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Genetics- how many of each genotype?

  1. Jan 16, 2012 #1
    1. The problem statement, all variables and given/known data

    A germ cell in the male part of a plant (anther) (n=2) produces 4 haploid pollen cells. Assume that the gene for flower color (F) is on chromosome 1 and the gene for leaf color (L) is on chromosome 2. If a plant contains two different alleles of the flower color gene (F and f) and 2 different alleles of the leaf color gene (L and l):

    A) what are the genotype of all possible pollen grains?
    B) when a single germ cell undergoes meiosis to produce 4 pollen grains, how many will have each genotype?
    C) what will be the ratio of F to f in the entire population of pollen grains?
    D) if a cross-over even occurs in meiosis I, will this influence the genotypes of the pollen grains?

    3. The attempt at a solution

    I just posted the whole question so it's clear. But I know that (assuming there isn't any cross-over) A is 2^2, or 4 different genotypes.

    I'm a little unsure about B- I think that for a single germ cell, since each pair of the four haploid cells it produces are identical to each other (again assuming no cross-over, since I don't know any other way to approach the question) then the 4 pollen grains will have 2 genotypes (i.e., 2 will have one genotype and 2 will have another)

    For C, I just thought it was 1:1? It doesn't mention a phenotype or anything, so I don't think dominance has anything to do with it. I'm a bit confused about the question.

    For D, I'm confused because I don't really understand how crossing-over affects the genotypes or phenotypes of a population. I know that homologous chromosomes synapse and exchange genetic information like my book says, yadda yadda, but what does that actually do? Does it create a brand new allele (which would then change the genotype and possibly the phenotype)? So if cross-over occurs, the plants will no longer be L or l, for example, but some new hybrid of M or m or something? So, in a sense, are there a huge number of possible alleles? Or is that not true at all?

    Thanks in advance for any help.
     
  2. jcsd
  3. Jan 24, 2012 #2

    Moonbear

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    For B, with the two genes on different chromosomes, and being heterozygous for both, I don't think there's just one answer to that question. Yours is one right answer, but I think there's another to consider.

    Regarding D, in this context (just learning genetics), usually they are talking about exchanging an entire gene or set of genes (a whole piece of a "leg" of a chromosome) in crossing over. In reality, you can also introduce other mutations that way, so you are probably best off asking your instructor for clarification of how you should learn that in your course.
     
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