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Crossing Over and Linkage Prophase I lasts such a long time because the chromosomes deliberately tie themselves in knots and then untangle (using topoisomerase II, page 74). Figure 19.4 shows what happens. At the top we see the paternal and maternal chromosomes, each composed of two chromatids, lined up side by side. As before we show the chromosome that originated from the father in light green and the maternal one in dark green. During prophase 1 the chromosomes are cut and resealed at points called chiasmata (singular chiasma) so that lengths of paternal chromosome are transferred to a maternal one and vice versa. The rest of meiosis I proceeds, followed by meiosis II, and the end result is that some gametes contain chromosomes that are neither completely paternal nor completely maternal but are a recombination of the two. The biological advantage of sexual reproduction is that it allows organisms to possess a random selection of the genes from their ancestors. Those individuals with a complement of genes that makes them better suited to their environment tend to do better, allowing evolution by natural selection of the individuals posessing the better genes. Without crossing over this could not happen: Those genes that are located on the same chromosome would remain linked down the generations, greatly reducing the number of gene permutations possible at each generation. Crossing over allows a child to inherit, for example, his grandmother’s green eyes without also inheriting her defective sodium channel gene (page 331), although both genes are on chromosome 19. Even with crossing over, genes on the same chromosome are inherited together more than they would be if they were on different chromosoomes. The closer the genes, the less likely it is that a chisma will form between them, and therefore the greater the probability that they will be inherited together. This phenomenon is used to help identify the genes responsible for specific diseases such as cystic fibrosis (page 426). Fertilization and Inheritance At fertilization the sperm cell fuses with the much larger egg cell. The nuclei that originated in sperm and egg, now called pronuclei, move together and eventually fuse to restore the normal somatic cell chromosome number. This diploid cell then divides many times by mitosis to generate the multicellular organism.