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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.

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The correct answer is explained below:

Explanation:

  • 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] %.

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