Info

A1B1

Pill

Su = (1 - r)/2

1 - r is the frequency of all coupling gametes.

A2B2

p2q2

022 = (1 - r)/2

A1B2

Pl<h

g12 = r/2

r is the frequency of all recombinant gametes.

A2B1

P2qi

g21 = r/2

The recombination fraction, r, can be thought of as the probability that a recombination event will occur between two loci. With independent assortment, the coupling and repulsion gametes are in equal frequencies and r equals V2 (like the chances of getting heads when flipping a coin). Values of r less than V2 indicate that recombination is less likely than non-recombination, so coupling gametes are more frequent. Values of r greater than V2 are possible and would indicate that recombinant gametes are more frequent than non-recombinant gametes (although such a pattern would likely be due to a process such as natural selection eliminating coupling gametes from the population rather than recombination exclusively).

The expected frequencies of gametes produced by all possible genotypes for two diallelic loci including the contributions of recombination are derived in Table 2.12. Summing the frequencies of each gamete produced by all genotypes gives the gamete frequencies that will found the next generation. This gives expected gamete frequencies for the more general case of a randomly mating population rather than for a single genotype. The table shows how only two of 10 possible genotypes contribute to the production of recombinant gametes. Most genotypes produce recombinant gametes that are identical to non-recombinant gametes (e.g. the A1B1/A1B2 genotype produces A1B1 and A1B2 coupling gametes and A1B1 and A1B2 repulsion gametes).

We can utilize observed gamete frequencies to develop a measure of the degree to which alleles are associated within gamete haplotypes. This quantity is called the gametic disequilibrium (or sometimes linkage disequilibrium) parameter and can be expressed by:

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