Question

Why do we get a 9:3:3:1 ratio in ${F_2}$ of dihybrid cross instead of the expected 3:1 ratio for yellow round and green wrinkled respectively?

Answer 1

Hint:A dihybrid cross- is one in which two traits are studied simultaneously. Monohybrid cross – A cross or mating between two individuals in which only one genetic trait is studied is called a monohybrid cross.

Complete answer: Definition –
Dihybrid cross – A cross or mating between two individuals where two genetic traits are studied simultaneously is called a dihybrid cross.
Genotype – The genetic makeup of an organism is known as its genotype.
Phenotype – The expressed characteristics in an organism are known as its phenotype.
In any monohybrid cross, only one character is studied. The ratios of the possible genotypes and phenotypes of the offsprings are then calculated. In a monohybrid cross, the ${F_2}$ generation has a phenotypic ratio of 3:1.
Now, when we come to the dihybrid cross, the ${F_2}$ generation has the phenotypic ratio of 9:3:3:1. This is because more than one character is being studied and there are four possible phenotypes.
For example, when a dihybrid cross is performed for seed color and seed shape, the following are the dominant and recessive phenotypes –
Seed color –
Dominant allele – Yellow ( represented by ‘Y’)
Recessive allele – Green (represented by ‘y’)
Seed shape –
Dominant allele – Round (represented by ‘R’)
Recessive allele – Wrinkled (represented by ‘r’)
So, the possible phenotypes in this cross are –
Yellow round
Yellow wrinkled
Green round
Green wrinkled
When there are four possible phenotypes, it means there are four different genotypes also involved in producing each one of these phenotypes respectively. They are –
Yellow round – YR
Yellow wrinkled – Yr
Green round – yR
Green wrinkled – yr

Therefore, the ratio that we get is 9:3:3:1 instead of the expected 3:1.

Additional information:
The concept of performing crosses was introduced by Gregor Mendel when he observed the pea plants (Pisum sativum) growing in the garden of a church. He observed that seven visible characters of the pea plant were present in two alternating forms. That is when he began studying them and gave the concept of the dominance of one trait over the other. He also gave three laws of inheritance, namely –
Law of Dominance
Law of Segregation
Law of Independent Assortment

Note: The genotypic and phenotypic ratios are an important factor while studying crosses. These ratios tell us about the possible traits that might appear in the offsprings.