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Gene linkage

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.

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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