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Name: Debbie
Status: Educator
Age: 30s
Location: N/A
Country: N/A
Date: October 2003


Question:
What is the best explanation for a BbCc multiply bbcc cross producing offspring in a 5:5:1:1 phenotypic ratio? linked genes polygenic inheritance codominance incomplete dominace Mendel's principles of segregation and independent assortment

I asked this question just like this on a test. And none of my students got it right so I am seeking an outside opinion.



Replies:
Linkage is the best explanation for a segregation ratio of 5:5:1:1.

The bbcc parent will produce only one type of gamete, namely bc.

The BbCc parent can produce 4 types of gametes, namely BC, Bc, bC, bc. If the locus of B and the locus of C are not linked, that is, if they are on different chromosomes, then we would expect (by Mendel's principles of segregation and independent assortment) equal amounts of the four gametes, giving a phenotypic ratio of 1:1:1:1 in the offspring.

The 5:5:1:1 ratio tells us that the gene loci are linked, and the distance between them is great enough to allow crossing over to take place one-sixth of the time.

Your question is correct in not specifying which phenotypes are which in the ratio presented, because this will depend on the cis or trans arrangement of genes on the heterozygous parent BbCc.

If this parent has BC on one chromosome and bc on the other, the offspring of the back-cross mating with a bbcc parent will be:
5 BbCc: 1 Bbcc: 1 bbCc: 5 bbcc.

On the other hand, if this parent has Bc on one chromosome and bC on the other, the offspring of the back-cross mating with a bbcc parent will be:
1 BbCc: 5 Bbcc: 5 bbCc: 1 bbcc.

Sarina Kopinsky, MSc, HED, CGC



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