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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

Parents:

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

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Map Distances

Time of entry mapping between a streptomycin-sensitive, prototrophic Hfr strain and a streptomycin- resistant Fminus strain that was Trp- Pyr- Cys- Nic- did not allow the order of the trp,pyr,cys genes to be determined, but these three genes were always transferred before nic. To determine the relative order of the trp,pyr,cys genes and the distances between them, the Hfr and Fminus strains were mated long enough to allow the transfer of the nic gene, after which Trp+ recombinants were selected. The unselected pyr and cys genes were then scored in the Trp+ recombinants , yielding the following results:

phenotype number of recombinants
Trp+ Pyr+ Cys+ 790
Trp+ Pyr+ Cys- 145
Trp+ Pyr- Cys+ 60
Trp+ Pyr- Cys- 5

Draw a map of the trp, pyr, cys genes relative to each other.Express map distances between adjacent genes as the frequency of crossing over between them.

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