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    Gene Linkage Map

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    Three recessive mutations in DROSOPHILA cause black body color(b), dumpy wings(dp), and hooked bristles(hk). From a cross of dumpy female with a black, hooked male, all the F1 were wild type for all three characters. The testcross of an F1 female with a dumpy, black, hooked male gave the following phenotypic results:

    wild type 169
    black 19
    black, hooked 301
    dumpy, hooked 21
    hooked 8
    hooked, dumpy, black 172
    dumpy, black 6
    dumpy 304

    Construct a linkage map of the three genes indicating the gene order and distance between each.

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    Solution Preview


    Female: B dp HK
    B dp HK
    Male: b DP hk
    b DP hk

    How do we know this? Because the F1 were all wildtype for each characteristic, it means the parents must have been homozygous for each gene, otherwise crossing over would of led to a small number of F1 offspring with on or more recessive characteristics.

    So, the F1 generation are all: B dp HK
    b DP hk

    Now, cross female F1: B dp HK
    b DP hk
    with male: b dp hk
    b dp hk

    The unrecombined offspring ...

    Solution Summary

    How to develop a linkage map when you only know the proportions of phenotypes