Now consider linkage. If the A and B loci were on the same chromosome, to
determine the proportions of the four
gametes we would have to know the probability
of recombination between the two loci. This probability is
r
, so "given" an A allele,
(1r) of the gametes would be AB, and r of the gametes would be Ab (i.e.,
recombinants). Similarly, given the a allele, (1r) of the gametes would be ab, and r of
the gametes would be aB (recombinants). Since there are two kinds of recombinant
gametes resulting from crossover (e.g., reciprocals: Ab and aB), these two types are
split evenly within the proportion
r
of recombinants (r/2 of each reciprocal). For
example, if
r
= 0.02 (i.e., 2%), then the gametes produced by a "double heterozygote"
such as the one three paragraphs above will result in the following proportions
(1r/2)AB : (r/2)Ab : (r/2)aB : (1r/2)ab, e.g., 49%:1%:1%:49% respectively. For
comparison, if there was no linkage between A and B, then the proportions of the four
possible gametes would be 25%:25%:25%:25%. By definition then,
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This note was uploaded on 11/05/2011 for the course BIOLOGY MCB2010 taught by Professor Jessicadigirolamo during the Fall '10 term at Broward College.
 Fall '10
 JessicaDigirolamo
 Microbiology

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