mouse breeding- double KOs - (Dec/01/2011 )

Not really a problem, just seeing if anyone out there has an idea of what's going with my mouse breeding.

I'm trying to create double KOs, mice that are knockouts for both gene A and gene B. I started by breeding single knockouts, so I crossed a knockout for gene A (A -/-) with a knockout for gene B (B -/-). Therefore the first generation of pups was hetero for each gene. Then, I crossed this first generation of pups with itself, trying to get double KOs.

The interesting thing is what happened when I genotyped the second generation of offspring. The cross was hetero (A+/- w/ B+/-) so I expect 25% homozygous +/+, 50% hetero +/-, and 25% homozygous -/-. BUT what I get from 14 offspring is:

ALL mice homozygous -/- for gene B
7 mice homozygous +/+ for gene A
and 7 mice either heterozygous +/- or homozygous -/- for gene A (can't tell the difference between +/- and -/- with the PCR protocol)

So does anyone know why ALL the second generation mice are genotyped to be -/- for gene B but only potentially 7 out of 14 mice are genotyped to be -/- for gene A? Why the difference in frequency between gene A and gene B?

Thanks!

Janey

This may or may not be helpful, but I had fun playing with it

http://www.changbioscience.com/genetics/punnett.html