Question

2) Two true-breeding varieties of cucumbers (long and miniature) have been obtained.  Long cucumbers are on average 12 inches upon reaching maturity, whereas short ones are about 2 inches long.

1. Performing a cross between these varieties, you find that the F1 cucumbers are all around 7 inches.  What single-locus mode of inheritance would lead to this outcome?

2. In the F2 generation, you obtain cucumbers in five discrete size categories: 2, 4.5, 7, 9.5, and 12 inches.  Is this what you would have expected in F2 if a singlegenewas acting to determine cucumber length (under the mode of inheritance you reported in a)?  Explain the mode of inheritance of cucumber length in this cross

3. In addition to cucumber length, the cucumbers’ color also varied.  Although the parents of the cross were true breeding for white and green color respectively, all cucumbers in the F1 generation were green, whereas in the F2 generation, 56% were green, 25% white, and 19% yellow. How many genes are involved and what type of gene interaction is involved in determining color?

Answer 1

a. F1 cucumbers are hybrids of long and miniature sized varieties. So Dominant-recessive mode of inheritance of cucumber length gene leads to this outcome.

b. If a single gene was acting, then the expected lengths would have been 12 inches (homozygous dominant), 7 inches (heterozygotes) and 2 inches (homozygous recessive).

Pleotropic genes (more than one gene) are responsible for these ranges of cucumber length, other gene locus also may have epistatic gene effect on length locus.

c. All F1 individuals are green which means green allele is completely dominant on white allele. Then in F2 generation also, individuals should be either green or white, but yellow cucumbers are also occurring here which means gene from other locus is having an epistatic effect on cucumber length gene locus.