Answers to Problems for Calculating Expected Ratios From a Cross Involving Two or More Genes.

Problem 1:    BbCc x BbCc

Again I use the expected F₂ distributions. These have already been calculated for the lethality of the CC genotype:

(3/4 bicorn: 1/4 unicorn) * (2/3 fuzzy:1/3 wispy) =

3/4 * 2/3 = 6/12 bicorn, fuzzy
3/4 * 1/3 = 3/12 bicorn, wispy
1/4 * 2/3 = 2/12 unicorn, fuzzy
1/4 * 1/3 = 1/12 unicorn, wispy

Problem 2

AaDd x aadd 

This is a test cross, not an F₂ cross. It always pays to check the cross that you are given. Perhaps one of the most common mistakes on problem sets, quizzes and exams is solving for the wrong cross.

From the table we expect a 1:1 ratio at the body color locus and a 2:3 ratio at the temperament
locus.  Their product is:

(1/2 gray : 1/2 white) * (2/5 shy : 3/5 bold) =

1/2 * 2/5 = 2/10 gray, shy
1/2 * 3/5 = 3/10 gray, bold
1/2 * 2/5 = 3/10 white, shy
1/2 * 3/5 = 2/10 white, bold

Problem 3:  CcDd x CcDd 

Another F₂ cross where one locus shows lethality and the other locus has penetrance problems. Neither of these problems affects independent assortment.  From the table we expect a 2:1 ratio at the body type locus and a 2:3 ratio at the temperament locus.  Their product is:

(2/3 fuzzy:1/3 wispy) * (3/5 shy : 2/5 bold) =

2/3 * 3/5 = 6/15 fuzzy, shy
2/3 * 2/5 = 4/15 fuzzy, bold
1/3 * 3/5 = 3/15 wispy, shy
1/3 * 2/5 = 2/15 wispy, bold