Recombination

This cell has two pairs of homologous chromosomes, one large pair and one small pair, shown here in the unduplicated state. In this example, each member of a homologue pair carries a different allele. Before meiosis, the chromosomes are duplicated. In prophase I the homologues pair up and in metaphase I they align on the metaphase plate. When two homologues pair up, the maternal chromosome may be on either the left or the right. In this cell, for example, the maternal chromosome is on the right for both pairs of homologues. In this case, the gametes formed will have either the genotype A D or a d. But it is equally likely that when the small pair of homologues comes together, the maternal chromosome will be on the left, as shown here. In this second case, the gametes formed will have either the genotype A d or a D. When a large number of gametes are made, all four outcomes are equally likely. This is called independent assortment. It is as if we made gametes by first drawing one chromosome from a pool containing maternal and paternal chromosomes of the large type and combining that with one chromosome drawn from a pool containing maternal and paternal chromosomes of the small type.


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