Genetics - A. V. Sivolob 2008
Formal Genetics: Patterns of Trait Inheritance
Crossing Over
Unequal Crossing Over
Homologous Recombination is a highly precise process that occurs between homologous DNA regions (see Section 1). As a result, typically during Crossing-over as in all previous Examples, segments of equal length containing the same number of genes are exchanged. However, recombination sometimes occurs between homologous regions located at different positions on the chromosome, resulting in the exchange of segments of unequal length—unequal crossing-over. As shown in Fig. 3.15, such an exchange leads to a duplication of the region in one chromosome and a deletion in the other. Thus, unequal crossing-over is one of the causes of chromosomal rearrangements (see Section 4).
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Fig. 3.15. Scheme of unequal crossing-over
Precisely this situation is observed for a chromosomal locus located on the X chromosome of Drosophila, which affects the number of eye facets: wild-type flies contain a single copy of this locus (designated as B+), whereas duplication of the locus (designated as the Bar or B mutation) leads to a reduced number of facets. Repeated unequal crossing-over between the single and duplicated loci leads to triplication—the BB mutation—resulting in an even smaller number of eye facets.
Last update: 11/08/2026
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