Question

1. The expression of FOXP2 protein has been linked to the development of speech in humans. We know that FOXP2 is a transcription factor that turns on a multitude of genes to direct this development. Recent investigations have shown that activation of FOXP2 is linked to the expression of SCN3A (a sodium voltage gated channel in neurons).

If you cross pure line of FOXP2 deficient mice (homozygous recessive) with one that is deficient in SCN3A, what will be the proportion of mice with proper brain development to those with malformation? What would you predict if you self-crossed the F1 generation? Provide your reasoning. (5 pts)

Answer 1

Okay, this is a problem about both correct crossing understanding and correct reasoning about the relation between these 2 genes. Let us address first the crossing.

The parentals are purebreeds for both traits but they complement each other, these are loss of function mutations and they tend to be recessive, so the genotypes are:

Number 1: f-f- S+S+

Number 2: F+F+ s-s-

The cross would end up like this:

	f-S+
F+s-	F+f-S+s-

The whole progeny is heterozygous for both traits, that means the whole offspring is going to show a proper brain development, beause all of them have at least one functional copy for each of our genes.

Now, let us make the F2:

	F+S+	F+s-	f-S+	f-s-
F+S+	F+F+S+S+ (proper)	F+F+S+s- (proper)	F+f-S+S+ (proper)	F+f-S+s- (proper)
F+s-	F+F+S+s- (proper)	F+F+s-s-	F+f-S+s- (proper)	F+f-s-s-
f-S+	F+f-S+S+ (proper)	F+f-S+s- (proper)	f-f-S+S+	f-f-S+s-
f-s-	F+f+S+s- (proper)	F+f-s-s-	f-f-S+s-	f-f-s-s-

Now we will find every combination of genotypes. But remember that the correct expression of FOXP2 depends on the correct expression of SCN3A, that means that the first factor to consider is to have at least one SCN3A healthy copy, that means that the s-s- homozygous will not develop a proper brain function independently from the FOXP2 genotype.