^ In a translocation, a chromosome segment moves from one chromosome to a non-homologous chromosome or to another place on the same chromosome.

I 9.5 The four basic types of chromosome rearrangements are duplication, deletion, inversion, and translocation.

A chromosome duplication is a mutation in which part of the chromosome has been doubled (see Figure 9.5a). Consider a chromosome with segments AB^CDEFG, in which • represents the centromere. A duplication might include the EF segments, giving rise to a chromosome with segments AB^CDEFEFG. This type of duplication, in which the duplicated region is immediately adjacent to the original segment, is called a tandem duplication. If the duplicated segment is located some distance from the original segment, either on the same chromosome or on a different one, this type is called a displaced duplication. An example of a displaced duplication would be AB^CDEFGEF. A duplication can either be in the same orientation as the original sequence, as in the two preceding examples, or be inverted: AB^CDEFFEG. When the duplication is inverted, it is called a reverse duplication.

An individual homozygous for a rearrangement carries the rearrangement (the mutated sequence) on both homologous chromosomes, and an individual heterozygous for a rearrangement has one unmutated chromosome and one chromosome with the rearrangement. In the heterozygotes (IFIGURE 9.6a), problems arise in chromosome pairing at prophase I of meiosis, because the two chromosomes are not homologous throughout their length. The homologous regions will pair and undergo synapsis, which often requires

Normal chromosome

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