Question

Mendelian monohybrid cross ratio is :-
A. 1 : 1
B. 3 : 1
C. 1 : 2 : 1
D. 9 : 3 : 3 : 1

Answer 1

Monohybrid cross is breeding between two individuals having either completely dominant genotype or recessive genotype and then there is formation of heterozygous offspring.

Complete answer:
First of all, you should know about monohybrid cross to solve this question. Monohybrid cross is the genetic mix between two individuals having homozygous genotype i.e., whether the genotype is completely dominant or the genotype if completely recessive and this further results in opposite phenotypes for a genetic trait. For example, breeding of long-stemmed pea plants with short-stemmed pea plants.
Mendel performed monohybrid cross and he did crosses with all the seven pairs of contrasting characters separately due to which he found that only of the two characters were expressed in ${F_1}$ generation but these characters show in ${F_1}$ generation are large number whereas in ${F_2}$ generations, characters are in small number which makes the ratio to be 3 : 1 which is called as monohybrid ratio.
If we look at options now, option A is 1 : 1. It is an incorrect option as there are more characters seen in ${F_1}$ generation than ${F_2}$ which makes the ratio to be 3 : 1 not 1 : 1. Therefore, this option gets eliminated.
Moving to option B i.e., 3 : 1. This option is correct since the characters shown in ${F_1}$ generation are large numbers whereas in ${F_2}$ generations, characters are in small numbers which makes the ratio to be 3 : 1.
Moving to option C i.e., 1 : 2 : 1. This option is also incorrect since in monohybrid crosses, the ratio found is 3 : 1 not 1 : 2 : 1. Therefore, this option also gets eliminated.
Last option D is 9 : 3 : 3 : 1. This option is also incorrect since in monohybrid crosses, the ratio found is 3 : 1 not 9 : 3 : 3 : 1. Therefore, this option also gets eliminated.

Thus, the correct answer is option B which is 3 : 1.

Note:
In monohybrid cross, the ratio is found to be 3 : 1 because the characters are shown in both ${F_1}$ and ${F_2}$ generation but more characters are shown in ${F_1}$ as compared to ${F_2}$ which makes the ratio to be 3 : 1. This is given in option B and thus B is the correct option.