Question

Mapping traits to a chromosome

Fruit flies have four chromosomes. Fruit flies also have a number of body markers that are useful for mapping genes to given chromosomes. Chromosome I is the X chromosome, which has eye color alleles w+ (red eyes, dominant) and w- (white eyes, recessive). Curly O (Cyo), located on chromosome II, is a dominant marker mutation which causes the wings to curl on the ends instead of beings straight (WT). Stubble (Sb), a dominant marker mutation located on chromosome III, causes the fly’s normally (WT) long and slender bristles to be short and stubbly. (Chromosome IV is very tiny, and has very few genes on it, so we'll ignore it for this problem.)

You have a new mutation that causes the fly to have four antennae instead of two, which you've named double. This mutation is dominant to the wild type allele (two antennae). You want to know which chromosome it is on, so you perform tests for linkage of double to the three markers on the three major chromosomes. Assume all parents are pure-breeding.

Cross I: Test for linkage with white You cross a white-eyed female with two antennae to a red-eyed male with four antennae. Offspring: 405 females with red eyes and four antennae 399 males with white eyes and four antennae

Cross 2: Testing for linkage with CyO and Sb You cross a fly with four antennae to a fly with curly wings and stubbly bristles. The offspring all have four antennae, curly wings, and stubbly bristles. You cross one of these F1 hybrid offspring to a WT fly (all recessive), and get the following outcomes: 226 curly wings, stubbly bristles, two antennae 224 straight wings, stubbly bristles, two antennae 223 curly wings, long bristles, four antennae 227 straight wings, long bristles, four antennae 23 curly wings, stubbly bristles, four antennae 24 straight wings, stubbly bristles, four antennae 26 curly wings, long bristles, two antennae 27 straight wings, long bristles, two antennae

To which of the three markers is double linked?

Which chromosome is it on?

Calculate the distance between double and that marker, showing your calculations below.


Answer 1

Cross I

Upon crossing we got 405 females with red eyes and four antennae 399 males with white eyes and four antennae

So antenna gene in not located on the X-chromosome.

Cross 2 -

226 curly wings, stubbly bristles, two antennae

224 straight wings, stubbly bristles, two antennae  

223 curly wings, long bristles, four antennae

227 straight wings, long bristles, four antennae

23 curly wings, stubbly bristles, four antennae

24 straight wings, stubbly bristles, four antennae

26 curly wings, long bristles, two antennae

27 straight wings, long bristles, two antennae

We look closely at the progeny we will find that

Stubbly bristles prefer two antennae and long bristles prefer four antennae.

So bristles gene and antennae genes are linked.

Antenna gene is on chromosome III

the distance between two genes can be measured by

Number of recombinants *100 / total progeny

100*100/1000

10cM or 10 map unit

So the distance between bristles gene and antennae gene is 10 map unit or 10cM