Question

Generate a Punnett square for a heterozygous individual crossed with a heterozygous individual. Make the mutation recessive. What percentage of the offspring contain the mutation?

Answer 1

Answer: let the normal allele of the gene be designated as ‘A’

And let the mutant allele of the same gene be designated as ‘a’

A allele is dominant means it will show the phenotype(expressed character) in both the conditions homozygous( when both alleles are same) AA

As well as heterozygous (When both alleles are different) Aa

The mutant allele(a) is recessive hence it will not show the effect in presence of the dominant allele(A) hence it will express the phenotype only in homozygous ‘aa’ condition.

Consider the cross between two heterozygous individuals Aa and Aa

The gamates produced by each of them will be ‘A’ and ‘a’

The cross is shown by Punnett square, as follows

gamates	A	a
A	AA Dominant phenotype	Aa Dominant phenotype
a	Aa Dominant phenotype	aa Recessive phenotype

The genotypic ratio is 1(AA):2(Aa):1(aa) i.e. 1:2:1

And the phenotypic ratio will be 3(dominant):1(recessive) i.e.3:1

This is a typical Mendelian ratio obtained when there are two alleles for a character and one allele is completely dominant over the other allele.

The percentage of offspring containing the mutation will be 3/4 i.e.75%

And the percentage of offspring showing the effect of mutation will be 1/4 i.e. 25%