Question

You are studying two different unrelated organisms. In the first, you cross a true-breeding colorless individual with a true-breeding red individual. All F1 offspring are red. When you cross two F1 individuals, you get 70 reds and 58 colorless. The reciprocal cross gives similar results. In the second organism, you cross a true-breeding colorless with a true-breeding blue. All F1 offspring are colorless. When you cross the F1 offspring you get 23 blue individuals and 105 colorless individuals. Again the reciprocal cross gives the same results. Hypothesize a genetic explanation for each result and a possible biochemical pathway leading to the phenotypes.

Answer 1

You are studying two different unrelated organisms. In the first, you cross a true-breeding colorless individual with a true-breeding red individual. All F1 offspring are red. When you cross two F1 individuals, you get 70 reds and 58 colorless. The reciprocal cross gives similar results.

Parents: RR (Red) x rr (colorless)

F1: Rr (Red)

F2: 70 reds : 58 colorless = 1.2 : 1 ratio whereas it should be 3:1 if the genes were independently segragating and assorted. This results suggests that the there is effect of some other genes on controlling the color in F2 generation where genomes of both the unrelated organisms have come together. So, there might be some other gene which is masking the effect of 'R' gene and producing equal red and colorless individuals.

r --------- colorless protein

R -------- intermediate protein ------------- red protein

R -------- intermediate protein -----^epistatis-------- colorless protein

In the second organism, you cross a true-breeding colorless with a true-breeding blue. All F1 offspring are colorless. When you cross the F1 offspring you get 23 blue individuals and 105 colorless individuals. Again the reciprocal cross gives the same results. Hypothesize a genetic explanation for each result and a possible biochemical pathway leading to the phenotypes.

In second case, the phenotypic ratio in the F2 is 105 colorless : 23 blue = 4.56 : 1 ratio. Again this is not following the Mendel's law. Here, the blue color is formed via an intermediate protein which is the check point in the pathway.

w --------- colorless protein

b -------- intermediate protein ------------- blue protein

b -------- intermediate protein -----^epistatis-------- colorless protein