FRUIT FLIES Best answer on the web

Author: mike     07 29th, 2010 in xn--00t609f.com     edit

  • In fruit flies, eye color is a sex-linked trait determined by a single gene with two alleles. Red color is dominant to white eye color. Body color is an autosomal trait determined by a single gene with two alleles, where brown is dominant to yellow. if a homozygous red eyed female, homozygous of yellow body color is crossed to a white-eyed male homozygous for brown body color, what proportion of the second-generation male offsping will have red eyes and brown bodies?


  • OK, for the sex-linked eye-color trait, the alleles are:
    XW - dominant red color
    Xw - recessive white eye color
    Y - male's Y chromosome doesn't contain eye-color gene at all

    for the other trait, there are just two alleles:
    B - brown body color
    b - yellow body color

    so if a homozygous red eyed female, homozygous of yellow body color is crossed to a white-eyed male homozygous for brown body color, we're combining XWXWbb with XwYBB, so the offspring will all have Bb for body color genes, and will thus all be brown, and their sex genes will be half XWXw and half XWY, so they will be half male, half female, and all red-eyed.
    in the second generation, we'll be mating XWXwBb females with XWYBb males.
    considering only body color for a moment, mating Bb with Bb will produce the normal equal mixture of BB, Bb, bB, and bb, so three-fourths of the offspring will display the dominant brown body phenotype.
    as far as the sex-linked eye color trait, the male offspring will all get a Y chromosome from their fathers, and either an XW or Xw from their mothers, so half of them will be XWY and red-eyed, and the other half XwY and white-eyed.
    so, combining the two traits, we'll have:

    1/8th XwYbb white-eyed, yellow-bodied
    1/8th XWYbb red-eyed, yellow-bodied
    3/8ths XwY(BB, Bb, bB) white-eyed, brown-bodied
    3/8ths XWY(BB, Bb, bB) red-eyed, brown-bodied.

    so, in particular, three eighths of the second-generation males will have red eyes and brown bodies.
    one web page I checked to make sure I wasn't forgetting the details was http://library.thinkquest.org/18258/noframes/morgan2.htm?tqskip1=1&tqtime=0408, which I found by searching for "fruit flies sex linked eye color".
    again, if you have further questions, or my explanation isn't clear, please ask for a clarification.
    Thanks again,
    David









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