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Name: Debbi
Status: Educator
Age: 30s
Location: N/A
Country: N/A
Date: July 2003

What is the best explanation for a BbCc multiply bbcc cross producing offspring in a 5:5:1:1 phenotypic ratio?

I really had to think about that one! Linkage is the only one I think is convincing. Here's my reasoning.

Simple independent assortment would give you 1:1:1:1 phenotypic ratios for the 4 possible genotypes.

Polygenic inheritance would imply that you would only see the phenotype when both recessive genotypes are present, and would give you 3:1 from the sum of the two independently assorting loci.

Codominance would probably give you an intermediate phenotype so would appear 1:1:1:1.

Incomplete dominance might work if either B or C (not both) were incompletely dominant alleles (I prefer calling them incompletely penetrant) with a lower penetrance for one dominant allele. If for example, C were 20% penetrant, the frequencies would look like this:

BbCc 1/2*1/2=0.25*0.2(penetrance of C)=0.05
Bbcc 1/2*1/2=0.25
bbCc 1/2*1/2=0.25*0.2(penetrance of C)=0.05
bbcc 1/2*1/2=0.25

Overall, this would look like 1:5:1:5. (I admit this is off a little since I haven't adjusted the cc genotype frequencies to account for the C penetrance issue, but you get the idea.)

But by far the simplest explanation is that the two loci are each a simple dominant & recessive that are linked.

Christine Ticknor, Ph.D.
Ireland Cancer Center
Case Western Reserve University

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