Cross Aa x Aa. What is the expected phenotype ratio in the offspring?

Step 1 — Work out the gametes

Each parent is heterozygous, so each one produces two kinds of gamete in equal numbers:

Aa → A or a

Step 2 — Draw the punnett square

Step 3 — Count the genotypes

The genotype ratio is 1 AA : 2 Aa : 1 aa.

Step 4 — Convert genotypes to phenotypes

The dominant allele A masks a, so AA and Aa look the same. Only aa shows the recessive phenotype:

1 + 2 = 3 dominant against 1 recessive → 3 : 1

Check

The ratio is an expectation, not a guarantee. In a litter of four you may see any split; across hundreds of offspring the counts converge on 3:1.