Most flour beetles are black, but several color variants areknown. Crosses of pu

Question

Most flour beetles are black, but several color variants areknown. Crosses of pure-breeding parents produced the followingresults in the F1 generation, and intercrossing theF1 from each cross gave the ratios shown for theF2 generation. The phenotypes are abbreviated Bl, black;Br, brown; Y, yellow; and W, white.

Cross Parents F1 F2

1 Br x Y Br 3 Br: 1Y

2 Bl x Br Bl 3 Bl: 1 Br

3 Bl x Y Bl 3 Bl: 1Y

4 W x Y Bl 9 Bl: 3Y: 4W

5 W x Br Bl 9 Bl: 3Br: 4 W

6 Bl x W Bl 9 Bl: 3Y: 4W

a. From the results, deduce and explain the inheritance of these colors.

b. Write the genotypes of each of the parents, the F1 and the F2 in all crosses.

Explanation / Answer

Crosses 1-3 show that there is normal monohybrid dominance between the three alleles involved, with Black dominant to Brown, and Yellow recessive to both. Calling the alleles B for black and R for brown, with lowercase for non-functional recessive alleles and - for any allele. A genotype of B--- gives black phenotype, a genotype of bbR- gives brown and a genotype of bbrr gives yellow. For crosses 4-6, a third allele with an epistatic role is introduced, W- is normal and ww is no colouration (i.e. white). So for all crosses, the parental white genotype is BBrrww Cross 4: x bbrrWW (so we can ignore the r allele). This gives an F1 of BbWw, which is black. The F2 is then 9 B-W-, 3x bbW-, and 4--ww. Cross 5: x bbRRWW. This gives an F1 of BbRrWw, which is black. The F2 is then 9 B---W-, 3x bbR-W-, and 4--ww. For cross 6 the white genotype must be different, bbrrww (otherwise the progeny ratios don't make sense): x BBrrWW (so we can ignore the r allele). This gives an F1 of BbWw, which is black. The F2 is then 9 B-W-, 3x bbW-, and 4--ww.

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