Genetics - A. V. Sivolob 2008

Formal genetics: patterns of trait inheritance
Crossing over
Double crossing over

A crossover observed between two chromosomal loci (as shown in Fig. 3.11) is the result of multiple recombination events occurring within the region separating these two genes. Genetic analysis detects only the net outcome of all such events, which can be formally treated as a single exchange of chromosomal segments. The term "double crossover" (as well as triple or higher-order crossovers) simply reflects the fact that we are analyzing the Genetic consequences of recombination events across a chromosomal segment spanning three marker loci (four for a triple crossover, and so on).

For example, if we analyze recombination events between loci A and C in Fig. 3.12 (assuming that region B contains no marker Gene with a discernible phenotypic effect), then crossovers occurring between A and B, as well as between B and C, will be perceived simply as a single crossover somewhere in the interval between A and C. Meanwhile, a double crossover that restores locus C to the original chromosome will not be detected as a recombination event at all. However, if region B carries a scorable gene, all these individual recombination events can be distinguished: the eight types of Gametes produced by the triheterozygote in Fig. 3.12 will be observable in a testcross (with a homozygote for the three recessive alleles abc/abc).

The results of such a hypothetical testcross are presented in Table 3.11. Based on the frequency of crossover offspring, the distance between the A/a and B/b loci is 15 + 3 = 18 cM (since a double crossover also results in the reciprocal transfer of B/b to the homologous Chromosomes, these double crossovers contribute to the frequency of what appears to be a single crossover if only the two loci A/a and B/b are analyzed). Similarly, the distance between the B/b and C/c loci is 26 + 3 = 29 cM. Consequently, the distance between A/a and C/c (which equals the sum of the distances between A/a and B/b, and between B/b and C/c) can be calculated as the sum of single crossover frequencies plus twice the frequency of double crossovers: 15 + 26 + 2 × 3 = 47 cM.

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Fig. 3.12. Single and double crossovers in a region containing three genes, and the gametes produced by a triheterozygote

Table 3.11. Progeny segregation in the testcross of the triheterozygote from Fig. 3.12

It should be noted that in the absence of the B/b locus, the distance between A/a and C/c would be estimated as 18 + 29 - 2 × 3 = 41 cM (Table 3.11): double crossovers occurring between A/a and C/c reduce the observed frequency of recombinant phenotypes. In general, the greater the distance between two genes, the higher the probability of double (and higher-order) crossovers, and the less accurately map distances can be measured. Conversely, the shorter the distance, the more accurately it can be determined. For example, in one of the classic experiments conducted by Morgan's research group, relative distances were determined among three genes located on the X chromosome of Drosophila: y (yellow body color), w (the previously mentioned white gene), and bi (bifid wings). No double crossovers were observed in this case, and the crossover frequency between the y and bi genes (4.7 cM) exactly equaled the sum of the crossover frequencies between y and w (1.2 cM) and between w and bi (3.5 cM). Such observations formed the empirical basis for establishing METABOLISM/2.html">THE CONCEPT OF linear gene arrangement on chromosomes.