NCERT Solutions For Class 12 Biology Chapter 4 (Principles of Inheritance and Variation )| Download Free PDF

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NCERT Solutions for Class 12 Biology Chapter 4 – Principles of Inheritance and Variation 

The chapter provides information about the principles of Inheritance and Variation. Heredity is the transfer of character from parents to their offspring. These hereditary characteristics are present on the chromosomes in the form of genes.  These gene combinations express characters that may be more similar to one of its two parents. The differences in characters of offspring mainly depend upon uthe nique process of crossing over that occurs during meiosis.  This is one of the main reasons for producing recombination.

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Important Questions of Ncert Solutions for Class 12 Biology Chapter 4

Question 1. Mention the advantages of selecting the pea plant for the experiment by Mendel.

Answer:  The advantages of selecting garden pea (Pisum sativum) for experiments by Mendel were –

1. Pea has many visibly distinct, contrasting characters.
2. The life span of the pea plant is short, and they produce many seeds in one generation.
3. Pea flowers are bisexual and show self-pollination, with reproductive whorls being enclosed by the corolla.
4. It is easy to artificially cross-pollinate the pea flowers. The hybrids thus produced were fertile.

Question 2.  Differentiate between the following –

(a) Dominance and Recessiveness recessive alleles,

(b) Homozygous and Heterozygous

(c) Monohybrid and Dihybrid.

Answer:  (a) Dominance and Recessive

(b) Homozygous and Heterozygous

(c) Monohybrid and Dihybrid

Question 3. A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?

Answer: Locus is a fixed position on a chromosome, which is occupied by a single or more genes. Heterozygous organisms contain different alleles for an allelic pair. Hence, a diploid organism, which is heterozygous at four loci, will have four different contrasting characters at four different loci. For example, if an organism is heterozygous at four loci with four characters, say Aa, Bb, Cc, Dd, then during meiosis, it will segregate to form 8 separate gametes.

Types of gametes produced by organism=  wherein n= number of loci for which the organism is heterozygous. The given diploid organism is heterozygous for 4 loci, types of gametes produced =  = 16.

Question 4. Explain the Law of Dominance using a monohybrid cross.

Answer:  Mendels law of dominance states that a dominant allele expresses itself in a monohybrid cross and suppresses the expression of recessive allele. However, this recessive allele for a character is not lost and remains hidden or masked in the progenies of F1generation and reappears in the next generation.

For example, when pea plants with round seeds (RR) are crossed with plants with wrinkled seeds (rr), all seeds in F1 generation were found to be round (Rr). When these round seeds were self-fertilized, both the round and wrinkled seeds appeared in F2 generation in 3: 1 ratio. Hence, in F1 generation, the dominant character (round seeds) appeared and the recessive character (wrinkled seeds) got suppressed, which reappeared in F2 generation.

Question 5. Define and design a test-cross.

Answer:  Test cross is a cross between an organism with unknown genotype and a recessive parent. It is used to determine whether an individual is homozygous or heterozygous for a trait.

Example:

Suppose you have a violet and white flower and violet color (P) is dominant to white (p). The white flower must be homozygous for the recessive allele, but the genotype of the violet flower is unknown. It could be either PP or Pp.

A testcross will determine the organism's genotype. The unknown genotype can be determined by observing the phenotypes of the resulting offspring.

If crossing the unknown dominant phenotype (PP or Pp genotype) individual with the recessive phenotype individual produces only dominant phenotypes (no recessive), then the unknown individual is homozygous dominant.

If any recessive phenotypic individuals result from the cross, then the unknown individual must carry the recessive allele, and have the heterozygous genotype

Question 6. Using a Punnett Square, work out the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.

Answer:  The phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus, say height, is as follows:

Question 7.  When a cross is made between a tall plant with yellow seeds (TtYy) and a tall plant with green seeds (Ttyy), what proportions of phenotype in the offspring could be expected to be

(a) tall and green.

(b) dwarf and green.

Answer:  As given phenotype of TtYy parent, tallness is dominant over dwarfism and yellow seed is dominant over green seeds. The genotype of tall and green progeny will be Ttyy and TTyy, and that of dwarf and green progeny will be ttyy. Proportion of tall and green progeny= 3

Proportion of dwarf and green progeny= 1

 Question 8. Two heterozygous parents are crossed. If the two loci are linked, what would be the distribution of phenotypic features in F₁ generation for a dihybrid cross?

Answer:  In this case, the distribution of phenotypic features would follow the same pattern as in a dihybrid cross done by Mendel. It is important to recall the Law of Independent Assortment while answering this question. This law says that different traits segregate independently of each other during gamete formation.

Question 9. Briefly mention the contribution of T.H. Morgan in genetics.

Answer:  The contributions of T.H. Morgan are:

1. Morgan was the first to provide experimental verification for the chromosomal theory of inheritance by his work on Drosophila melanogaster.
2. He defined linkage between two genes located close together, which are always inherited together.
3. The identified linkage is of two types: tightly linked genes, where both genes are passed onto the next generation, and loosely linked genes, in which recombination may take place due to a large distance between the two genes.
4. He also defined the term recombination of non – parental gene recombination.
5. His findings of linked genes paved the path for genome mapping that is done today.
6. He also contributed to the understanding of sex – linked inheritance vii. He has also worked on mutations.

Question 10. How is sex determined in human beings?

Answer: Sex determination in human beings

In humans, out of 23 pairs of chromosomes, 22 pairs are exactly the same in males and female called autosomes. However, a pair of X chromosomes is present in females and XY in males. During spermatogenesis, males produce two types of gametes (sperms), 50% carries Y chromosome, and the remaining 50% contain the X chromosome. On the other hand, female produces only one kind of gamete (ovum) with X chromosomes only. When a sperm with a Y chromosome fertilises the ovum, the sex of the baby is male, and when a sperm with an X chromosome fertilises the egg, the sex of the baby is female.