FIGURE 8-2 Exon and gene duplication. (a) Exon duplication results from unequal crossing over during meiosis. Each parental chromosome (top) contains one ancestral gene containing three exons (blue) and two introns (orange). Homologous noncoding sequences called L1 long interspersed elements lie 5′ and 3′ of the gene as well as in the intron between exons 2 and 3. As we will see later in the chapter, L1 elements have been repeatedly transposed to new sites in the genome over the course of human evolution, so that all chromosomes are peppered with them. The parental chromosomes are shown displaced relative to each other, so that the L1 elements are aligned. Homologous recombination between these L1 elements as shown would generate one recombinant chromosome in which the gene now has four exons (two copies of exon 3) and one chromosome in which the gene is missing exon 3. (b) The same process can generate duplications of entire genes. Each parental chromosome (top) contains one ancestral β-globin gene. After unequal recombination between L1 elements, subsequent independent mutations in the resulting duplicated genes could lead to slight changes in sequence that might result in slightly different functional properties of the encoded proteins. Unequal crossing over can also result from rare recombinations between unrelated sequences. See D. H. A. Fitch et al., 1991, Proc. Natl. Acad. Sci. USA 88:7396.