Unusual Characteristics Of Sex Chromosomes And Mitochondria

In mammals a female carries two copies of the X chromosome; males have one X and one Y. However, this simple difference in karyotype (the set of chromosomes) has profound effects that go beyond just the establishment of sex. The first thing to consider is why we need sex at all. In species with just one sex, each organism can reproduce clon-ally. The offspring of that organism may be identical. If the organism inhabits a wide ecological range, different selection processes will produce a geographical pattern of genetic differences, but there is no rapid way to combine these in response to a shifting environment. Sex, on the other hand, demands continual outbreeding, so it leads to much more efficient mixing of the gene pool of a species.

Figure 2.22 Generation of partial aneuploidy by a reciprocal translocation, followed by segregation of the rearranged chromosomes to gametes and, after fertilization, generation of individuals that are usually phenotypically abnormal.

We are so used to the notion of two sexes, that it often goes unquestioned why two and only two? Current speculation is that this is the result of the smallest component of mammalian genomes, the mitochondrial DNA. Mitochondria have a circular chromosome, much like the typical bacterial genome from which they presumably derived. This genome codes for key cellular metabolic functions. In the human it is 16,569 kb in size, and the complete DNA sequence is known. A map summarizing this sequence is shown in Figure 2.23. Mitochondria have many copies of this DNA. What is striking is that all of an individual's mitochondria are maternally inherited. The sperm does contain a few mitochondria, and these can enter the ovum upon fertilization, but they are somehow destroyed.

Bacterial DNAs carry restriction nucleases that can destroy foreign (or incompatible) DNA (see Box 1.2). Perhaps, from their bacterial origin, mitochondria also have such properties.

Figure 2.23 Map of human mitochondrial DNA. The tRNAs are indicated by their cognate amino acid letter code. The genes encoded by the G-rich heavy (H) strand are on the outside of the circle, while those for the C-rich light (L) strand are on the inside. The H- and L-strand origins (OH and OL) and promoters (PH and PL) are shown. The common 5-kb deletion associated with aging is shown outside the circle. (Adapted from Wallace, 1995.)

Figure 2.23 Map of human mitochondrial DNA. The tRNAs are indicated by their cognate amino acid letter code. The genes encoded by the G-rich heavy (H) strand are on the outside of the circle, while those for the C-rich light (L) strand are on the inside. The H- and L-strand origins (OH and OL) and promoters (PH and PL) are shown. The common 5-kb deletion associated with aging is shown outside the circle. (Adapted from Wallace, 1995.)

If they do, this would explain why one sex must contribute all of the mitochondria. It can be used as an argument that there should only be two sexes. In fact, however, cases are known where organisms have more than two sexes. The slime mold, Physarum polycephalum, has 13 sexes. However, these turn out to be hierarchical. When two sexes mate, the higher one on the hierarchy donates its mitochondria to the offspring. This ensures that only one parental set of mitochondria survive. So one important thing about sex is who you get your mitochondria from.

In the human and other mammals, the Y chromosome is largely devoid of genes. The long arm is a dark G band (Fig. 2.13), and the short arm is small. However, an exception is the gene-rich tip of the short arm, which is called the pseudoautosomal region.

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