Original chromosomes e+ st+ ss+
st ss e
Chromosomes after crossing over e+ st ss+
7.13 The results of a three-point testcross can be used to map linked genes. In this three-point testcross in Drosophila melanogaster, the recessive mutations scarlet eyes (st), ebony body color (e), and spineless bristles (ss) are at three linked loci. The order of the loci has been designated arbitrarily, as has the sex of the progeny flies.
The only gene order that produces chromosomes with alleles for the traits observed in the double crossovers (st+ e+ ss and st e ss+) is the third one, where the locus for bristle shape lies in the middle. Therefore, this order ( st ss e ) must be the correct sequence of genes on the chromosome.
With a little practice, it's possible to quickly determine which locus is in the middle without writing out all the gene orders. The phenotypes of the progeny are expressions of the alleles inherited from the heterozygous parent. Recall that, when we looked at the results of double crossovers (see Figure 7.13), only the alleles at the middle locus differed from the nonrecombinants. If we compare the nonrecombi-nant progeny with double-crossover progeny, they should differ only in alleles of the middle locus.
Let's compare the alleles in the double-crossover progeny st+ e+ ss with those in the nonrecombinant progeny st+ e+ ss+ . We see that both have an allele for red eyes (st+) and both have an allele for gray body (e+), but the nonrecombinants have an allele for normal bristles (ss+), whereas the double crossovers have an allele for spineless bristles (ss). Because the bristle locus is the only one that differs, it must lie in the middle. We would obtain the same results if we compared the other class of double-crossover progeny ( st e ss+ ) with other nonrecombinant progeny ( st e ss ). Again the only trait that differs is the one for bristles. Don't forget that the nonrecombinants and the double crossovers should differ only at one locus; if they differ in two loci, the wrong classes of progeny are being compared.
To determine the middle locus in a three-point cross, compare the double-crossover progeny with the nonrecombinant progeny. The double crossovers will be the two least-common classes of phenotypes; the nonrecombinants will be the two most-common classes of phenotypes. The double-crossover progeny should have the same alleles as the nonrecombinant types at two loci and different alleles at the locus in the middle.
Determining the locations of crossovers When we know the correct order of the loci on the chromosome, we should rewrite the phenotypes of the testcross progeny in Figure 7.13 with the loci in the correct order so that we can determine where crossovers have taken place ( FIGURE 7.14).
Among the eight classes of progeny, we have already identified two classes as nonrecombinants ( st+ ss+ e+ and st ss e ) and two classes as double crossovers ( st+ ss e+ and st ss+ e ). The other four classes include progeny that resulted from a chromosome that underwent a single crossover: two underwent single
7.14 Writing the results of a three-point testcross with the loci in the correct order allows the locations of crossovers to be determined.
These results are from the testcross illustrated in Figure 7.13, with the loci shown in the correct order. The location of a crossover is indicated with a slash (/). The sex of the progeny flies has been designated arbitrarily.
crossovers between st and ss, and two underwent single crossovers between ss and e.
To determine where the crossovers took place in these progeny, compare the alleles found in the single-crossover progeny with those found in the nonrecombinants, just as we did for the double crossovers. Some of the alleles in the single-crossover progeny are derived from one of the original (nonrecombinant) chromosomes of the heterozygous parent, but at some place there is a switch (due to crossing over) and the remaining alleles are derived from the homologous non-recombinant chromosome. The position of the switch indicates where the crossover event took place. For example, consider progeny with chromosome st+ ss e . The first allele (st+) came from the nonrecombinant chromosome st+ ss+ e+ and the other two alleles (ss and e) must have come from the other nonrecombinant chromosome st ss e through crossing over:
This same crossover also produces the st ss+ e+ progeny.
This same method can be used to determine the location of crossing over in the other two types of single-crossover progeny. Crossing between ss and e produces st+ ss+ e and st ss e+ chromosomes:
st ss st ss
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