Gametic disequilibrium

• Estimating gametic disequilibrium with D.

• Approach to gametic equilibrium over time.

• Causes of gametic disequilibrium.

In 1902 Walter Sutton and Theodor Boveri advanced the chromosome theory of heredity. They observed cell division and hypothesized that the discrete bodies seen separating into sets at meiosis and mitosis contained hereditary material that was transmitted from parents to offspring. At the time the concept of chromosomal inheritance presented a paradox. Mendel's second law says that gamete haplotypes (haploid genotype) should appear in frequencies proportional to the product of allele frequencies. This prediction conflicted with the chromosome theory of heredity since there are not enough chromosomes to represent each hereditary trait.

To see the problem, take the example of Homo sapiens with a current estimate of around 30,000 genes in the nuclear genome. However, humans have only 23 pairs of chromosomes. There are a large number of loci but a small number of chromosomes. So if chromosomes are indeed hereditary molecules, many genes must be on the same chromosome (on average about 1300 genes per chromosome for humans if there are 30,000 genes). This means that some genes are physically linked by being located on the same chromosome (see Fig. 2.17). The solution to the paradox is the process of recombination. Sister chromatids touch at random points during

Figure 2.17 Maps for human chromosomes 18 (left) and 19 (right) showing chromosome regions, the physical locations of identified genes and open reading frames (labeled orf ) along the chromosomes, and the names and locations of a subset of genes. Chromosome 18 is about 85 million bp and chromosome 19 is about 67 million bp. Maps from NCBI Map Viewer based on data as of January 2008.





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