Genes That Assort Independently and Those That Dont

Chapter 3 introduced Mendel's principles of segregation and independent assortment. Let's take a moment to review these two important concepts. The principle of segregation states that each diploid individual possesses two alleles that separate in meiosis, with one allele going into each gamete. The principle of independent assortment provides additional information about the process of segregation: it tells us that the two alleles separate independently of alleles at other loci.

The independent separation of alleles produces recombination, the sorting of alleles into new combinations. Consider a cross between individuals homozygous for two different pairs of alleles: AABB X aabb. The first parent, AABB, produces gametes with alleles AB, and the second parent, aabb, produces gametes with the alleles ab, resulting in F1 progeny with genotype AaBb ( FIGURE 7.2). Recombination means that, when one of the F1 progeny reproduces, the combination of alleles in its gametes may differ from the combinations in the gametes of its parents. In other words, the F1 may produce gametes with alleles Ab or aB in addition to gametes with AB or ab.

Mendel derived his principles of segregation and independent assortment by observing progeny of genetic crosses, but he had no idea of what biological processes produced these phenomena. In 1903, Walter Sutton proposed a biological basis for Mendel's principles, called the chromosome theory of heredity (Chapter 3). This theory holds that genes are found on chromosomes. Let's restate Mendel's two principles in terms of the chromosome theory of heredity. The principle of segregation states that each diploid individual possesses two alleles for a trait, each of which is located at the same position, or locus, on each of the two homologous chromosomes. These chromosomes segregate in meiosis, with each gamete receiving one homolog. The principle of independent assortment states that, in meiosis,

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