Nick Crawford Evolution and more…

Chapter 5 Question 14(i): Complete linkage

Another question:

I understand the cross between AB/ab x AB/ab creates 2 phenotypes. 

But...  with the 1.Ab/2.aB x 3.Ab/4.aB cross, however, I get 3 different phenotypes, A-bb (1x3), A-B- (1x4) and aaB- (2x4). I do not see where I went wrong.

(I have numbered the allele sets to show how I crossed them)

The first cross is essentially a mono-hybrid cross because of the complete linkage causes A and B to act as one gene.

The second cross makes more sense if you remember that A and B are always linked and on the same chromosome.  So Ab/aB could be written as [AB]b/a[AB] where [AB] is essentially one gene.  This cross is really a mono-hybrid with two homozygous parents for AB (e.g. AB x AB).

update 2.9.09

The second portion could also be interpreted as two sets of linkages: A linked b and B linked a.  If this is the case then you would end up with three possible phenotypes in the F1 progeny.

1. AbBa
2. AbAb
3. BaBa