Genetics - A. V. Sivolob 2008

Formal Genetics: Patterns of Trait Inheritance
Chromosomes as Linkage Groups of Genes

The fact that genes reside on Chromosomes, which are passed directly from parents to offspring, significantly limits Mendel’s third law: chromosomes—large groups of genes known as linkage groups—are inherited independently of one another. Naturally, this limitation leads to DEVIATIONS FROM MENDELIAN segregation ratios in polyhybrid crosses. For example, when crossing dihybrids AaBb, assuming that genes A/a and B/b are located on the same chromosome, instead of the expected 9 : 3 : 3 : 1 ratio, we obtain two phenotypic classes in a 3 : 1 ratio (Fig. 3.8). Another way to test whether genes are on the same chromosome is a testcross AaBb × aabb: if the two genes assort independently, four phenotypic classes of offspring are produced in a 1 : 1 : 1 : 1 ratio (using the monohybrid testcross scheme given above and the product rule of probability); if the genes are linked on the same chromosome, there will be only two phenotypic classes (AaBb, aabb) in equal proportions.

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Fig. 3.8. Crossing dihybrids assuming two genes are located on the same chromosome

Observations of this type, along with the establishment that the number of linkage groups equals the haploid chromosome number, eventually enabled Thomas Hunt Morgan and his coworkers to provide genetic proof that genes are located on chromosomes (see Historical Background).

However, as was also discovered by Morgan's group, expected ratios like the ones just mentioned are almost never precisely realized: the exchange of segments between homologous chromosomes during Meiosis—Crossing Over (see Chapter 1)—leads to the "shuffling" of genes within homologous linkage groups (homologous chromosomes) and thus partially restores the independent transmission of genes to offspring.

Fig. 3.9. Chromosome sets of female and male Drosophila melanogaster: three pairs of autosomes and one pair of sex chromosomes

The latter statement requires a qualification: crossing over, driven by Homologous Recombination, occurs only between pairs of homologous chromosomes that share identical Morphology and Gene content in both sexes—the so-called autosomes. In most sexually reproducing organisms, one pair of chromosomes, the sex chromosomes, consists of two non-homologous types (Sex Genetics is discussed in detail in Chapter 6). One of the sexes (the homogametic sex) possesses two identical sex chromosomes, while the other (the heterogametic sex) has two different ones (for example, in mammals and some insects, two X chromosomes determine the female, whereas an X and a Y chromosome determine the male, Fig. 3.9). Recombination between a pair of non-homologous sex chromosomes is impossible (or severely restricted); consequently, sex chromosomes are transmitted as a single intact unit from the heterogametic parent to the offspring.