遗传分析-第二十四章Population Genetics.doc

Chapter 24 Population GeneticsKey Concepts The goal of population genetics is to understand the genetic composition of a population and the forces that determine and change that composition. In any species, a great deal of genetic variation within and between populations arises from the existence of various alleles at different gene loci. A fundamental measurement in population genetics is the frequency at which the alleles are found at any gene locus of interest. The frequency of a given allele in a population can be changed by recurrent mutation, selection, or migration or by random sampling effects. In an idealized population, in which no forces of change are acting, a randomly interbreeding population would show constant genotypic frequencies for a given locus. Introduction So far in our investigation of genetics, we have been concerned with processes that take place in individual organisms and cells. How does the cell copy DNA and what causes mutations? How do the mechanisms of segregation and recombination affect the kinds and proportions of gametes produced by an individual organism? How is the development of an organism affected by the interactions between its DNA, the cell machinery of protein synthesis, cellular metabolic processes, and the external environment? But organisms do not live only as isolated individuals. They interact with each other in groups, populations, and there are questions about the genetic composition of those populations that cannot be answered only from a knowledge of the basic individual-level genetic processes. Why are the alleles of the protein Factor VIII and Factor IX genes that cause hemophilia so rare in all human populations, whereas sickle-cell anemia is very common in some parts of Africa? What changes in the frequency of sickle-cell anemia are to be expected in the descendants of Africans in North America as a consequence of the change in environment and of the interbreeding between Africans and Europeans and Native Ame