NCERT Class 12 Biology: Chapter –5 Principle of Inheritance and Variation Part 3

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Very Short Answer Type Questions

Question 1:

What is the cross between the progeny of and the homozygous recessive parent called? How is it useful?

Answer:

The cross between the progeny of and the homozygous recessive parent is called test cross. The progenies of a test cross are studies to determine the phenotype in generation

Question 2:

Do you think Mendel’s laws of inheritance would have been different if the characters that he chose were located on the same chromosome.

Answer:

Mendel’s laws of inheritance would not have been different even if the characters were located on the same chromosome. Because there is no change in the law of dominance that characters are controlled by discrete units called factors, factors open in pairs, and in a pair of dissimilar pair of factors one member of the pair dominates the other. And the universally applicable law of segregation that even though the parents contain two alleles during gamete formation, the alleles of a pair segregate from each other such that a gamete receives only one of the two factors or alleles.

Question 3:

Enlist the steps of controlled cross pollination. Would emasculation be needed in a cucurbit plant? Give reasons for your answer.

Answer:

Steps of controlled cross pollination are:

• Selection of parents with desired characters

• Protection of stigma from the contamination ( unwanted pollen grains ) by emasculation and bagging techniques

Emasculation is the removal of anther using forceps from bisexual flowers before the anther dehiscence. Cucurbits are monoecious (bisexual); having both male and female flowers on the same plant. Thus the chance of self- pollination is very high. This can prevent the desired pollen from landing on stigma. Hence emasculation is needed in cucurbit plant for cross pollination.

Question 4:

A person has to perform crosses for the purpose of studying inheritance of a few traits / characters. What should be the criteria for selecting the organisms?

Answer:

The criterion for selecting the organisms is true breeding. A true breeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations.

For example 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, tall or dwarf plants etc.

Question 5:

The pedigree chart given below shows a particular trait which is absent in parents but present in the next generation irrespective of sexes. Draw your conclusion on the basis of the pedigree.

Chapter-3- Question -5- Image of Pedigree Chart

Chapter-3- Question -5- Image of Pedigree Chart

Chapter-3- Question -5- Image of Pedigree Chart

Answer:

The first row shows mating between parents. The next row shows the number of progenies. There are three male and 2 female children. One boy and one girl are affected with some genetic disorder. Thus, the trait is autosome linked and recessive in nature.

Question 6:

In order to obtain the generation Mendel pollinated a pure-breeding tall plant with a pure breeding dwarf plant. But for getting the generation, he simply self-pollinated the tall plants. Why?

Answer:

To obtain the generation Mendel pollinated a pure-breeding tall plant with a pure breeding dwarf plant because a true breeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations. On doing so he obtained the generation with phenotype as tall and genotype to be Tt.

For getting the generation, he simply self-pollinated the tall plants because already the two alleles say T from one parent from pollen and t from the other parent through egg are united to produce zygote having one T allele and one t allele. Now to interpret the possible genotype and phenotype of progenies, Mendel self-pollinated the F1 plant of genotype Tt. When fertilisation takes place, the pollen grains of genotype T have chance to pollinate eggs of the genotype t, as well as of genotype t. Also the pollen grains of genotype t have chance to pollinate eggs of the genotype T, as well as of genotype t. As a result of random fertilisation the resultant zygote can be of genotypes TT, Tt, or tt.

Question 7:

“Genes contain the information that is required to express a particular trait.” Explain.

Answer:

During Mendel’s period, genes were not known to the scientists, But after the discovery of chromosomes, scientists could discover that genes are present on chromosomes and they were responsible for inheritance of characters. Hence, it is said that genes contain the information that is required to express a particular trait.

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