Genetic maps can be constructed from a series of testcrosses for pairs of genes, but this approach is not particularly efficient, because numerous two-point crosses must be carried out to establish the order of the genes and because double crossovers are missed. A more efficient mapping technique is a testcross for three genes (a three-point testcross, or three-point cross). With a three-point cross, the order of the three genes can be established in a single set of progeny and some double crossovers can usually be detected, providing more accurate map distances.
Consider what happens when crossing over takes place among three hypothetical linked genes. I FIGURE 7.12 illustrates a pair of homologous chromosomes from an a a b c b c
Pair of homologous chromosomes
(a) Single crossover (b) between A and B
Single crossover (c) Double between B and C crossover
I 7.12 Three types of crossovers can take place among three linked loci.
individual that is heterozygous at three loci (AaBbCc). Notice that the genes are in the coupling configuration; that is, all the dominant alleles are on one chromosome
( A_B_QJ and all the recessive alleles are on the other chromosome (_a_b_L_). Three types of crossover events can take place between these three genes: two types of single crossovers (see Figure 7.12a and b) and a double crossover (see Figure 7.12c). In each type of crossover, two of the resulting chromosomes are recombinants and two are nonrecombinants.
Notice that, in the recombinant chromosomes resulting from the double crossover, the outer two alleles are the same as in the nonrecombinants, but the middle allele is different. This result provides us with an important clue about the order of the genes. In progeny that result from a double crossover, only the middle allele should differ from the al-leles present in the nonrecombinant progeny.
To examine gene mapping with a three-point testcross, we will consider three recessive mutations in the fruit fly Drosophila melanogaster. In this species, scarlet eyes (st) are recessive to red eyes (st+), ebony body color (e) is recessive to gray body color (e+), and spineless (ss) — that is, the presence of small bristles — is recessive to normal bristles (ss+). All three mutations are linked and located on the third chromosome.
We will refer to these three loci as st, e, and ss, but keep in mind that either recessive alleles (st, e, and ss) or the dominant alleles (st+, e+, and ss+) may be present at each locus. So, when we say that there are 10 m.u. between st and ss, we mean that there are 10 m.u. between the loci at which these mutations occur; we could just as easily say that there are 10 m.u. between st+ and ss+.
To map these genes, we need to determine their order on the chromosome and the genetic distances between them. First, we must set up a three-point testcross, a cross between a fly heterozygous at all three loci and a fly homozygous for recessive alleles at all three loci. To produce flies heterozygous for all three loci, we might cross a stock of flies that are homozygous for normal alleles at all three loci with flies that are homozygous for recessive alleles at all three loci:
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