant, then in the first generation only tall plants appear.
(a) What happens to the traits of the dwarf plant?
(b) In the second generation, the dwarf trait reappears. Why? [2 Marks]

Answer:
1. (a) The dwarf trait remains hidden or suppressed in the F1 generation as a recessive trait.
2. (b) It reappears in the second generation because alleles segregate during gamete formation, allowing recessive alleles to pair up (tt).

Teacher's Note:
a) Recessive traits do not blend or get destroyed; they are simply masked.
b) This demonstrates Mendel's law of segregation.

78. How was it established that genes are located on the chromosomes? [2 Marks]

Answer:
1. Cytological studies showed parallel behavior between Mendelian factors and chromosomes during meiosis.
2. Sutton and Boveri formulated the Chromosomal Theory of Inheritance, proving genes are located on chromosomes.

Teacher's Note:
a) Chromosomes segregate and assort independently during gamete formation, mirroring gene behavior.
b) This unified Mendelian genetics with cellular cytology.

79. Clarify the terms 'haploid' and 'diploid'. What is the relation between the two terms? [2 Marks]

Answer:
1. Haploid refers to a cell having a single set of unpaired chromosomes (n), while diploid has two complete sets of chromosomes (2n).
2. The relation is that a diploid cell is formed by the fusion of two haploid gametes during fertilization.

Teacher's Note:
a) Gametes are haploid, whereas somatic cells are diploid.
b) Meiosis reduces diploid chromosome numbers to haploid.

80. Explain the law of segregation by taking an example. [3 Marks]

Answer:
1. The law of segregation states that alleles separate during gamete formation so that each gamete carries only one allele for each gene.
2. For example, in a tall pea plant with genotype Tt, the T and t alleles separate so that half the gametes receive T and the other half receive t.
3. These alleles recombine randomly at fertilization to restore the diploid state in the next generation.

Teacher's Note:
a) This is Mendel's first law of inheritance.
b) Segregation ensures genetic diversity and purity of gametes.

81. If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same species, which trait is likely to have arisen earlier? [1 Mark]

Answer:
1. Trait B is likely to have arisen earlier than trait A.
2. In an asexually reproducing species, variations are generated due to inaccuracies in DNA copying.
3. A trait present in a higher percentage of the population (60%) has had more time to accumulate and spread through generations than a rare trait present in only 10%.

Teacher's Note:
a) Asexual reproduction leads to identical clones, so new traits arise solely through mutations.
b) Higher frequency indicates older origin because progeny inherit parental traits.

82. How does creation of variations in a species promote survival? [2 Marks]

Answer:
1. Variations provide adaptability to organisms when environmental conditions undergo drastic changes.
2. If a sudden change like global warming or a new disease kills off a uniform population, individuals with beneficial variations are more likely to survive and continue the species.

Teacher's Note:
a) Genetic diversity is the primary shield against extinction during environmental stress.
b) Emphasize that natural selection acts on pre-existing variations in populations.

83. 'Variations that confer an advantage to an individual organism only will survive in a population.' Justify. [2 Marks]

Answer:
1. Beneficial variations help an organism adapt better to its environment, improving its chances of survival and reproduction.
2. As these individuals reproduce successfully, the advantageous trait gets passed on to their progeny, gradually increasing its frequency in the population through natural selection.

Teacher's Note:
a) Favorable traits accumulate in gene pools over generations via differential reproduction.
b) Detrimental or neutral traits without advantage often die out or remain minimal.

84. Suggest three similarities between Mendel's 'factors' and 'chromosomes'. [3 Marks]

Answer:
1. Both occur in pairs in somatic or diploid cells.
2. Both segregate during gamete formation so that only one of each pair enters a gamete.
3. Both maintain their independent identity and do not blend or mix when they come together in the zygote.

Teacher's Note:
a) Walter Sutton and Theodor Boveri noted parallel behaviors between Mendelian factors and chromosomes.
b) This realization led to the formulation of the Chromosomal Theory of Inheritance.
c) Focus on the exact pairing and separation rules established during meiosis.

85. Justify logically that many genes are present on one chromosome. [3 Marks]

Answer:
1. The total number of distinct traits and inherited characteristics in an organism is vastly greater than the total number of chromosomes present in its cells.
2. For example, humans have thousands of essential genes controlling various metabolic and structural traits, but only 23 pairs of chromosomes.
3. Therefore, multiple genes must be physically linked and linearly arranged on a single chromosome.

Teacher's Note:
a) Chromosomes are long DNA molecules packed with multiple functional protein-coding segments called genes.
b) Linkage groups correspond directly to the number of chromosome pairs.
c) Connect this concept clearly with gene linkage and independent assortment exceptions.