When exposed to bright sunlight, some people involuntarily sneeze. This reaction is known as a photic response. It is controlled by a single gene with two alleles, H and h. A man and a woman who sneeze when exposed to bright sunlight are both heterozygous for the photic response trait. Draw a Punnett Square using the allele symbols H and h to show the expected genotypes of the man and woman’s biological children. Identify all possible phenotypes of the offspring and their expected percentages.

According to the question,
The dominant allele for the photic response gene is: H
The recessive allele for the photic response gene is: h.
It is said that both the man and the woman are heterozygous for the photic response trait, that is, both of them have the Hh genotype.
Both sneeze when exposed to bright sunlight, hence both of them show the photic response phenotype.
This means the photic response trait is a dominant trait.
Gametes produced by both the man and woman are: H and h.
Mating the man with the woman, we get offspring with the following genotypes and phenotypes.

H h

H HH Hh

Photic Response Positive Positive

h Hh hh

Photic Response Positive Negative

From the above Punnett square we get,
Percentage of offspring expected to show photic response trait = [tex]\frac{3}{4} = 75[/tex] %.
Percentage of offspring expected to not show any photic response trait = [tex]\frac{1}{4} = 25[/tex] %.