Question

In: Biology

AABB and aabb individuals were crossed to each other, and the F1 generation was backcrossed to...

AABB and aabb individuals were crossed to each other, and the F1 generation was backcrossed to the aabb parent. 993 AaBb, 995 aabb, 5 Aabb, and 7 aaBb offspring resulted.

a. How far apart are the a and b loci?

b. What progeny and in what frequencies would you expect to result from testcrossing the F1 generation from a AAbb X aaBB crosss to aabb?

c. In a typical meiosis, how many crossovers occur between genes A and B?

d. Assume that the A and B loci are on the same chromosome, but the offspring from the testcross described above were 494 AaBb, 506 aabb, 505Aabb, and 495 aaBb. How would your answer to part (c) change.

Expert Solution

When F1 generation AaBb was test crossed with aabb

following genotypes were formed in respective frequencies

1st AaBb 993 out of 2000 offsprings

2nd aabb 995 out of 2000 offsprings

3rd Aabb 5 out of 2000 offsprings

4th aaBb 7 out of 2000 offsprings

ans to part a.)

Now 1st and 2nd genotypes are parental combination and 3rd and 4th are new combination which are result of crossing over

so the frequency of organisms with new combination are 12 out of 2000

so the recombination or crossing over frequency is 0.6 %

so the distance between gene a and b is 0.6 centimorgan

ans to part b .)

result of cross in segment b is

Still the recombinants ( offsprings formed after crossing over ) will be 0.6 %

but recombinants in this part would be AaBb and aabb

results of this cross

1st AaBb 0.3% of total offsprings

2nd aabb 0.3 % of total offsprings

3rd Aabb 49.7% of total offsprings

4th aaBb 49.7% of total offsprings

ans to part c.)

as distance between loci a and b or A and B is 0.6 centimorgan

frequency of crossing over in meiosis will be 0.6 %

ans to part d .)

in this case offsprings and their relative frquencies are given as

1st AaBb 494 out of 2000 offsprings

2nd aabb 506 out of 2000 offsprings

3rd Aabb 505 out of 2000 offsprings

4th aaBb 495 out of 2000 offsprings

now in this case no of recombinants are equal to offsprings with parental combinatons

so the frequency of recombinants is 50 %

so the distance between 2 loci is around 50 centimorgan ( or more ) and frequency of crossing over is 50 %.

gladiator answered 2 years ago

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