Introduction to Genetic Analysis 49

Introduction to Genetic Analysis 49 - 44200_02_p27-72...

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marks to help geneticists find their way around the chro- mosomes of complex organisms. The population and evolutionary genetics of polymorphisms is considered in Chapters 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 at specific base sequences in the genome. The target se- quences have no biological significance in organisms other than bacteria—they occur purely by chance. Al- though the target sites generally occur quite consistently at specific locations, sometimes, on any one chromo- some, a specific target site is missing or there is an extra site. If the presence or absence of such a restriction site flanks the sequence hybridized by a probe, then a South- ern hybridization will reveal a length polymorphism, or RFLP. Consider this simple example in which one chro- mosome of one parent contains an extra site not found in the other chromosomes of that type in that cross: 2.2 Sex chromosomes and sex-linked inheritance
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