CBSE Class 10 Science Heredity Sure Shot Questions Set 02

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ASSIGNMENT QUESTIONS SET - 2

CHAPTER - 9

HEREDITY AND EVOLUTION

1. Exchange of genetic material takes place in [1 Mark]
(a) vegetative reproduction
(b) asexual reproduction
(c) sexual reproduction
(d) budding

Answer: (c) sexual reproduction

1. Sexual reproduction involves the fusion of male and female gametes.
2. This fusion brings together genetic material from two different parents, leading to the exchange and recombination of genetic material.
3. Hence, the correct option is (c).

Teacher's Note:
a) Sexual reproduction introduces genetic variation through crossing over and gamete fusion.
b) Asexual methods like budding and vegetative propagation produce clones with identical genetic material.

2. Two pink coloured flowers on crossing resulted in 1 red, 2 pink and 1 white flower progeny. The nature of the cross will be [1 Mark]
(a) double fertilisation
(b) self pollination
(c) cross fertilisation
(d) no fertilisation

Answer: (c) cross fertilisation

1. The phenotypic ratio of 1 red : 2 pink : 1 white is characteristic of incomplete dominance in a monohybrid cross.
2. To obtain offspring from two flowers, pollen from one flower must be transferred to the stigma of another flower.
3. Hence, the nature of the cross is cross fertilisation.
4. Therefore, the correct option is (c).

Teacher's Note:
a) Incomplete dominance shows an intermediate phenotype in the heterozygote.
b) Controlled cross-pollination is required to cross two specific parent plants safely.

3. A cross between a tall plant (TT) and short pea plant (tt) resulted in progeny that were all tall plants because [1 Mark]
(a) tallness is the dominant trait
(b) shortness is the dominant trait
(c) tallness is the recessive trait
(d) height of pea plant is not governed by gene ‘T’ or ‘t’

Answer: (a) tallness is the dominant trait

1. The parent plants are homozygous tall (TT) and homozygous short (tt).
2. The F1 progeny receives one T allele and one t allele, making its genotype Tt.
3. Since the T allele for tallness expresses itself and masks the t allele, tallness is the dominant trait.
4. Hence, the correct option is (a).

Teacher's Note:
a) Mendel's Law of Dominance states that in a heterozygote, one trait masks the presence of another for the same characteristic.
b) Always identify the heterozygous condition in F1 generation to determine dominance.

4. Which of the following statement is incorrect? [1 Mark]
(a) For every hormone there is a gene.
(b) For every protein there is a gene.
(c) For production of every enzyme there is a gene.
(d) For every molecule of fat there is a gene

Answer: (d) For every molecule of fat there is a gene

1. Genes code for proteins, enzymes, and polypeptide hormones through transcription and translation.
2. Fats (lipids) are synthesized by specific enzyme pathways, but there is no specific gene dedicated to creating every single individual fat molecule.
3. Therefore, the statement regarding fat molecules is incorrect.
4. Hence, the correct option is (d).

Teacher's Note:
a) Biological macromolecules like proteins and RNA are direct gene products.
b) Lipids and carbohydrates are synthesized by enzyme-catalyzed reactions rather than direct genetic templates.

5. If a round, green seeded pea plant (RR yy) is crossed with wrinkled, yellow seeded pea plant, (rr YY) the seeds produced in F1 generation are [1 Mark]
(a) round and yellow
(b) round and green
(c) wrinkled and green
(d) wrinkled and yellow

Answer: (a) round and yellow

1. The alleles for seed shape are R (round, dominant) and r (wrinkled, recessive).
2. The alleles for seed color are Y (yellow, dominant) and y (green, recessive).
3. The F1 generation genotype is Rr Yy, combining the dominant traits from both parents.
4. Therefore, the F1 seeds are round and yellow, making the correct option (a).

Teacher's Note:
a) Dihybrid crosses demonstrate independent assortment of traits in the F1 generation.
b) Always combine alleles from each parent gene pair to form the dihybrid F1 genotype.

6. In human males all the chromosomes are paired perfectly except one. This/these unpaired chromosome is/are [1 Mark]
(i) large chromosome
(ii) small chromosome
(iii) Y-chromosome
(iv) X-chromosome
(a) (i) and (ii)
(b) (iii) only
(c) (iii) and (iv)
(d) (ii) and (iv)

Answer: (d) (ii) and (iv)

1. Human males possess one X chromosome and one Y chromosome as sex chromosomes (XY).
2. The Y chromosome is significantly smaller in size compared to the X chromosome.
3. Because they are morphologically different and non-identical, they form an unpaired or mismatched pair in male karyotypes.
4. Hence, the correct option is (d) (ii) and (iv).

Teacher's Note:
a) Human sex chromosomes determine gender, with females having XX and males having XY.
b) The Y chromosome lacks many genes found on the larger X chromosome.

7. The maleness of a child is determined by [1 Mark]
(a) the X chromosome in the zygote
(b) the Y chromosome in zygote
(c) the cytoplasm of germ cell which determines the sex
(d) sex is determined by chance

Answer: (b) the Y chromosome in zygote

1. Human males have XY chromosomes and females have XX chromosomes.
2. If a sperm carrying a Y chromosome fertilizes an egg with an X chromosome, the resulting zygote is XY and develops into a boy.
3. Hence, the correct option is (b).

Teacher's Note:
a) The presence of the Y chromosome is decisive for male sex determination in humans.
b) Students often confuse maternal and paternal chromosomal contributions.

8. A zygote which has an X-chromosome inherited from the father will develop into a [1 Mark]
(a) boy
(b) girl
(c) X- chromosome does not determine the sex of a child
(d) either boy or girl

Answer: (b) girl

1. A father contributes either an X chromosome or a Y chromosome to the zygote.
2. If the father contributes an X chromosome along with the mother's X chromosome, the zygote becomes XX, which develops into a girl.
3. Hence, the correct option is (b).

Teacher's Note:
a) Females always contribute an X chromosome, so the father's contribution determines the sex.
b) Remember that XX results in female and XY results in male.

9. Select the incorrect statement [1 Mark]
(a) Frequency of certain genes in a population change over several generations resulting in evolution
(b) Reduction in weight of the organism due to starvation is genetically controlled
(c) Low weight parents can have heavy weight progeny
(d) Traits which are not inherited over generations do not cause evolution

Answer: (b) Reduction in weight of the organism due to starvation is genetically controlled

1. Starvation leads to a drop in weight, which is an acquired trait and not genetically controlled.
2. Acquired traits cannot be passed on to the progeny and do not cause evolution.
3. Hence, the incorrect statement is option (b).

Teacher's Note:
a) Distinguish clearly between inherited traits and acquired traits caused by environmental factors.
b) Option (b) is scientifically false since starvation is environmental, not genetic.

10. New species may be formed if [1 Mark]
(i) DNA undergoes significant changes in germ cells
(ii) chromosome number changes in the gamete
(iii) there is no change in the genetic material
(iv) mating does not take place
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii), (iii) and (iv)
(d) (i), (ii) and (iii)

Answer: (a) (i) and (ii)

1. Speciation requires significant genetic variations such as DNA changes in germ cells or variations in chromosome number.
2. Statements (iii) and (iv) would not lead to the formation of new species.
3. Therefore, statements (i) and (ii) are correct.
4. Hence, the correct option is (a).

Teacher's Note:
a) Genetic drift, mutation, and natural selection drive the formation of new species.
b) Look out for negative statements like no change in genetic material which stop evolution.

11. Two pea plants one with round green seeds (RRyy) and another with wrinkled yellow (rrYY) seeds produce F1 progeny that have round, yellow (RrYy) seeds. When F1 plants are selfed, the F2 progeny will have new combination of characters. Choose the new combination from the following [1 Mark]
(i) Round, yellow
(ii) Round, green
(iii) Wrinkled, yellow
(iv) Wrinkled, green

Answer: (c) (ii) and (iii)

1. The parental combinations are round green and wrinkled yellow.
2. During independent assortment in a dihybrid cross, new combinations appear in the F2 generation.
3. These new combinations are round, green (ii) and wrinkled, yellow (iii).
4. Hence, the correct option group is (ii) and (iii).

Teacher's Note:
a) Dihybrid crosses yield a phenotypic ratio of 9:3:3:1 in the F2 generation.
b) Parental traits are round yellow and wrinkled green, while round green and wrinkled yellow are recombinant traits.

12. A basket of vegetables contains carrot, potato, radish and tomato. Which of them represent the correct homologous structures? [1 Mark]
(A) Carrot and potato
(B) Carrot and tomato
(C) Radish and carrot
(D) Radish and potato

Answer: (C) Radish and carrot

1. Carrot and radish are modified roots that share a similar basic structural design, making them homologous.
2. Potato is a modified stem and tomato is a fruit, so they do not share homologous root structures with carrot and radish.
3. Hence, the correct option is (C).

Teacher's Note:
a) Homologous organs have similar structures but may perform different functions.
b) Do not confuse modified stems like potatoes with underground roots like carrots and radishes.

13. Select the correct statement [1 Mark]
(A) Tendril of a pea plant and phylloclade of Opuntia are homologous
(B) Tendril of a pea plant and phylloclade of Opuntia are analogous
(C) Wings of birds and limbs of lizards are analogous
(D) Wings of birds and wings of bat are homologous

Answer: (B) Tendril of a pea plant and phylloclade of Opuntia are analogous

1. Tendrils of pea plants are modified leaves, whereas phylloclades of Opuntia are modified stems.
2. Both perform the function of support or reduced transpiration, but they have different origins, making them analogous.
3. Hence, the correct option is (B).

Teacher's Note:
a) Analogous organs have different basic structures but perform similar functions.
b) Pay close attention to whether plant parts are modifications of leaves, stems, or roots.

14. If the fossil of an organism is found in the deeper layers of earth, then we can predict that [1 Mark]
(A) the extinction of organism has occurred recently
(B) the extinction of organism has occurred thousands of years ago
(C) the fossil position in the layers of earth is not related to its time of extinction
(D) time of extinction cannot be determined

Answer: (B) the extinction of organism has occurred thousands of years ago

1. Deeper layers of the earth represent older geological strata.
2. Fossils buried deeper down were deposited long before those found in upper layers.
3. Hence, the correct option is (B).

Teacher's Note:
a) The law of superposition states that older layers are at the bottom and newer layers are at the top.
b) Fossil depth is directly related to the relative age of the organism.

15. Which of the following statements is not true with respect to variation? [1 Mark]
(A) All variations in a species have equal chance of survival
(B) Change in genetic composition results in variation
(C) Selection of variants by environmental factors forms the basis of evolutionary processes.
(D) Variation is minimum in asexual reproduction

Answer: (A) All variations in a species have equal chance of survival

1. Not all variations are equally beneficial; some may be disadvantageous depending on environmental changes.
2. Natural selection favors advantageous variations, giving them a higher chance of survival.
3. Hence, the correct option is (A).

Teacher's Note:
a) Environmental pressure determines whether a particular variation aids survival.
b) Asexual reproduction produces very few variations compared to sexual reproduction due to DNA copying errors.

16. A trait in an organism is influenced by [1 Mark]
(A) paternal DNA only
(B) maternal DNA only
(C) both maternal and paternal DNA
(D) neither by paternal nor by maternal DNA

Answer: (C) both maternal and paternal DNA

1. Offspring inherit one set of chromosomes from the mother and one set from the father.
2. Each parent contributes equally to the genetic makeup of the offspring.
3. Hence, the correct option is (C).

Teacher's Note:
a) Both parents contribute genetic material, leading to the inheritance of traits from both sides.
b) Mendel's laws of inheritance demonstrate that traits are determined by gene pairs contributed by both parents.

17. Select the group which shares maximum number of common characters [1 Mark]
(A) two individuals of a species
(B) two species of a genus
(C) two genera of a family
(D) two genera of two families

Answer: (A) two individuals of a species

1. Species is the lowest taxonomic category in biological classification.
2. Individuals belonging to the same species share the highest degree of genetic and morphological similarity.
3. Hence, the correct option is (A).

Teacher's Note:
a) As you move up the taxonomic hierarchy, common characters decrease.
b) Individuals within the same species can successfully interbreed and share maximum traits.

18. According to the evolutionary theory, formation of a new species is generally due to [1 Mark]
(a) sudden creation by nature
(b) accumulation of variations over several generations
(c) clones formed during asexual reproduction
(d) movement of individuals from one habitat to another

Answer: (b) accumulation of variations over several generations

1. Speciation occurs when subpopulations are isolated and accumulate genetic variations over long periods.
2. Natural selection and genetic drift drive these gradual changes until interbreeding is no longer possible.
3. Hence, the correct option is (B).

Teacher's Note:
a) Speciation is a gradual process driven by accumulation of variations and reproductive isolation.
b) Watch out for distractors suggesting sudden changes or cloning, which do not explain natural speciation.

19. From the list given below, select the character which can be acquired but not inherited [1 Mark]
(A) colour of eye
(B) colour of skin
(C) size of body
(D) nature of hair

Answer: (C) size of body

1. Acquired traits are those that develop during the lifetime of an organism and are not passed on to progeny.
2. Body size can be affected by environmental factors like nutrition and is an acquired trait, unlike eye color, skin color, and hair nature which are genetically inherited.
3. Hence, the correct option is (C).

Teacher's Note:
a) Distinguish between inherited traits (DNA level) and acquired traits (somatic level).
b) Do not confuse body size variations with genetic dwarfism or height.

20. The two versions of a trait (character) which are brought in by the male and female gametes are situated on [1 Mark]
(A) copies of the same chromosome
(B) two different chromosomes
(C) sex chromosomes
(D) any chromosome

Answer: (A) copies of the same chromosome

1. Each gene is located on a specific chromosome position called a locus.
2. Diploid organisms possess homologous chromosomes, where one set comes from the male gamete and the other from the female gamete.
3. The two alleles or versions of a trait occupy corresponding loci on the same pair of homologous chromosomes.
4. Hence, the correct option is (A).

Teacher's Note:
a) Homologous chromosomes carry genes for the same characters at the same loci.
b) Gametes carry only one copy of each chromosome.

21. Select the statements that describe characteristics of genes [1 Mark]
(i) genes are specific sequence of bases in a DNA molecule
(ii) a gene does not code for proteins
(iii) in individuals of a given species, a specific gene is located on a particular chromosome
(iv) each chromosome has only one gene
(A) (i) and (ii)
(B) (i) and (iii)
(C) (i) and (iv)
(D) (ii) and (iv)

Answer: (B) (i) and (iii)

1. Genes are functional segments of DNA composed of specific base sequences.
2. Statement (ii) is incorrect because genes specifically code for proteins.
3. Statement (iv) is incorrect because a single chromosome carries many genes.
4. Statements (i) and (iii) are correct characteristics of genes.
5. Hence, the correct option is (B).

Teacher's Note:
a) A gene is the basic physical and functional unit of heredity.
b) Each chromosome contains hundreds to thousands of genes.

22. In peas, a pure tall plant (TT) is crossed with a short plant (tt). The ratio of pure tall plants to short plants in F2 is [1 Mark]
(A) 1 : 3
(B) 3 : 1
(C) 1 : 1
(D) 2 : 1

Answer: (C) 1 : 1

1. Crossing TT with tt gives all heterozygous tall plants (Tt) in the F1 generation.
2. Selfing F1 (Tt × Tt) gives genotypes in F2 as TT, Tt, Tt, and tt.
3. The pure tall plants (TT) are 1, hybrid tall (Tt) are 2, and pure short (tt) are 1.
4. The ratio of pure tall plants (TT) to short plants (tt) in F2 is 1 : 1.
5. Hence, the correct option is (C).

Teacher's Note:
a) Pay close attention to the word "pure" tall versus total tall plants.
b) Total tall to short in F2 is 3 : 1, but pure tall to short is 1 : 1.

23. The number of pair (s) of sex chromosomes in the zygote of humans is [1 Mark]
(A) one
(B) two
(C) three
(D) four

Answer: (A) one

1. Human somatic cells and zygotes contain 23 pairs of chromosomes in total.
2. Out of these 23 pairs, 22 pairs are autosomes and exactly 1 pair is sex chromosomes (XX in females or XY in males).
3. Hence, the correct option is (A).

Teacher's Note:
a) Humans have 46 individual chromosomes, forming 23 pairs.
b) Only the 23rd pair determines the sex of the individual.

24. The theory of evolution of species by natural selection was given by [1 Mark]
(A) Mendel
(B) Darwin

Answer: (B) Darwin

1. Charles Darwin proposed the theory of evolution by natural selection along with Alfred Russel Wallace.
2. Gregor Mendel worked on the laws of inheritance using pea plants.
3. Hence, the correct option is (B).

Teacher's Note:
a) Natural selection explains how populations adapt to their environment over generations.
b) Gregor Mendel is known as the father of genetics.

25. Some dinosaurs had feathers although they could not fly but birds have feathers that help them to fly. In the context of evolution this means that [1 Mark]
(A) reptiles have evolved from birds
(B) there is no evolutionary connection between reptiles and birds
(C) feathers are homologous structures in both the organisms
(D) birds have evolved from reptiles

Answer: (D) birds have evolved from reptiles

1. Dinosaurs were reptiles that possessed feathers for insulation initially, which later got adapted for flight in birds.
2. This establishes an evolutionary relationship showing that birds evolved from reptilian ancestors.
3. Hence, the correct option is (D).

Teacher's Note:
a) Morphological adaptations provide crucial clues about common ancestry.
b) Do not confuse the chronological appearance of traits with homology.

26. State one advantage of variation of a species. [2009] [1 Mark]

Answer:
1. Variations increase the chances of survival of a species in a changing environment.
2. It prevents the wipeout of a population during drastic environmental changes like extreme temperature fluctuations.

Teacher's Note:
a) Variations form the raw material for organic evolution.
b) Emphasize how adaptability protects populations from sudden extinction.

27. What is the effect of DNA copying which is not perfectly accurate on the reproduction process? [2008] [1 Mark]

Answer:
1. Inaccurate DNA copying introduces variations in the offspring.
2. It leads to the creation of new traits that are essential for evolution and survival under altered environmental conditions.

Teacher's Note:
a) DNA replication is a biochemical process prone to minor errors.
b) Highlight that not all variations resulting from copying errors are beneficial.

28. What decides that humans give rise to humans? (Imp.) [1 Mark]

Answer:
1. Specific DNA molecules present in the chromosomes inside the nucleus of human cells decide that humans give rise to humans.

Teacher's Note:
a) Hereditary material ensures the transmission of species-specific traits.
b) Connect this concept directly to genetic continuity across generations.

29. What are hereditary characteristics? [1 Mark]

Answer:
1. Hereditary characteristics are traits that are genetically passed down from parents to their offspring across generations.

Teacher's Note:
a) These traits are encoded in the DNA within germ cells.
b) Distinguish clearly between inherited traits and acquired traits.

30. Are the variations created by sexual reproduction heritable or non-heritable? (Imp.) [1 Mark]

Answer:
1. The variations created by sexual reproduction are heritable because they involve changes in the DNA of germ cells.

Teacher's Note:
a) Sexual reproduction involves gamete formation and fertilization.
b) Somatic variations are non-heritable, whereas germline variations are heritable.

31. What are the components of a chromosome? (Imp.) [1 Mark]

Answer:
1. A chromosome is composed of Deoxyribonucleic Acid (DNA) and proteins.

Teacher's Note:
a) Histone proteins provide structural support to tightly pack the long DNA molecules.
b) State both components precisely to secure full credit.

32. What is a retrovirus? [1 Mark]

Answer:
1. A retrovirus is a type of virus that uses RNA as its genetic material and converts it into DNA inside a host cell using the enzyme reverse transcriptase.

Teacher's Note:
a) HIV is a classic example of a retrovirus.
b) Mention the reverse transcription mechanism for completeness.

33. What is a sex chromosome? [1 Mark]

Answer:
1. A sex chromosome is a chromosome that determines the biological sex of an organism.

Teacher's Note:
a) Humans possess X and Y chromosomes.
b) Contrast sex chromosomes with autosomes.

34. How many chromosomes are there in a human ovum? [1 Mark]

Answer:
1. There are 23 chromosomes in a human ovum.

Teacher's Note:
a) An ovum is a haploid gamete containing half the somatic chromosome number.
b) Remind students not to confuse somatic cells with gametes.

35. Who coined the term 'factor'? (Imp.) [1 Mark]

Answer:
1. Gregor Johann Mendel coined the term 'factor' for units of inheritance, which are now known as genes.

Teacher's Note:
a) Mendel used factors to explain the transmission of traits.
b) Wilhelm later named these factors as genes.

36. Give the monohybrid ratio. [1 Mark]

Answer:
1. The phenotypic monohybrid ratio is 3:1 and the genotypic ratio is 1:2:1.

Teacher's Note:
a) Mention both phenotypic and genotypic ratios if not specified.
b) 3:1 represents dominant to recessive physical appearance.

37. Write the dihybrid ratio. [1 Mark]

Answer:
1. The phenotypic dihybrid ratio is 9:3:3:1.

Teacher's Note:
a) This ratio arises from the independent assortment of two pairs of contrasting traits.
b) Ensure numbers add up correctly to sixteen total phenotypic combinations.

38. Define the term 'speciation'. [1 Mark]

Answer:
1. Speciation is the evolutionary process by which new species arise from existing populations due to genetic drift, isolation, or natural selection.

Teacher's Note:
a) Reproductive isolation is a key driver of speciation.
b) Emphasize the formation of interfertile distinct groups.

39. What is a factor? [1 Mark]

Answer:
1. A factor is a discrete unit of heredity proposed by Mendel, which controls the expression of a specific inherited character.

Teacher's Note:
a) Factors exist in pairs for every character.
b) Modern genetics equates factors with genes located on chromosomes.

40. Name the most accepted theory of evolution. [1 Mark]

Answer:
1. The Modern Synthetic Theory of Evolution (or Neo-Darwinism) is the most accepted theory.

Teacher's Note:
a) It combines Darwinian natural selection with Mendelian genetics.
b) Population genetics forms the bedrock of this modern synthesis.

41. Define the term 'evolution'. [1 Mark]

Answer:
1. Evolution is the gradual unfolding of new organisms from pre-existing simpler forms over millions of years.

Teacher's Note:
a) It involves descent with modification.
b) Link evolution directly to biodiversity and adaptation.

42. Whose theory influenced Darwin? What did Darwin fail to explain? (Imp.) [2 Marks]

Answer:
1. Charles Darwin was influenced by Thomas Malthus's essay on population growth.
2. Darwin failed to explain the origin of variations and the mechanism of their inheritance.

Teacher's Note:
a) Malthus's work highlighted the struggle for existence among competing organisms.
b) Mendel's subsequent discoveries filled the genetic gaps left unexplained by Darwin.

43. Define 'recessive characteristic'. [1 Mark]

Answer:
1. A recessive characteristic is a trait that remains masked in the presence of a dominant allele and expresses itself only in the homozygous condition.

Teacher's Note:
a) Both alleles must be identical for a recessive trait to appear.
b) Use standard notation like 'tt' to illustrate homozygosity.

44. What is the basis of sex determination in most plants and animals? (Imp.) [1 Mark]

Answer:
1. Genetic makeup inherited from parents, specifically through sex chromosomes, is the basis of sex determination in most plants and animals.

Teacher's Note:
a) Mention environmental cues as alternative mechanisms in some reptiles.
b) Highlight the role of heterogametic sex in determining offspring sex.

45. What are fossils? What do they tell about the process of evolution? [2008] [2 Marks]

Answer:
1. Fossils are the preserved remains or impressions of ancient organisms found in sedimentary rocks.
2. They provide direct evidence of evolutionary history by showing gradual changes in species and connecting links between different groups.

Teacher's Note:
a) Carbon dating helps determine the absolute age of fossils.
b) Explain how fossils establish evolutionary timelines.

46. What do you understand by the term heredity? [2008] [1 Mark]

Answer:
1. Heredity is the transmission of physical and mental characters from parents to their progeny.

Teacher's Note:
a) It accounts for similarities between parents and offspring.
b) Genetic material acts as the vehicle for hereditary transmission.

47. What constitutes the link between one generation and the next? [2008] [1 Mark]

Answer:
1. Gametes (sperm and ovum) constitute the link between one generation and the next.

Teacher's Note:
a) Gametes carry the parental DNA to the zygote.
b) Emphasize the cellular bridge formed during sexual reproduction.

48. "The sex of the children is determined by what they inherit from their father and not from the mother." Justify. [2008] [3 Marks]

Answer:
1. Human females possess two X chromosomes (XX) and produce only one type of ovum carrying an X chromosome.
2. Human males possess an X and a Y chromosome (XY) and produce two types of sperms, fifty percent carrying an X chromosome and fifty percent carrying a Y chromosome.
3. If a sperm carrying an X chromosome fertilizes the egg, the child is a girl; if a sperm carrying a Y chromosome fertilizes the egg, the child is a boy, proving the father's contribution determines sex.

Teacher's Note:
a) Use a simple chromosomal cross diagram to clarify gamete combinations.
b) Address the common misconception blaming mothers for child sex.

49. Explain the terms analogous and homologous organs with one example of each. [2008] [3 Marks]

Answer:
1. Homologous organs are those that have similar basic structural design and developmental origin but perform different functions; example: forelimbs of humans and wings of birds.
2. Analogous organs are those that have different basic structural origins but perform similar functions due to adaptation; example: wings of birds and wings of insects.
3. Homologous organs indicate divergent evolution from a common ancestor, whereas analogous organs indicate convergent evolution.

Teacher's Note:
a) Clear differentiation between origin and function is essential for full marks.
b) Provide correct biological pairs for both anatomical terms.

50. A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits blood group A or O is dominant? Why? [2008] [2 Marks]

Answer:
1. No, this information alone is not enough to tell which trait is dominant.
2. Both blood group A and blood group O can be explained genetically because blood group O is recessive, but the father could be heterozygous (AO) or the mother could carry recessive alleles.
3. To determine dominance conclusively, we need multi-generation pedigree analysis or crosses involving homozygous parents.

Teacher's Note:
a) ABO blood grouping involves multiple alleles where I^A and I^B are dominant over i.
b) Students often assume the trait appearing in the offspring is automatically dominant without checking parental genotypes.

51. Define variation in relation to a species. Why is variation beneficial to the species? [2008] [2 Marks]

Answer:
1. Variation refers to differences in characters or traits among individuals of the same species.
2. It is beneficial because it enables organisms to adapt to changing environmental conditions and prevents the wipeout of a species during drastic changes.

Teacher's Note:
a) Genetic variations are the raw material for evolutionary processes and natural selection.
b) Emphasize survival value during environmental stress like sudden temperature rises.

52. Describe briefly four ways in which individuals with a particular trait may increase in a population. [2008] [4 Marks]

Answer:
1. Natural selection where individuals with advantageous traits survive and reproduce better.
2. Genetic drift where random events alter allele frequencies in small populations.
3. Geographical isolation leading to adaptive radiation and accumulation of specific traits.
4. Migration or gene flow introducing new individuals with specific traits into a population.

Teacher's Note:
a) Mention evolutionary forces that dynamically shift gene pools over generations.
b) Avoid confusing genetic drift with directed selection pressures.
c) Highlight how isolation creates distinct localized populations.
d) Mention mutation as the ultimate source of novel traits.

53. What are acquired characteristics? (Imp.) [1 Mark]

Answer:
1. Acquired characteristics are non-inherited traits developed by an organism during its lifetime due to environmental influence, use and disuse of organs, or life experiences.

Teacher's Note:
a) These traits do not cause changes in the DNA of germ cells and hence are not passed to progeny.

54. What is variation? [1 Mark]

Answer:
1. Variation is the occurrence of differences among individuals of the same species arising due to errors in DNA copying, mutations, or sexual reproduction.

Teacher's Note:
a) Variations can be somatic (not inherited) or germinal (inherited).

55. Why is variation less common in asexually reproducing organisms? [1 Mark]

Answer:
1. Asexually reproducing organisms involve a single parent and cellular division via mitosis where DNA is copied with minimal errors, leading to offspring that are exact clones.

Teacher's Note:
a) The absence of gametic fusion and crossing over restricts genetic recombination.

56. Clarify the term heredity and variation. (Imp.) [2 Marks]

Answer:
1. Heredity is the transmission of traits and characteristics from parents to their offspring through genes.
2. Variation refers to the structural and functional differences present between parents and offspring or among individuals of the same species.

Teacher's Note:
a) Heredity maintains parental identity, while variation introduces diversity.
b) Both opposing yet complementary forces drive biological evolution.

57. Define variation in relation to a species. Why is variation beneficial to the species? (Imp.) [2 Marks]

Answer:
1. Variation means the degree of dissimilarity in morphological, physiological, or genetic traits among individuals of a species.
2. It ensures species survival by providing resilience against environmental disasters, diseases, or climate shifts.

Teacher's Note:
a) Uniform populations are vulnerable to total extinction when conditions change adversely.
b) Diverse traits guarantee that at least some organisms possess survival advantages.

58. What are autosomes? [1 Mark]

Answer:
1. Autosomes are chromosomes that do not determine the sex of an individual and are identically paired in both males and females.

Teacher's Note:
a) Humans have 22 pairs of autosomes and 1 pair of sex chromosomes.

59. What is the reason that a male is called heterogametic? (Imp.) [1 Mark]

Answer:
1. A human male produces two different types of gametes, one carrying an X chromosome and another carrying a Y chromosome, hence called heterogametic.

Teacher's Note:
a) Females produce only one type of gamete containing an X chromosome and are homogametic.

60. What was the basic study material of Mendel? How did he bring in the term 'factor'? [2 Marks]

Answer:
1. Mendel's basic study material was the garden pea plant (Pisum sativum).
2. He deduced that discrete units of inheritance control traits and called them factors, which are now known as genes.

Teacher's Note:
a) Mendel chose pea plants due to clear contrasting characters and easy self and cross-pollination.
b) Factors occur in pairs and segregate during gamete formation.

61. Why are the traits acquired during lifetime of an individual not inherited? [2009] [1 Mark]

Answer:
1. Acquired traits cause changes only in non-reproductive somatic tissues and do not bring about any modifications in the DNA of germ cells.

Teacher's Note:
a) Only changes in germ cell DNA can be transmitted to the next generation.

62. How is the sex of a newborn determined in humans? [2 Marks]

Answer:
1. Human sex is determined by the type of sex chromosome contributed by the male gamete at fertilization.
2. If a sperm carrying an X chromosome fertilizes an egg, the zygote develops into a female (XX), and if a sperm carrying a Y chromosome fertilizes an egg, it develops into a male (XY).

Teacher's Note:
a) Egg cells always contribute an X chromosome.
b) The father's sperm dictates the genetic sex of the child.

63. Do genetic combination of mothers play a significant role in determining the sex of a new born? [1 Mark]

Answer:
1. No, the mother's genetic combination does not determine the sex because human females produce only one type of egg carrying an X chromosome.

Teacher's Note:
a) Mothers contribute an X chromosome irrespective of whether a boy or a girl is born.

64. Mention three important features of fossils which help in the study of evolution. [3 Marks]

Answer:
1. Fossils provide direct evidence of extinct organisms and historical life forms.
2. They help establish evolutionary relationships and connecting links between different groups.
3. Stratigraphic fossil positioning helps determine the exact chronological timeline of species evolution.

Teacher's Note:
a) Hard body parts like bones and teeth form fossils easily.
b) Carbon dating helps calculate the precise age of fossils.
c) Archaeopteryx is a classic fossil example showing reptilian and avian features.

65. Why do all the gametes formed in human females have an X chromosome? [1 Mark]

Answer:
1. Human females possess two identical sex chromosomes (XX), so every normal haploid egg cell formed after meiosis receives one X chromosome.

Teacher's Note:
a) Meiotic segregation distributes one X to each mature ovum.

66. In human beings, the statistical probability of getting either a male or female child is 50 : 50. Give a suitable explanation. [2 Marks]

Answer:
1. Males produce equal proportions of X-bearing and Y-bearing sperms, while females produce only X-bearing eggs.
2. Fusion of an X egg with an X sperm gives XX (female) and with a Y sperm gives XY (male), resulting in an equal 1:1 probability.

Teacher's Note:
a) Random fertilization ensures equal chance for both sperm types.
b) The ratio remains statistical and does not guarantee strict alternation.

67. A very small population of a species faces a greater threat of extinction than a larger population. Provide a suitable genetic explanation. [2 Marks]

Answer:
1. Small populations exhibit reduced genetic diversity and higher chances of inbreeding depression.
2. Any sudden environmental catastrophe or disease can wipe out the entire gene pool easily due to lack of diverse adaptive traits.

Teacher's Note:
a) Genetic drift has a severe random impact on small gene pools.
b) Loss of advantageous alleles compromises evolutionary resilience.

68. What are homologous structures? Give an example. Is it necessary that homologous structures always have a common ancestor? [3 Marks]

Answer:
1. Homologous structures are organs having similar basic structural design and embryonic origin but performing different functions.
2. Example: Forelimbs of humans, birds, and whales.
3. Yes, homologous structures invariably indicate common ancestry and divergent evolution.

Teacher's Note:
a) Homology reflects divergent evolution adapting to different needs.
b) Contrast this with analogous structures which show convergent evolution.
c) Comparative anatomy relies heavily on identifying homologous organs.

69. Does the occurrence of diversity of animals on earth suggest their diverse ancestry also? Discuss this point in the light of evolution. [3 Marks]

Answer:
1. No, the immense diversity of animals does not imply diverse ancestry.
2. Evolutionary studies and shared biochemical similarities reveal that all life forms originated from a common ancestral root.
3. Diversity arises through divergent evolution, adaptation to distinct habitats, and natural selection over geological timescales.

Teacher's Note:
a) Universal genetic code and common cellular machinery point to monophyletic origin.
b) Speciation splits a single lineage into multiple diverse branches.
c) Structural variations are modifications of pre-existing ancestral blueprints.

70. Give the pair of contrasting traits of the following characters in pea plant and mention which is dominant and recessive (i) yellow seed (ii) round seed [2 Marks]

Answer:
1. For seed color, yellow seed is dominant and its contrasting trait is green seed which is recessive.
2. For seed shape, round seed is dominant and its contrasting trait is wrinkled seed which is recessive.

Teacher's Note:
a) Mendel studied seven pairs of contrasting traits in pea plants.
b) Dominant traits express in heterozygous conditions while recessives appear only in homozygous states.

71. Why did Mendel choose pea plant for his experiments? [3 Marks]

Answer:
1. Pea plants have clear-cut contrasting visible characters that are easy to identify.
2. They have a short life cycle, producing many offspring per generation, making statistical analysis reliable.
3. They normally undergo self-pollination but can be easily cross-pollinated artificially.

Teacher's Note:
a) Scientific name of garden pea is Pisum sativum.
b) Controlled pollination allowed Mendel to perform precise hybridization experiments.
c) The bisexual flowers facilitate both natural selfing and artificial emasculation.

72. A woman has only daughters. Analyse the situation genetically and provide a suitable explanation. [2 Marks]

Answer:
1. Sex is determined by the sperm contributed by the father, carrying either an X or a Y chromosome.
2. The birth of only daughters is a matter of chance where every time an X-bearing sperm fertilized the egg, which is governed by probability.

Teacher's Note:
a) Blaming the mother for the sex of the child is genetically incorrect.
b) Each conception is an independent statistical event with a 50 percent chance.

73. Does geographical isolation of individuals of a species lead to formation of a new species? Provide a suitable explanation. [3 Marks]

Answer:
1. Yes, geographical isolation often leads to speciation.
2. Physical barriers prevent interbreeding between isolated subpopulations, causing genetic drift and accumulation of independent mutations.
3. Over generations, reproductive isolation sets in, creating new species incapable of interbreeding.

Teacher's Note:
a) Geographical barriers include rivers, mountains, or oceans.
b) Natural selection acts differently on isolated niches.
c) This mechanism forms the foundation of allopatric speciation.

74. Bacteria have a simpler body plan when compared with human beings. Does it mean that human beings are more evolved than bacteria? Provide a suitable explanation. [3 Marks]

Answer:
1. No, having a simpler body plan does not mean that human beings are more evolved than bacteria.
2. Evolution simply generates diversity and complexity over time, but it does not inherently mean progress or superiority from a simpler form to a more complex form.
3. Bacteria inhabit some of the most inhospitable habitats on Earth like hot springs, deep-sea vents, and ice, proving that their simple body design is extremely efficient for their survival and successful proliferation.

Teacher's Note:
a) Evolution is not a direct ladder of progress, but rather a branching tree with multiple successful adaptations.
b) Students often confuse body complexity with evolutionary success; clarify that survival and reproduction define success.
c) Bacteria are among the most resilient and populous organisms on the planet.

75. All the human races like Africans, Asians, Europeans, Americans and others might have evolved from a common ancestor. Provide a few evidences in support of this view. [3 Marks]

Answer:
1. DNA studies and genetic tracking of mitochondrial DNA and the Y chromosome trace the earliest human lineages back to Africa.
2. Despite superficial differences in skin color, height, and facial features, all modern humans belong to a single biological species, Homo sapiens, and can interbreed successfully.
3. Archaeological excavations and fossil records of early human ancestors consistently point to an African origin before human migration across the globe.

Teacher's Note:
a) Genetic studies serve as the strongest modern evidence for common ancestry.
b) Avoid confusing cultural or racial classifications with distinct biological species.
c) Mentioning mitochondrial DNA studies helps secure full marks in board evaluations.

76. Differentiate between inherited and acquired characters. Give one example for each type. [3 Marks]

Answer:
1. Inherited characters are genetically controlled traits passed down from parents to offspring, whereas acquired characters are developed during an individual's lifetime and are not genetically controlled.
2. Inherited traits cause changes in DNA germ cells and are transmitted to the next generation, whereas acquired traits affect only somatic cells and cannot be inherited.
3. Example of an inherited character is human eye color or attached earlobes, and an example of an acquired character is learning to swim or muscle mass built through exercise.

Teacher's Note:
a) Clearly distinguish between germ cells and somatic cells as the basis of the difference.
b) Students often lose marks by failing to provide valid examples for both categories.
c) Remember that acquired traits die with the organism and do not lead to evolution.

77. Give reasons why acquired characters are not inherited. [2 Marks]

Answer:
1. Acquired characters develop in the somatic tissues or body cells of an organism during its lifetime due to environmental influence, practice, or injury.
2. These changes do not cause any alterations in the DNA sequence of the germ cells or reproductive cells, which are solely responsible for passing genetic information to the progeny.

Teacher's Note:
a) Emphasize the separation between somatic cells and germ cells.
b) Mentioning that DNA remains unaltered in reproductive cells ensures complete conceptual clarity.

78. Evolution has exhibited a greater stability of molecular structure when compared with morphological structures. Comment on the statement and justify your opinion. [3 Marks]

Answer:
1. Molecular structures such as DNA and core proteins show remarkable conservation and stability across diverse groups of organisms separated by millions of years of evolution.
2. Morphological structures undergo rapid and extensive modifications through natural selection to adapt to changing environments, resulting in diverse external body forms.
3. Small baseline changes in DNA accumulate slowly over generations, whereas external body designs adapt directly to ecological niches, making molecular changes far more stable and fundamental.

Teacher's Note:
a) Connect molecular stability to the conservation of genetic material like DNA and RNA.
b) Contrast this with outward physical adaptations driven by environmental pressures.
c) This concept is often used to establish evolutionary relationships using comparative biochemistry.

79. Give the basic features of the mechanism of inheritance. [3 Marks]

Answer:
1. Traits are controlled by genes, which are specific segments of DNA located on chromosomes.
2. An organism receives two sets of genes for every trait, one inherited from each parent.
3. The two inherited gene versions may be identical or different, with dominant traits expressing themselves and recessive traits remaining masked unless paired in homozygous form.

Teacher's Note:
a) Highlight the role of genes and DNA as the hereditary units.
b) State the principle of equal contribution from both mother and father clearly.
c) Mention dominance and recessiveness as core pillars of Mendelian inheritance.

80. Give reasons for the appearance of new combinations of characters in the F2 progeny. [2 Marks]

Answer:
1. The appearance of new combinations in the F2 generation is primarily due to the independent assortment of genes controlling different traits during gamete formation.
2. Recombination and crossing over during meiosis further mix maternal and genetic material, producing novel phenotypic traits not seen in the pure parental generation.

Teacher's Note:
a) Link the phenomenon directly to Mendel's Law of Independent Assortment.
b) Emphasize that independent separation of alleles creates genetic diversity in offspring.

(c) Morgan (d) Lamarck