Punnett squares are used to show possible combinations of alleles or to predict the probability of a trait occurring in offspring. A parakeet that is heterozygous for blue feathers (dominant) is crossed with a parakeet that is homozygous for white feathers (recessive).
According to the Punnett square for this cross, what percentage of offspring is predicted to have blue feathers?

To get started, first determine an arbitrary letter for the allele. I chose the letter f for feathers. We know it is standard in genetics to use capital letter to indicate the dominant allele, and lowercase letter for recessive allele.

F - dominant f - recessive

We now determine the following:

F/f - heterozygous (phenotype: blue)

F/F - homozygous dominant (phenotype: blue)

f/f - homozygous recessive (phenotype: white)

The parakeet crossing is between F/f x f/f.

Draw a Punnet Square.

When done, you should get two possible zygotes TWICE:

F/f x 2
f/f x 2

The question asks for offspring percentage to have BLUE feathers, which are the heterozygotes F/f. Since there are two offspring with this genotype for every four offspring, the ratio is:

2 offspring/ 4 offspring = 1/2 or 0.5

Convert to percentage by multiplying by 100%:

0.5 * 100% = 50%

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