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14 October, 17:42

In Drosophila, the genes for body coloration and eye size are on different chromosomes. Normal-colored bodies are dominant to ebony-colored (very dark) bodies, and normal-sized eyes are dominant to being eyeless. Line A is true breeding for normal body and normal eye, whereas line B is true breeding for ebony bodies and eyeless. Individuals from lines A and B are crossed. From a dihybrid cross between the F1 generation, 400 flies are scored. How many of these F2 flies are expected to have both normal body color and normal eyes?

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  1. 14 October, 19:41
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    225 flies

    Explanation:

    Let gene for body colour : A

    Let gene for eye type : B

    Parent 1 : normal body and normal eyes = AABB

    Parent 2: ebony body and eyeless = aabb

    F1 : AABB X aabb = AaBb (all have normal body and normal eye)

    F1 progeny is self crossed given that the two genes are on different chromosomes which means they show independent assortment:

    AaBb X AaBb =

    A_B_ = normal body, normal eyes = 9

    aaB_ = ebony body, normal eyes = 3

    A_bb = normal body, eyeless = 3

    aabb = ebony body, eyeless = 1

    Hence the ratio is 9 : 3 : 3 : 1

    Out of the 400 flies:

    (9/16) * 400 = 225 flies have normal body colour and normal eyes
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