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    Gene linkage

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    Eye color in Drosophila depends on the synthesis of two pigments, a red pigment and a brown pigment, resulting in the wild-type (WT) dull red eye color. Mutations blocking production of the brown pigment result in bright red eyes, while mutations blocking production of the red pigment result in brown eyes. If both pathways are blocked, the eyes are white. Thus, recessive cinnabar (cn) homozygotes have bright red eyes, while recessive brown (bn) homozygotes have brown eyes, and double mutants homozygous for both both cn and bn have white eyes. Flies with WT eyes that were heterozygous for both cn and bn were crossed with white-eyed flies that are homozygous for both cn and bn, and the following results were obtained:

    Eye Phenotype # of flies
    bright red 135
    WT 78
    brown 145
    white 82

    Are the cn and bn genes linked? Test using a chi square test. If they are linked, what is the map distance? What was the arrangement of alleles on the chromosome homologs in the heterozygous parent?

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