Page 154 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

146 Chapter 5 Linkage, Recombination, and the Mapping of Genes on Chromosomes

Figure 5.14 Inferring the location of a crossover event. Once you establish the order of genes involved in a three-point cross, it
is easy to determine which crossover events gave rise to particular recombinant gametes. Note that double crossovers are needed to
generate gametes in which the gene in the middle has recombined relative to the parental combinations for the genes at the ends. Such
events include the 2-strand DCO in part (d) as well as 3-strand DCOs (not shown).
vg pr b
(a) Parental chromosomes
vg pr b Sister chromatids
Region 1 Region 2 Homologous
chromosomes
vg + pr + b + of F females
1
vg + pr + b + Sister chromatids
(b) Crossover in region 1 Resultant chromatids
vg pr b vg pr b
vg pr b vg pr + b +
vg + pr + b + vg + pr b
vg + pr + b + vg + pr + b +
(c) Crossover in region 2 Resultant chromatids
vg pr b vg pr b
vg pr b vg pr b +
vg + pr + b + vg + pr + b
vg + pr + b + vg + pr + b +
(d) Double crossover; one crossover in each region Resultant chromatids
vg pr b vg pr b
vg pr b vg pr + b
vg + pr + b + vg + pr b +
vg + pr + b + vg + pr + b +

outside genes vg and b, even though not one but two ex- ambiguous. A two-point cross involving y and r gave a re-
changes must have occurred between them. combination frequency of 42.9, but the sum of all the inter-
Because of the existence of double crossovers, the vening distances was 55.0 (review Fig. 5.12). This discrepancy
vg ↔ b distance of 17.7 m.u. calculated in the previous sec- occurred because the two-point cross ignored double cross-
tion does not reflect all of the recombination events pro- overs that might have occurred in the large interval between
ducing the gametes that gave rise to the observed progeny. the y and r genes. The data summing the smaller intervening
To correct for this oversight, it is necessary to adjust the distances accounted for at least some of these double cross-
recombination frequency by adding the double crossovers overs by catching recombinations of gene pairs between y and r.
twice, because each individual in the double crossover Moreover, each smaller distance is less likely to encompass
groups is the result of two exchanges between vg and b. The a double crossover than a larger distance, so each number for
corrected distance is a smaller distance is inherently more accurate.
Note that even a three-point cross like the one for vg,
252 + 241 + 131 + 118 + 13 + 13 + 9 + 9 × 100 pr, and b ignores the possibility of two recombination
4197 events taking place in, say, region 1. For greatest accuracy,
= 18.7 m.u. it is always best to construct a map using many genes sepa-
rated by relatively short distances.
This value makes sense because you have accounted for all
of the crossovers that occur in region 1 as well as all of the
crossovers in region 2. As a result, the corrected value of Interference: Fewer double crossovers
18.7 m.u. for the distance between vg and b is now exactly than expected
the same as the sum of the distances between vg and pr In a three-point cross following three linked genes, of the
(region 1) and between pr and b (region 2). eight possible genotypic classes, the two parental classes
As previously discussed, when Sturtevant originally contain the largest number of progeny, while the two double
mapped several X-linked genes in Drosophila by two-point recombinant classes, resulting from double crossovers, are
crosses, the locus of the rudimentary wings (r) gene was always the smallest (see Fig. 5.11). We can understand why