A female Drosophila with a yellow body and white eyes is crossed with a male tha

ID: 163716 • Letter: A

Question

A female Drosophila with a yellow body and white eyes is crossed with a male that has normal (tan) body color and normal (red) eyes. Both lines are true-breeding. The resulting offspring are as follows. All females have normal (tan) bodies and normal (tan) eyes, all males have yellow bodies and white eyes. What genetic interaction seems to be the case here? Based on these results alone, can you tell if yellow bodies and white eyes are caused by one, or two loci? Now, imagine that the F1 females from the above cross is crossed to the males with yellow bodies and white eyes. The resulting F2 offspring are as follows. MALES Females 44 Yellow body white eyes 45 Yellow body white eyes 6 Yellow body red eyes 5 Yellow body red eyes 3 Tan body white eyes 5 Tan body white eyes 47 Tan body red eyes 45 Tan body red eyes Diagram this cross, starting with the P1 and proceeding to the F2. Are the loci sex linked? Are the loci linked? Do a chi-square test of goodness of fit, and determine whether independent assortment is falsified. If so, how many map units apart are these two loci? Look up the actual genetic loci and state whether this hypothetical cross makes sense.

Explanation / Answer

Given result is: males have yellow bodies and white eyes and females with normal body and eyes. This means that the genes for body color and eyes are X-linked. Males have only one X-chromosome.

The male has normal body and normal eyes, so, its genotype will be (XTR).

Females have two X-chromosomes. The female has yellow body and white eyes, so, its genotype will be (XtrXtr).

The cross is: XtrXtr crossed to XTR

Females will have the resulting genotype XTR Xtr. So, its phenotype is normal body and eyes.

Males will have the genotype Xtr. So, its genotype will be yellow body and white eyes.

Answer 1.

This type of interaction is known as X-linked interaction.

Answer 2.

There is no crossing over seen here. This is because the cross is between pure lines. There is no result with yellow body and normal eyes or tan body and white eyes. This does not mean that both genes are located on the same loci. Infact the crossed results will be seen in the F2 generation. So, white eyes and yellow bodies are caused by two different loci.

F2 cross is: F1 females (XTR Xtr) crossed to males with yellow body and white eyes (Xtr)

Expected result will be: Females with genotype (XtrXtr) and (XTRXtr) in ratio 1:1; and males with genotype (XTR) and (Xtr) in ratio 1:1.

But the observed result is:

MALES

Females

44 Yellow body white eyes

45 Yellow body white eyes

6 Yellow body red eyes

5 Yellow body red eyes

3 Tan body white eyes

5 Tan body white eyes

47 Tan body red eyes

45 Tan body red eyes

Total = 100

Answer 3. The loci are X-linked. This has already been proved.

Answer 4.

% recombination = (number of recombinants/total) *100

= [(6+3+5+5)/200]*100 = 9.5%. There is a distance of 9.5mu between the two loci.

Please note that two loci are linked if distance between the two loci is less than 50mu.

Note:

Please post the chi-square part separately.