Figure WN21.1. (A) The SD haplotype consists of a cluster of alleles near the centromere of the second chromosome of Drosophila melanogaster. The Sd allele destroys sperm that carry the Rsp⁺ allele; those carrying Rspⁱ are not affected. Thus, in a Sd Rspⁱ/Sd⁺ Rsp⁺ heterozygote, the Sd Rspⁱ combination has a strong segregation advantage over Sd⁺ Rsp⁺. (B) Sd lies near the centromere (green oval), just outside a large block of heterochromatin (blue), which contains the Rspⁱ allele. The lower line shows the genetic map in more detail, with distances measured in centiMorgans (cM). As well as Sd and Rsp, there are alleles that enhance the segregation distortion (E(SD), M(SD)). There is no recombination within the heterochromatin, so the whole region is compressed to a point in the genetic map at 55 cM (B). (B, Redrawn from Fig. 1 in Palopoli and Wu 1996.)

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