Class 12 Botany Chapter 4 Principles of Inheritance and Variation Solutions. Explore Class 12 Botany Chapter 4 Principles of Inheritance and Variation Solutions with accurate answers, clear explanations, and exam-oriented notes for effective preparation and better scores.

Class 12 Biology Chapter 4 Principles of Inheritance and Variation Solutions

A. Very Short Answer Questions (1 mark each)

1. What is genetics?

Ans: Genetics is the study of how offspring inherit hereditary characteristics from their parents and ancestors.

2. What is the monohybrid ratio given by Mendel?

Ans: The phenotypic ratio in the F2 generation of a typical monohybrid cross is 3:1.

3. What are inheritance and variation?

Ans: Inheritance is the transmission of hereditary information from parents to offspring across generations. Variation refers to differences between parents and offspring or among individuals of the same species.

4. What is a gene?

Ans: A gene is a unit of heredity. It is a specific segment of DNA that carries information influencing a characteristic of an organism.

5. What is codominance?

Ans: Codominance occurs when both alleles of a gene are expressed in a heterozygous individual, so both parental traits are observable together.

6. Define the following: allele or allelomorph, hybrid, dominant and recessive traits, parental generation, and reciprocal cross.

Ans: Allele/allelomorph: One of two alternative forms of a gene occupying the same locus on homologous chromosomes. Hybrid: Offspring produced by crossing individuals with different hereditary characteristics.

Dominant trait: A trait expressed in a heterozygous individual. Recessive trait: A trait masked by a dominant allele in a heterozygote and generally expressed in the homozygous condition. Parental generation: The original parents used in a genetic cross. Reciprocal cross: Two crosses in which the sexes of the parents carrying the contrasting traits are reversed.

7. What is a frameshift mutation?

Ans: A frameshift mutation occurs when one or more nucleotides are inserted into or deleted from a DNA sequence in a number not divisible by three, changing the reading frame and potentially altering the amino-acid sequence of a protein.

8. Write one characteristic of the principle of dominance.

Ans: Hereditary factors occur in pairs. When two contrasting alleles are present in a heterozygote, one may express itself (dominant allele), while the other is masked (recessive allele).

9. What are holandric genes?

Ans: Holandric genes are genes located on the Y chromosome and therefore typically transmitted from father to son.

10. Give two examples of incomplete dominance.

Ans: Four-o’clock plant (Mirabilis jalapa) and snapdragon (Antirrhinum).

11. What is codominance?

Ans: Codominance is the expression of both alleles in a heterozygote, with neither allele completely masking the other.

12. What carries DNA?

Ans: Chromosomes carry DNA.

13. Which Mendelian disorder is commonly called a royal disease?

Ans: Haemophilia.

14. What is linkage?

Ans: Linkage is the tendency of genes located on the same chromosome to be inherited together from one generation to the next.

15. What is the principle proposed by Bateson and Punnett called?

Ans: The principle of coupling and repulsion, also called gametic coupling and repulsion.

16. What is crossing over or recombination?

Ans: Crossing over is the exchange of corresponding DNA segments between non-sister chromatids of homologous chromosomes during the pachytene stage of meiosis.

17. How can the order of genes on a chromosome be determined?

Ans: It can be inferred from linkage studies and recombination frequencies between genes.

18. What are autosomes?

Ans: Autosomes are chromosomes that carry genes for most body characteristics and are not sex chromosomes.

19. In a dihybrid cross, which of the following genotypes is homozygous for both traits: (a) AaBB, (b) AaBb, (c) AABb, (d) aabb?

Ans: (d) aabb.

20. A 9:3:3:1 phenotypic ratio is obtained in which cross? (a) Test cross, (b) monohybrid cross, (c) dihybrid cross, (d) none of these.

Ans: (c) Dihybrid cross.

20. Mendelian variation in traits is explained by which of the following? (a) Linkage, (b) crossing over, (c) mutation, (d) law of independent assortment.

Ans: (d) Law of independent assortment.

Fill in the Blanks

21. A heterozygous parent produces ______ types of gametes.

Ans: Two types of gametes for a single gene.

22. Morgan called the association of genes located on the same chromosome ______.

Ans: Linkage.

23. Alternative forms of a gene that occupy the same locus are called ______.

Ans: Alleles.

24. In a pair of contrasting alleles, the allele expressed in a heterozygote is called ______.

Ans: Dominant.

25. The gain or loss of one or more chromosomes due to abnormal chromosome segregation is called ______.

Ans: Aneuploidy.

26. Sickle-cell anaemia results from the replacement of glutamic acid by ______ in haemoglobin.

Ans: Valine.

B. Short-Answer Questions (2 marks each)

27. What is mutation? H S ‘24

Ans: A mutation is a sudden, heritable change in genetic material, usually DNA, that may alter an organism’s genotype and sometimes its phenotype.

28. What is a monohybrid cross? How does it explain Mendel’s law of segregation?

Ans: In a monohybrid cross, parents differing in one pair of contrasting traits are crossed. For example, a pure tall pea plant (TT) crossed with a pure dwarf plant (tt) produces F1 plants that are all tall and heterozygous (Tt). During gamete formation, the two alleles separate, so each gamete carries either T or t. This demonstrates the law of segregation.

29. Under what conditions is the law of independent assortment observed, and why?

Ans: The law generally applies when the genes concerned are on different chromosomes or are sufficiently far apart on the same chromosome. During gamete formation, alleles of different gene pairs assort independently, producing different combinations of alleles in the gametes.

30. Explain Mendel’s principle of dominance or first law of inheritance.

Ans: When a pure-breeding tall pea plant (TT) is crossed with a pure-breeding dwarf plant (tt), all F1 offspring are tall (Tt). Although each F1 plant carries both alleles, only the tall trait is expressed. The allele for tallness is dominant, while the allele for dwarfness is recessive.

31. Describe codominance with an example.

Ans: In codominance, both alleles are expressed in the heterozygote. A standard example is the human AB blood group, in which both the A and B alleles are expressed. Note: The source’s cattle-coat example describes incomplete dominance rather than codominance; the scientifically correct codominance example is given here.

32. Distinguish between: (a) homozygous and heterozygous, (b) dominant and recessive traits, and (c) phenotype and genotype.

Ans:

(a) Homozygous: Has two identical alleles for a trait (e.g., TT or tt) and usually produces one type of gamete for that gene. Heterozygous: Has two different alleles (e.g., Tt) and produces two types of gametes for that gene.

(b) Dominant trait: Expressed in both homozygous and heterozygous conditions, e.g., T in TT and Tt. Recessive trait: Usually expressed only in the homozygous condition, e.g., dwarfness in tt.

(c) Phenotype: Observable characteristics of an organism. Genotype: The genetic constitution or allele combination of an organism. Individuals with the same phenotype may have different genotypes; individuals with the same genotype for a trait generally show the same phenotype under comparable conditions.

33. Why did Mendel choose pea plants for his hybridisation experiments?

Ans: Pea plants are easy to grow and require little space; have a short life cycle; possess clear contrasting traits; normally self-pollinate but can be cross-pollinated artificially; produce many seeds; and their hybrids are fertile. These features made controlled experiments and statistical analysis easier.

34. What factors contributed to the success of Mendel’s hybridisation experiments?

Ans: Mendel used pea plants with a short life cycle and clearly distinguishable traits; selected true-breeding parental varieties; studied one pair of contrasting traits at a time; planned his experiments carefully; recorded results systematically; and analysed large numbers of offspring quantitatively. These practices helped reveal consistent inheritance patterns.

35. Why did Thomas Hunt Morgan choose Drosophila melanogaster for his experiments?

Ans: Drosophila can be raised easily in the laboratory on a simple medium, completes a generation in about two weeks, produces many offspring, has readily distinguishable sexes, and shows numerous heritable variations that can be observed with a simple microscope.

36. Describe the role of chromosomes in heredity.

Ans: Chromosomes contain DNA and carry hereditary information. DNA is transmitted from one generation to the next through chromosomes. Chromosomal DNA directs many biochemical and developmental processes. During mitosis, chromosomes are distributed to daughter cells, preserving chromosome number. During meiosis, crossing over and chromosome assortment create new gene combinations and contribute to variation.

37. Write some characteristics of linkage.

Ans: Genes are arranged linearly on chromosomes. Genes close together on the same chromosome tend to be inherited together. Linked genes tend to travel together into gametes, although crossing over may separate them. Strong linkage therefore results in more parental-type combinations than recombinant combinations.

38. Describe: (a) phenylketonuria, (b) sickle-cell anaemia, and (c) thalassaemia.

Ans:

(a) Phenylketonuria (PKU): An inherited disorder usually caused by recessive variants affecting phenylalanine hydroxylase. Phenylalanine accumulates and, without dietary management, can damage the developing brain. The enzyme normally converts phenylalanine to tyrosine.

(b) Sickle-cell anaemia: An inherited disorder caused by a variant in the beta-globin gene. It produces abnormal haemoglobin (HbS), causing red blood cells to become sickle-shaped under certain conditions and impairing oxygen transport.

(c) Thalassaemia: An inherited disorder in which production of alpha- or beta-globin chains is reduced or absent. This leads to anaemia and, in severe cases, abnormal red blood cells and other complications.

39. Briefly describe: (a) Klinefelter syndrome and (b) Turner syndrome.

Ans:

(a) Klinefelter syndrome: Typically involves a 47,XXY chromosome complement in a person with a male phenotype. Features may include small testes, reduced fertility, sparse facial/body hair, and sometimes breast development. Intellectual ability varies.

(b) Turner syndrome: Typically involves one X chromosome (45,X) in a person with a female phenotype. Features may include short stature, underdeveloped ovaries, delayed puberty, and infertility; other features vary among individuals.

C. Descriptive Questions (3–5 marks each)

40. What are the causes or categories of human hereditary disorders?

Ans: Hereditary disorders may arise from changes in genes or in chromosome structure or number. Major categories include: (i) gene mutations; (ii) disorders caused by dominant or recessive genes on autosomes; (iii) metabolic disorders caused by defects in enzymes or other gene products; (iv) disorders due to changes in the number of autosomes; and (v) disorders due to changes in the number of sex chromosomes.

41. What is meant by the law of independent assortment? Explain it with a dihybrid cross.

Ans: The law of independent assortment states that the alleles of different gene pairs assort independently during gamete formation, provided the genes are unlinked or sufficiently far apart. Suppose yellow seed colour is represented by Y, green by y, round seed shape by R, and wrinkled by r. Crossing a pure-breeding yellow, round plant (YYRR) with a green, wrinkled plant (yyrr) produces F1 plants with genotype YyRr and yellow, round seeds. These plants form four types of gametes in equal proportions: YR, Yr, yR, and yr. Random fusion of these gametes can produce 16 combinations in the F2 generation, giving the typical phenotypic ratio 9:3:3:1.

42. Write Mendel’s laws of inheritance.

Ans:

(1) Law of dominance: When parents differing in a pair of contrasting traits are crossed, only one trait is usually expressed in the F1 generation; this is the dominant trait, while the masked trait is recessive.

(2) Law of segregation: The two alleles of a gene remain distinct and separate during gamete formation, so each gamete receives only one allele. The alleles reunite at fertilisation.

(3) Law of independent assortment: Alleles of different gene pairs assort independently during gamete formation when the genes are unlinked or sufficiently far apart.

43. Write a short note on phenylketonuria.

Ans: Phenylketonuria is usually an autosomal recessive inherited disorder associated with deficient phenylalanine hydroxylase activity. Phenylalanine is not converted efficiently into tyrosine and accumulates in the body. If untreated, the excess can damage the brain and cause developmental and intellectual disability. Early diagnosis and a carefully managed low-phenylalanine diet can help prevent serious complications.

44. What is the relationship between Mendel’s hereditary factors and chromosomes?

Ans: Mendel proposed that each trait is controlled by hereditary factors occurring in pairs. In diploid organisms, homologous chromosomes also occur in pairs. During meiosis, homologous chromosomes separate, and alleles segregate into different gametes. At fertilisation, one chromosome set comes from each parent, restoring the pair. The behaviour of chromosomes during meiosis therefore provides a physical explanation for Mendel’s laws of segregation and, for unlinked genes, independent assortment.

45. How is sex determined in humans?

Ans: Humans generally have the XX–XY sex-chromosome system. Typically, females have 44 autosomes and two X chromosomes (46,XX), and their ova carry 22 autosomes and one X chromosome. Typically, males have 44 autosomes, one X chromosome and one Y chromosome (46,XY), and produce two types of sperm: one carrying 22 autosomes plus X, and the other carrying 22 autosomes plus Y. Fertilisation of an X-bearing ovum by an X-bearing sperm usually produces an XX embryo; fertilisation by a Y-bearing sperm usually produces an XY embryo. Sex development is biologically more complex in some individuals.

D. Additional Questions and Answers (Class 12 Biology Chapter 4 Principles of Inheritance and Variation Solutions)

1. What is polygenic inheritance?

Ans: Polygenic inheritance is a form of quantitative inheritance in which a characteristic is influenced by two or more genes, often producing continuous variation.

2. What is the significance of recombination distance?

Ans: Recombination frequency helps estimate the distance between genes on a chromosome and is used to construct genetic maps. Genes with lower recombination frequency are generally closer together.

3. What was distinctive about Mendel’s experiments?

Ans: Mendel used large numbers of offspring, kept careful records, and applied quantitative and statistical analysis to explain inheritance patterns.

4. What is an allele?

Ans: An allele is one of two or more alternative forms of a gene found at the same locus.

5. Name two disorders associated with sex chromosomes.

Ans: Klinefelter syndrome and Turner syndrome.

6. Why is pedigree analysis important?

Ans: Controlled genetic crosses cannot generally be conducted in humans. Pedigree analysis traces the occurrence of a trait through several generations of a family and helps infer its inheritance pattern.

7. Why are eukaryotic chromosomes complex structures?

Ans: Eukaryotic chromosomes contain long DNA molecules associated with histone and other proteins, forming a compact and organised structure within the nucleus.

8. What are some advantages or uses of polyploidy?

Ans: Polyploidy is important in plant breeding and agriculture because it can contribute to larger organs, improved vigour, or useful crop traits in some species. Allopolyploidy, which combines chromosome sets from different species, has contributed to the formation of new plant species. Aneuploidy studies also help researchers understand the roles of individual chromosomes in development.

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