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Genetic Crosses

Genetics as it applies to evolution, molecular biology, and medical aspects.

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Genetic Crosses

Postby martini » Wed Oct 19, 2011 4:12 am

A lilly from a true-breeding line with white flowers and short corollas (line A) is crossed to a lilly from a true-breeding line with black flowers and long corollas (line B). The resulting offspring, the F1, are all red-flowered, with corollas of medium length.


a) Suppose an individual from the F1 is crossed to an individual from line A. The resulting offspring are as follows: 50% white flowers and short corollas, 50% red flowers and medium corollas. What type of allelic interaction is this? How many loci are involved? Diagram the cross.


b) What is the term for when one locus affects two or more traits? In the scenario above, how many types of gametes can the F1 plants make?

I have looked through my book and internet but Im still not sure how to do this problem. What confuses me the most is that since P1 are either black or white, that, if it was codominance, F1 would be grey not red, as stated in the problem. Why would it be red? Because of this Im not sure how to write the genotype for these. Please help! :(
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Postby aptitude » Thu Oct 20, 2011 1:24 am

a) Because the frequency of recombination is zero (all offspring resemble the parental types), the genes must be completely linked.

b) It is called pleiotropy. White flower and short corolla can produce only 1 gamete type since it is homozygous at both loci, and red flower and medium corolla can produce 9 (3x3) gametes since it is heterozygous at both loci.
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Postby JackBean » Thu Oct 20, 2011 8:14 am

how did you get 9 gametes? It should be 2.
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Postby Cat » Fri Oct 21, 2011 12:12 am

JackBean is right. It's 2 types because F1 is heterozygous and only 1 locus is involved. The interaction is codominance. There is no "black" and no "white" in nature. So "white" is probably = no pigment and "back" = a lot of pigment to the point it looks black. Therefore, middle ground is "red" = some pigment.
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