Question

Mendelian ratio 9:3:3:1 is due to
A. Law of segregation
B. Law of purity of gametes
C. Law of independent assortment
D. Law of unit characters

Answer 1

Hint: 9:3:3:1 is the ratio of F2 progeny conducted by Mendel in which two traits were studied namely seed colour and shape. He also conducted monohybrid cross studying one trait. According to both the cross he gave three laws namely law of dominance, the law of independent assortment and law of segregation.

Complete answer:
Mendel performed experiments on pea plants from 1856-1863. Pea plants were chosen because they are easy to grow, self-pollinating, annual plants and produce many generations in a short period, and also several contrasting characters can be studied. The ratio, i.e. 9:3:3:1 was obtained from the dihybrid cross which is explained in the law of independent assortment. Two experiments conducted by Mendel were:
i. Monohybrid cross experiment: Two pea plants of contrasting traits for height (tall and short) were crossed. F1 progeny was observed to have all tall plants and after crossing F1 plants F2 plants were tall and short in the ratio 3:1.
ii. Dihybrid cross experiment: Two pea plants with two contrasting traits i.e. seed shape and seed colour were crossed. The round yellow seeded plant was crossed with the plant having wrinkled green seeds. In F1 progeny, all plants were having round yellow seeds and when F1 plants were crossed, in F2 generation out of 16 plants 9 were plants having round yellow seeds 3 were having round green, seeds 3 were having yellow wrinkled seeds, and 1 was having yellow wrinkled seeds giving a ratio of 9:3:3:1
Three laws were given by Mendel after the experiment:
a. Law of dominance: Alleles determining the trait is called dominant and the allele that is suppressed in the trait is called recessive. For example, in ‘Tt’ heterozygous state, ‘T ‘is for tall height and ‘t’ is for short height. ‘T’ is dominant and ‘t’ is a recessive allele. ‘T’ determines the trait in the heterozygous state (Tt) and the plant has a tall height.
b. Law of segregation: At the time of gamete formation two alleles in parent separate and one allele enters one gamete. Hence the offspring acquires one allele from each parent. This law is also known as the purity of gametes.
c. Law of independent assortment At the time of gamete formation, one pair of trait segregates independently of another pair of the trait. This law is totally based on the dihybrid cross.
Hence, the correct answer is option (C).

Note: Gregor Mendel was an Austrian monk who discovered the basic principles of genetics. He chose to study variation in plants in his monastery's 2 hectares (4.9 acres) experimental garden. He is also regarded as the father of genetics.