Bio question from ADA 2009 test

Discussion in 'DAT Discussions' started by Dental2000, Aug 16, 2011.

  1. Dental2000

    7+ Year Member

    Jan 12, 2011
    Likes Received:
    Dental Student
    Suppose an animal has a gene for coat color that is sex-linked (X-linkage) and incompletely dominant, and in which females with an AA genotype have a black coat color; aa individuals have a yellow color; and those with Aa have a marble coloration. If a marble female was crossed with a yellow male, then each of the following might result under normal Mendelian conditions EXCEPT one. Which one is the EXCEPTION?

    1)Black male
    2)Black female
    3)Yellow male
    4)Yellow female
    5)Marble female
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  3. Demps

    Jul 26, 2011
    Likes Received:
    Dental Student
    Answer is B. Black Female. You will not get a black female from this cross.

    You're given marble-colored female - possessing X^A and X^a

    This female is crossed with yellow-colored male - possessing X^a and Y

    Everytime you do sex-linked cross. Male is hemizygous and only has one X chromosome as you see.

    if you do the punnett square, you will see that the resulting phenotypes are following

    25% marble female (Aa)
    25% yellow female (aa)
    25% black male (A)
    25% yellow male (a)

    Hence, you won't see any black female. Answer is choice B

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