Meiosis and its Consequences

Defining Meiosis

Writing in Science, Hirsch (1963) explained that sexual reproduction involves meiosis—a complex cellular process resulting in a meristic division of the nucleus and formation of gametes (reproductive cells) having single genomes (a haploid chromosome set). One homolog in every chromosome pair in our diploid complement is of paternal origin and the other is of maternal origin. In meiosis, the homologs of a pair segregate and a gamete receives one from each pair. The assortment to gametes of the segregating homologs occurs independently for each pair.

This process ensures diversity because it maximizes the likelihood that gametes will receive unique genomes. For example, gametogenesis in Drosophila[Page 696]willistoni produces eight alternative gametic genomes, which, if we represent the three chromosome pairs of this species by Aa, Bb, and Cc, we designate ABC, Abc, AbC, aBC, ABc, aBc, abC, abc.

In general, n pairs of chromosomes produce 2n genomes (if we ignore the recombination of gene linkages that actually occurs in crossover exchanges between chromosomes). Humans, with 23 chromosome pairs, produce gametes with any of 223 alternative genomes. Hirsch (1963) argued that these facts make vanishingly small the chances that even siblings (other than monozygotes) will be genetically identical. He concluded that, since the gamete contributed by each parent is chosen from 223 alternatives, the probability that the second offspring born to parents will have exactly the same genotype as their firstborn is (1/223)2, or less than 1 chance in more than 70 trillion (Hirsch, 1963). The probability that two unrelated individuals will have the same genotype, then, is effectively zero (Hirsch, 1963).

After this computation, the argument for the genotypic uniqueness of members of populations is even more compelling, since other conditions also contribute to diversity. Accordingly, Hirsch stressed that the organisms that the behavioral sciences study are intrinsically variable before they undergo differentiating experiences, and that the mechanisms responsible for this variety are mutation, recombination, and meiosis. If one adds to these factors individual experience, it then becomes evident why individuals differ in behavior. In fact, it was inferred that the more reliable our methods of observation become, the more evident will this variety be (Hirsch, 1963).

While Hirsch (1963) was correct that the facts of meiosis result in an impressive degree of human diversity, it is now apparent that the magnitude of this variation—while nevertheless vast—is less than specified in 1963. Understanding why this is the case illustrates that there are some surprising and important subtleties about the impact of meiosis on diversity.

Advances in the Conceptual Understanding of Meiosis and Diversity

The two extremes of diversity are, first, that each individual is unique, which was what researchers said for years (Hirsch, 1963, through Hirsch, 2004). However, this assertion turned out to be wrong. The other extreme of diversity is that, except for sex, all individuals are similar. This view is not true either. Individuals are not uniformly unique and, theoretically, replicates will occur. It turns out to be a “Never Never Land.” Individuals are unique but can fall into similar slots.

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