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Gene Linkage Map

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.

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