sex-linked and Multiple loci

Single Locus


As we've learned, if there are two alleles at one locus not on sex chromosome, the hybrid cross (Aa x Aa) must produce the 1:2:1 assortment. However, the 3:1 dominance is only predicted if there is a simple dominance relationship. aa x aa is boring. The other three types of crosses are shown below, AA x aa; Aa x aa and Aa x Aa
monohybridcross

Sex linked


Here's how that maps out for a simple dominance on the X chromosome:
Sex Linked
For incomplete dominance, the case is interesting because the heterozygous phenotype ONLY shows up in females.


Multiple Loci


The first think is how do we deal with multiple loci? There is a way that is done purely with math…that's what we do when there are more than two loci. However, I'm going to show you how to do it with a punnet square.
You can download a plot of the possible gametes and that
4x4 matrix here. It is also shown below.
Dihybrid&test cross-dihybrid
The expected 9:3:3:1 ratio assumes independent assortment (not on the same chromosome). We can use that prediction to ask whether there is linkage (are they really on the same chromosome) and test the model out using a statistical test we will cover next.