Figure 2.5.1 shows Punnett squares for two of the four possible test crosses. Fill in the Punnett squares in Figure 2.5.2 for the other two possible genotypes of the unknown that aren’t shown.
Based on meiosis, when dealing with two loci, there will always be four distinct gamete types. But if the organism is homozygous, like the tester, all those gametes will look the same. In this situation, when writing a Punnett square, is it necessary to write out the four similar gametes? How would you re-draw the Punnett square on the right in Figure 2.5.1?
If two loci assort independently, then the AABB x aabb cross will result in dihybrid progeny, which when crossed together will give ratios of 9:3:3:1 in the F2, assuming “A” and “B” are dominant to “a” and “b”, respectively. Now, assume that locus “A” and “B” are not assorting independently but are linked with a RF of 20%. That is the “AB” and “ab” combinations are more likely. How will this affect the 9:3:3:1 ratio? What if the combinations are “Ab” and “aB”?c
Perform the same first cross as in Question 3, but make the second cross a test cross (x aabb), with expectation of a 1:1:1:1 ratio. How would the ratio be changed if the two loci were not assorting independently but are somewhat linked?