Linked genes: You are studying Drosophila and you are interested in determining

ID: 59594 • Letter: L

Question

Linked genes: You are studying Drosophila and you are interested in determining whether

or not certain genes are linked. In particular you would like to look at two recessive traits:

vestigial wings and cinnabar eyes.

a. Your first cross involved two true breeders for both characters: a homozygous

wild type crossed with a homozygous recessive for vestigial wings and cinnabar

eyes. This allowed you to take heterozygotes and cross them with double

recessives in a testcross. You are doing this to observe phenotypic ratios that

would indicate whether or not the genes are linked.

i. If they are not linked, what would the expected genotypic and phenotypic

ratios be for the testcross offspring? What would the percentages of

parental to recombinant phenotypes be?

ii. If they are linked, what ratios would you expect? What would the

percentages of parental to recombinant phenotypes be?

iii. What is the recombination frequency and how could it help you determine

the relationship (if any) between these two genes?

iv. You perform a testcross that results in 2500 offspring with the following

phenotypes: 1104 wild type: 1158 cinnabar vestigial: 128 red vestigial:

110 cinnabar normal. Draw a linkage map that represents the distance

between these two genes.

Explanation / Answer

1. If the two genes are not linked, four different genotypes and phenotypes are observed, the frequency is shown below:

Genotype

Phenotype

Frequency

V C / v c

Wild type

0.4416

v c/ v c

Vestigial/cinnabar

0.4632

V c/ v c

Vestigial/red

0.044

v C / v c

Normal/cinnabar

0.0512

* If the genes are linked, the wild type and double recessive phenotype is expected at 1:1 ration. There will not be any recombinant phenotypes if the genes are linked

* The recombination frequency between the two genes is 0.10, which means the genes are 10 mu apart

* The distance between the two genes is 0.09 or approximately 0.1 mu