Page 153 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

5.3 Mapping: Locating Genes Along a Chromosome 145

Figure 5.13 Analyzing the results of a three-point cross. Similarly, because recombinants for the vg–pr gene pair are
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(a) Results from a three-point testcross of F 1 females simultaneously vg pr and vg pr, the interval between these two genes is
heterozygous for vg, b, and pr. (b) The gene in the middle must be
pr because the longest distance is between the other two genes: 252 + 241 + 13 + 9
vg and b. The most accurate map distances are calculated by × 100
summing shorter intervening distances, so 18.7 m.u. is a more 4197
accurate estimate of the genetic distance between vg and b than = 12.3 m.u. (vg ↔ pr distance)
is 17.7 m.u.
while the distance separating the b–pr pair is
(a) Three-point cross results 131 + 118 + 13 + 9
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P vg b pr / vg b pr vg b pr / vg b pr + 4197 × 100
= 6.4 m.u. (b ↔ pr distance).
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(all identical) vg b pr / vg b pr +
F 1
These recombination frequencies show that vg and b are
separated by the largest distance (17.7 m.u., as compared
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Testcross vg b pr / vg b pr + vg b pr / vg b pr with 12.3 and 6.4) and must therefore be the outside
genes, flanking pr in the middle (Fig. 5.13b). But as with
the X-linked y and r genes analyzed by Sturtevant, the
Testcross 1779 vg b pr Parental combinations for distance separating the outside vg and b genes (17.7)
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progeny 1654 vg b pr + all three genes does not equal the sum of the two intervening distances
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252 vg b pr Recombinants for vg relative to
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241 vg b pr + parental combinations for b and pr (12.3 + 6.4 = 18.7). In the next section, we learn that the
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131 vg b pr + Recombinants for b relative to reason for this discrepancy is the rare occurrence of dou-
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118 vg b pr parental combinations for vg and pr ble crossovers.
13 vg b pr + Recombinants for pr relative to
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9 vg b pr parental combinations for vg and b
4197 Correction for double crossovers
(b) Deduced genetic map Figure 5.14 depicts the homologous autosomes of the F 1
vg pr b females that are heterozygous for the three genes vg, pr, and b.
A close examination of the chromosomes reveals the kinds
12.3 m.u. 6.4 m.u. = 18.7 m.u. of crossovers that must have occurred to generate the
classes and numbers of progeny observed. In this and sub-
17.7 m.u.
sequent figures, the chromosomes depicted are in late pro-
phase/early metaphase of meiosis I, when there are four
phenotypes, indicating that the mutations are autosomal chromatids for each pair of homologous chromosomes. As
recessive. In a testcross of the F 1 females with males hav- we have suggested previously and demonstrate more rigor-
ing vestigial wings, black body, and purple eyes, the prog- ously in Chapter 6, prophase I is the stage at which recom-
eny were of eight different phenotypes reflecting eight bination takes place. Note that region 1 is the space between
different genotypes. The order in which the genes in each vg and pr, and the space between pr and b is region 2.
phenotypic class are listed in Fig. 5.13a is completely arbi- Recall that the progeny from the testcross performed
trary. Thus, instead of vg b pr, one could write b vg pr or vg earlier fall into eight groups (review Fig. 5.13). Flies in the
pr b to indicate the same genotype. Remember that at the two largest groups carry the same configurations of genes
outset we do not know the gene order; deducing it is the as did their grandparents in the P generation: vg b pr and
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goal of the mapping study. vg b pr ; they thus represent the parental classes
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In analyzing the data, we look at two genes at a time (Fig. 5.14a). The next two groups—vg b pr and vg b pr —
(recall that the recombination frequency is always a function are composed of recombinants that must be the recipro-
of a pair of genes). For the pair vg and b, the parental combi- cal products of a crossover in region 1 between vg and pr
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nations are vg b and vg b ; the recombinants are vg b and (Fig. 5.14b). Similarly the two groups containing vg b pr
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vg b. To determine whether a particular class of progeny is and vg b pr flies must have resulted from recombination
parental or recombinant for vg and b, we do not care whether in region 2 between pr and b (Fig. 5.14c).
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the flies are pr or pr . Thus, to the nearest tenth of a map But what about the two smallest groups made up of
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unit, the vg ↔ b distance, calculated as the percentage of re- rare vg b pr and vg b pr recombinants? What kinds of
combinants in the total number of progeny, is chromosome exchange could account for them? Most
likely, they result from two different crossover events oc-
252 + 241 + 131 + 118 × 100 curring simultaneously, one in region 1, the other in region
4197 2 (Fig. 5.14d). The gametes produced by such double
= 17.7 m. u. (vg ↔ b distance). crossovers still have the parental configuration for the