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Gene Duplication and Gene Families Reading MGAe2 Duplications p. 372 (ignore 2nd paragraph); Fig 11-19b Duplications p. 634-635 Gene Duplications Duplications may be "tandem" or "insertional" (non-tandem). The most likely scenario for tandem gene duplication is "unequal crossing over". Gene Duplication and Deletion by Unequal Crossing Over Meiotic Pairing In Duplication Heterozygote Tandemly repeated sequences facilitate unequal crossing over leading to expansion/contraction of the tandem array. The relatively high frequency of unequal crossing over is responsible for the hypervariability of VNTR, minisatellite, and microsatellite sequences. Meiotic Pairing In Duplication Homozygote. Amplification of duplication by unequal crossing over within "asymmetric pairing" region Gene duplication may lead to altered phenotypic expression due to change in relative gene dosage (imbalance). Natural selection on a duplicated copy may be relaxed, allowing it to accumulate mutations and either evolve modified functions or degenerate into a non-expressed pseudogene. Gene Families (Gene Clusters, Tandem Gene Arrays) A gene family is a set of genes created by duplication of an ancestral gene. Genes in a family may occur in tightly linked tandem arrays, or be dispersed on different chromosomes (following translocation events). Duplicated copies of genes in gene families may be freed from the constraints of natural selection and allowed to accumulate mutational changes without phenotypic effect. They may evolve modified, or more rarely, completely different functions. Even when genes in a family have similar functions they may be expressed in different tissues and at different times of development. Alternatively, and this may be more common, duplicated genes may degenerate into non-functional pseudogenes. In other gene families, all copies of the gene have essentially identical sequence and function. Such gene families always exhibit tight linkage of all genes in the family. The products of genes in such families are frequently required in large amounts. When the sequences of all genes in a family are identical, it does not mean that the shared sequence is identical to the sequence of the original gene, but that all sequences in the array are subject to concerted evolution and homogenization. The maintenance of such clusters of identical genes demands a mechanism to homogenize their sequences and prevent divergence by accumulation of incremental changes. Such mutations would be only weakly opposed by selection. Unequal crossing over is one possible mechanism to homogenize genes in a family. In the model shown to the right, abcde represent a tandem array of five repeats that, while they have diverged in sequence, still retain sufficient homology to exhibit asymmetric pairing. The scenario envisions expansion of b by unequal crossing over, leading to displacement of the other sequences. Examples Gene Family #Genes animal globin proteins 2-50 modified gene function/dispersed opsins (visual pigment proteins) 4-? mammalian lysozyme/lactalbumin rRNA histones conserved function 100-1,000 Immunoglobul;ins Actins 5-30 >500 modified gene function/dispersed novel function Thalassemia The thalassemias are a group of genetic defects with variable clinical manifestations due to derangements of hemoglobin production. The thalassemic conditions are associated with inheritance of chromosomes that lack one or more normal globin genes. In some thalassemias the aberrant chromosome contains a chimeric globin gene (such as "Lepore") that must have arisen by unequal crossing over. Draw a diagram of meiotic pairing between two normal copies of chromosome 11 to show how unequal crossing over could produce the Lepore globin gene. Lactalbumin The mammalian α-lactalbumin gene arose by duplication of the lysozyme gene, and subsequently evolved an entirely new function, although its regulation has apparently not changed.