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    Gene linkage

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    The following cross involving three genes was performed: Aa Bb Cc x aa bb cc, and the phenotypes and number of progeny are shown below. Each pair of alleles exhibits simple dominance and recessiveness.

    Progeny Phenotype Number of progeny
    A B C 181
    a b c 169
    a B C 77
    A b c 73
    A B c 69
    a b C 81
    a B c 173
    A b C 177

    Which pair of genes is clearly linked to each other, and which gene is clearly unlinked to the other two? What is the map distance between the two linked genes? HINT: Compare two genes at a time and check if they are linked or assort independently.

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    Solution Preview

    This is an example of a three-point test cross.

    First let's compare genes A and B. We will ignore C and recombine our groups.
    Phenotype Number
    AB 181+69=250
    ab 169+81 = 250
    Ab 73+177 = 250
    aB 77+173 = 250
    Obviously since each phenotype has exactly the same number of progeny A and B are not linked.

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