biology

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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  1. Assuming this is a recessive trait.

    HH = doesn't express trait

    Hh = carrier

    hh= express trait


    Genotype:

    1 HH

    2 Hh

    1 hh

    1:2:1

    Phenotype:

    3:1

    Doesn't express: express

    25% chance of expressing trait.

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  2. Cont.

    Assuming this is a dominant trait.

    HH = Express trait

    Hh = Express trait

    hh= doesn't express trait


    Genotype:

    1 HH

    2 Hh

    1 hh

    1:2:1

    Phenotype:

    3:1

    Express: doesn't express

    75% chance of expressing trait.


    The wording of the question indicates autosomal dominant. The trait is expressed although both are heterozygous.

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