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19 September, 02:12

In sheep, coat color is influenced by two genes. Gene A influences pigment production, while gene B produces black or brown pigment. If two heterozygous white sheep were crossed and offspring were produced in a ratio of 12 white sheep to 3 black sheep to 1 brown sheep, what are the genotypes of the offspring? A) The white sheep could be A-B - or aaB-.

B) The white sheep could be A_B_ or aabb.

C) The white sheep could be A_B_ or A_bb.

D) The white sheep must all be aabb.

E) The white sheep must all be A_B_.

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  1. 19 September, 02:42
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    C) The white sheep could be A_B_ or A_bb.

    Explanation:

    Given that in a certain population of sheep, coat color is influenced by two genes, in which:

    Gene A: influence pigment production

    Gene B: influence black or brown pigment.

    Since Gene B influences Black or Brown pigment production and we are given three coat color (white, black and brown), definitely, this implies that Gene A influences White pigment production.

    NOW, If two heterozygous sheep were crossed; definitely we knew our heterozygous genes will be AaBb, so if two of these were crossed;

    Offspring that were produced were in the ratio of 12 white: 3 black : 1 brown

    So, we are asked to deduce, what is the possible genotype of the offspring:

    If a cross occur between AaBb:

    i. e AaBb * AaBb

    we have the following gametes for the cross: (AB, Ab, aB, ab)

    AB Ab aB ab

    AB AABB AABb AaBB AaBb

    Ab AABb AAbb AaBb Aabb

    aB AaBB AaBb aaBB aaBb

    ab AaBb Aabb aaBb aabb

    So, since gene A influences white pigment production; if can either be A_B_ or A_bb.

    Since gene B influences black or brown pigment production;

    then aaB_ represent Black sheep

    and aabb represent homozygous recessive Brown sheep.

    So for white sheep; we have the following gene:

    (AABB, AABb, AaBB, AaBb, AABb, AAbb, AaBb, Aabb, AaBB, AaBb, AaBb, Aabb) = 12

    Black sheep = (aaBB, aaBb, aaBb) = 3

    Brown = (aabb) = 1

    Therefore, The white sheep could be A_B_ or A_bb.
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