NCERT Book Class 12 Biology Principles of Inheritance

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 notany 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 thevariation 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 by 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 characters. 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, recognise that though our ancestors knew about the inheritance of characters and variation, they had very little idea about the scientific basis of these phenomena.

5.1 MENDEL’S LAWS OF INHERITANCE

It was during the mid-nineteenth century that headway was made in the understanding of inheritance. Gregor Mendel, conducted hybridisation experiments on garden peas for seven years (1856-1863) and proposed the laws of inheritance in living organisms. DuringMendel’s investigations into inheritance patterns it was for the first time that statistical analysis and mathematical logic were appliedto problems in biology. His experiments had a large sampling size, which gave greatercredibility 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 characters in the garden pea plant that were 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.

Mendel conducted such artificial pollination/cross pollination experiments using several true-breeding pea lines. A truebreeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations. Mendel selected 14 true-breeding pea plant varieties, as pairs which were similar except for one character with contrasting traits. Some of the contrasting traits selected were smooth or wrinkled seeds, yellow or green seeds, smooth or inflated pods, green or yellow pods and tall or dwarf plants (Figure 5.1, Table 5.1).

5.2 INHERITANCE OF ONE GENE

Let us take the example of one such hybridisation experiment carried out byMendel where he crossed tall and dwarf pea plants to study the inheritance of one gene(Figure 5.2). He collected the seeds produced as a result of this cross and grew them to generate plants of the first hybrid generation. This generation is also called the Filial1 progeny or the F1. Mendel observed that all the F1 progeny plants were tall, like one of its parents; none were dwarf (Figure 15.3). He made similar observations for the other pairs of traits – he found that the F1 always resembled either one of the parents, and that the trait of the other parent was not seen in them.

EXERCISES

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

2. Differentiate between the following –

 (a) Dominance and Recessive

 (b) Homozygous and Hetrozygous

 (c) Monohybrid and Dihybrid.

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

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

5. Define and design a test-cross.

6. Using a Punnett Square, workout 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.

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

 (a) tall and green.

 (b) dwarf and green.

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 dibybrid cross ?

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

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

11. How is sex determined in human beings?

12. A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.

13. Explain the following terms with example

 (a) Co-dominance

 (b) Incomplete dominance

14. What is point mutation? Give one example.

15. Who had proposed the chromosomal theory of the inheritance?

16. Mention any two autosomal genetic disorders with their symptoms

Please refer to attached file for NCERT Class 12 Biology Principles of Inheritance