NCERT Class 12 Biology: Chapter – 5 Principle of Inheritance and Variation Part 5 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 15:

How does a mutagen induce mutation? Explain with example.


An agent such as chemical or physical factors, UV radiation which results in alterations in DNA sequences and consequently results in changes in phenotype and genotype of an organism is called mutagen. A mutagen can change the alignment and composition of nitrogen bases of DNA that results in changed product of gene (thus bring about mutation) .

For example chemical mutagens such as ethylmethanesulfonate (EMS) and X-rays can both be used to induce pseudorandom mutations into the genome of fly. EMS typically causes single base pair changes while X rays often results in deletion and gross chromosomal rearrangements.

Short Answer Type Questions

Question 1:

In a Mendelian monohybrid cross, the generation shows identical genotypic and phenotypic ratios. What does it tell us about the nature of alleles involved? Justify your answer.


In a monohybrid cross, starting with parents which are homozygous dominant and homozygous recessive, , at the same time with incomplete dominance can show identical genotypic and phenotypic ratios.

Sometimes had a phenotype that did not resembled either of the two parents and was in between them. The inheritance of flower colour in dog flower is a good example

In a cross between true breeding red flowered (RR) true breeding white flowered plants (rr) , the was pink. When was self-pollinated, the resulted in the following ratio 1 (RR) Red: 2 (Rr) Pink: 1 (rr) White. Here the genotype ratios were exactly as we would expect in any Mendelian monohybrid cross; but the phenotype ratio had changed from 3: 1 dominant: recessive ratio. Here R was not completely dominant over r and this made it possible to distinguish Rr as pink from RR (red) and rr (white) .

Question 2:

Can a child have blood group O if his parents have blood group ‘A’ and ‘B’ . Explain.


It is possible for a child to have blood group O if his parents have blood groups A and B. blood grouping in humans is controlled by the gene I. The gene (I) has three alleles and i. The genotype of blood group A is ; blood group is or and that of blood group O is ii.

So for a child to have O blood group, his parents should be with blood group genotype as and . Hence when the allele from parent 1 is “i” and allele from parent 2 is “i” the genotype of offspring becomes “ii” which means the blood group is .

Question 3:

What is Down՚s syndrome? Give its symptoms and cause. Why is it that the chances of having a child with Down՚s syndrome increases if the age of the mother exceeds forty years?


Down՚s syndrome

Caused due to presence of an additional copy of chromosome number 21 9trisomy of 21)

Affected individual is short statured with small round head, furrowed tongue and partially opened mouth. Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded.

Karyotype with 23 pairs of autosomes and 1 pair of sex chromosomes. (Total of 24 pairs i.e.. 47 chromosomes) .

The chances of having a child with Down՚s syndrome increases if the age of the mother exceeds forty years because ova are present in females since their birth and therefore older cells are more prone to chromosomal non-disjunction because of various Physico-chemical exposure during the mother՚s life time.

Question 4:

How was it concluded that genes are located on chromosomes?


Morgan confirmed the Mendelian laws of inheritance and the fact that genes are located on same chromosomes. Morgan had discovered that eye colour in Dorsophilia expressed a sex linked trait. All first generation offspring of a mutant white-eyed male and a normal red-eyed female would have red eyes because every chromosome pair would contain at least on copy of the X chromosome with dominant trait. The X chromosome is transferred to both male and female offspring because Dorsophilia exhibits XY type of sex determination.

Question 5:

A plant with red flowers was crossed with another plant with yellow flowers. If showed all flowers orange in colour, explain the inheritance.


Sometime, dominance of a particular trait over another trait may not be complete. This results in a situation that both the characters manifest together in some progeny. This condition is called incomplete dominance. Let us assume that red flowers have genotype RR and yellow flowers have genotype rr. All progenies in the generation will have Rr genotype. Since red colour fails to completely dominate the yellow colour, all plants in generation produce orange flowers.

Question 6:

What are the characteristic features of a true-breeding line?


A true breeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations. It increases homozygosity of the organism. This also helps to eliminate the harmful recessive genes through deletion. Thus promoting accumulation of superior genes.