Date: October 2003
What is the best explanation for a BbCc multiply bbcc
cross producing offspring in a 5:5:1:1 phenotypic ratio?
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.
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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Update: June 2012