marks to help geneticists ﬁnd their way around the chro-mosomes of complex organisms. The population andevolutionary genetics of polymorphisms is considered inChapters 19 and 21.One useful type of molecular chromosomal land-mark, or marker, is a restriction fragment length poly-morphism (RFLP). In Chapter 1, we learned that restric-tion enzymes are bacterial enzymes that cut DNA atspeciﬁc base sequences in the genome. The target se-quences have no biological signiﬁcance in organismsother than bacteria—they occur purely by chance. Al-though the target sites generally occur quite consistentlyat speciﬁc locations, sometimes, on any one chromo-some, a speciﬁc target site is missing or there is an extrasite. If the presence or absence of such a restriction siteﬂanks the sequence hybridized by a probe, then a South-ern hybridization will reveal a length polymorphism, orRFLP. Consider this simple example in which one chro-mosome of one parent contains an extra site not foundin the other chromosomes of that type in that cross:2.2 Sex chromosomes and sex-linked inheritance
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