Question

Answer the lab question, “What is the effect of the inheritance of one trait on the inheritance of a second trait?” with a hypothesis. Remember that you can either assume that the two genes are independent or that they are linked. What results would you expect in either case?
Hypothesis:

Answer 1

Hypothesis:

- If the two genes assort independently (are unlinked), then the inheritance of one trait will have no effect on the inheritance of the other. Prediction: a dihybrid cross (AaBb × AaBb) will yield a 9:3:3:1 phenotypic ratio; a testcross (AaBb × aabb) will give a 1:1:1:1 ratio. Example (400 offspring, dihybrid): expect ~225 : 75 : 75 : 25.

- If the two genes are linked (close together on the same chromosome), then the inheritance of one trait will affect the inheritance of the other: parental allele combinations will be transmitted together more often than recombinant combinations. Prediction: observed offspring frequencies will deviate from 9:3:3:1 (or 1:1:1:1 in a testcross) with parental phenotypes overrepresented and recombinant phenotypes underrepresented; if completely linked (no crossing over) only parental classes will appear. If crossing over occurs, the proportion of recombinants will be equal to the recombination frequency (<50%). Example (testcross, 400 offspring): if recombination frequency = 20%, expect ~160 parental A-B, 160 parental a-b, and ~40 + 40 recombinants.

Either outcome will be evident by comparing observed offspring ratios to these expected ratios.