Principles of Inheritance And Variation Class 12 Biology NCERT PDF

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Principles of Inheritance and Variation

Chapter 5: Principles of Inheritance and Variation

Have you ever wondered why an elephant always gives birth only to a baby elephant and not some other animal?

Or why a mango seed forms only a mango plant and not any other plant? Given that they do, are the offspring identical to their parents? Or do they show differences in some of their characteristics?

Have you ever wondered why siblings sometimes look so similar to each other? Or sometimes even so different?

These and several related questions are dealt with, scientifically, in a branch of biology known as Genetics.

This subject deals with the inheritance, as well as the variation of characters from parents to offspring. Inheritance is the process by which characters are passed on from parent to progeny; it is the basis of heredity.

Variation is the degree to which progeny differ from their parents. Humans knew from as early as 8000-1000 B.C. that one of the causes of variation was hidden in sexual reproduction.

They exploited the variations that were naturally present in the wild populations of plants and animals to selectively breed and select for organisms that possessed desirable characteristics. For example, through artificial selection and domestication from ancestral

wild cows, we have well-known Indian breeds, e.g., Sahiwal cows in Punjab. We must, however, recognize that though our ancestors knew about the inheritance of characters and variation, they had very little idea about the scientific basis of these phenomena.


It was during the mid-nineteenth century that headway was made in the understanding of

Gregor Mendel conducted hybridization experiments on garden peas for seven years (1856-1863) and proposed the laws of inheritance in living organisms.

During Mendel’s investigations into inheritance patterns, it was for the first time that statistical
analysis and mathematical logic were applied to problems in biology. His experiments had a large sampling size, which gave greater credibility to the data that he collected.

Also, the confirmation of his inferences from experiments on successive generations of his test plants proved that his results pointed to general rules of inheritance rather than being unsubstantiated ideas.

Mendel investigated characteristics in the garden pea plant that was manifested as two opposing traits, e.g., tall or dwarf plants, yellow or green seeds.

This allowed him to set up a basic framework of rules governing inheritance, which was expanded on by later scientists to account for all the diverse natural observations and the complexity inherent in them.

Language English
No. of Pages28
PDF Size18.4 MB

NCERT Solutions Class 12 Biology Chapter 5 Principles of Inheritance and Variation

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


Gregor Mendel demonstrated characteristics of inheritance acquired by offspring from parents. He selected a pea plant for this experiment for the following reasons:

  • Peas exhibit several visible contrasting features such as dwarf/tall plants, wrinkled/round seeds, yellow/green pods, white/purple flowers, and so on.
  • As they possess bisexual flowers, they can easily undergo self-pollination. This is why pea plants are able to produce offspring with the same traits over generations
  • Cross-pollination can easily be achieved through emasculation wherein the stamen of the flower is plucked without any disturbance to the pistil
  • These plants have a short life span wherein they produce plenty of seeds in one generation alone

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?


The co-existence of two or more genes in the same chromosome is termed linkage. If the genes are located close to each other and on the same chromosome, they are inherited together and are referred to as linked genes. If two heterozygous parents exhibit linkage, then the outcome is as follows:

Parents BbLl x BbLl

Genotype Blue long Blue long

Phenotypes in F1 in all the possible may exhibit parental characteristics as the genes are linked completely. All the possible genotypes in F1 progeny can display a blue long type of phenotype in the above-mentioned example. However, if there is an incomplete linkage, the parental combination will comparatively be more than the newer combinations which are less in number.

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


The contributions of T.H. Morgan in the field of genetics are as follows:

  • He proposed and established that genes are positioned on the chromosomes
  • He discovered the basis for variations as a result of sexual reproduction
  • He discovered the concept of linkage and discriminated against linked and unlinked genes
  • He stated the chromosomal theory of linkage
  • He carried out a study on sex-linked inheritance
  • Morgan stated the chiasma type hypothesis demonstrating that the chiasma causes crossing over
  • He observed that the frequency of recombination between two linked genes is directly proportional to the distance between them both
  • Proposed the theory of inheritance
  • He put forward the methodology for chromosome mapping
  • He carried out a study on mutation

10. What is pedigree analysis? Suggest how such an analysis, can be useful.


A pedigree is a record of inheritance of a specific genetic trait for two or more generations which is presented in the form of a diagram or family tree. Pedigree analysis is an analysis of several generations of a family which is used on human beings and domesticated animals.

The usefulness of pedigree analysis:

  • Serves as a powerful tool which can be used to trace the inheritance of a particular trait, disease or an abnormality
  • It is helpful for genetic counsellors to suggest to couples about the possibility of having children with genetic abnormalities such as colour blindness, haemophobia, thalassaemia, sickle cell anaemia, etc
  • The analysis is helpful in indicating the origination of a trait and its flow in ancestors
  • It is helpful in suggesting that Mendel’s principles can be applied to human genetics with some alterations such as quantitative inheritance, sex-related linkage and characters
  • Helpful in reasoning why marriage between close relatives is harmful
  • Helpful in extensive research in the field of medical science

11. How is sex determined in human beings?


The chromosomal mechanism of the determination of sex in human beings is of the genotype XX-XY. The nucleus of each cell consists of 23 pairs of chromosomes or 46 chromosomes out of which 22 pairs are autosomes and the last pair, the 23rd pair is the sex chromosome. Females are homomorphic, i.e., they possess two same-sex chromosomes XX whereas males are heteromorphic, they possess two different sex chromosomes XY. Females are homogametic, producing only one type of egg (22+X) whereas males are heterogametic producing two types of sperms – (22+X) & (22+Y).

In the process of fertilization, if the sperm containing X chromosome fertilizes the egg, then the developing offspring would be a female (XX) and if the sperm containing Y chromosome fertilizes the egg, then the developing offspring would be a male (XY).

Therefore, the sex ratio that is produced in the progeny is 1:1. Heterogamesis is the chromosomal mechanism of sex determination which can be male heterogamety or female heterogamety.

Principles of Inheritance and Variation NCERT Textbook With Solutions PDF Free Download

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