Variations of Sex Linkage Help
Variations of Sex Linkage
The sex chromosomes (X and Y) often are of unequal size, shape, and/or staining qualities. The fact that they pair during meiosis is an indication that they contain at least some homologous pseudoautosomal segments. Genes on the pseudoautosomal segments are said to be incompletely sex-linked or partially sex-linked and may recombine by crossing over in both sexes, just as do the gene loci on homologous autosomes. Special crosses are required to demonstrate the presence of such genes on the X chromosome, and few examples are known. Genes on the nonhomologous segment of the X chromosome are said to be completely sex-linked and exhibit the peculiar mode of inheritance described in the preceding sections. In humans, the nonrecombining region of theY chromosome (NRY) makes up about 95% of this chromosome, but only about a dozen active genes reside there. In such cases, the corresponding trait(s) would be expressed only in males and would always be transmitted from father to son. Such completely Y-linked genes are called holandric genes (Fig. 5-2).
Fig. 5-2. Generalized diagram of X and Y chromosomes. The relative size of these chromosomes and the size of homologous and nonhomologous regions, as well as location of the centromeres (not shown), vary according to the species.
Sex-Influenced and Sex-Limited Traits
Sex-influenced traits are traits whose phenotypic expression is affected by the sex of the individual. The genes governing sex-influenced traits may reside on any of the autosomes or on the homologous portions of the sex chromosomes. The expression of dominance or recessiveness by the alleles of sex-influenced loci is reversed in males and females due, in large part, to the difference in the internal environment provided by the sex hormones. Thus, examples of sex-influenced traits are most readily found in the higher animals with well-developed endocrine systems.
Some autosomal genes may only come to expression in one of the sexes, either because of differences in the internal hormonal environment or because of anatomical dissimilarities. For example, we know that bulls have many genes for milk production that they may transmit to their daughters, but they or their sons are unable to express this trait. The production of milk is therefore limited to variable expression in only the female sex. When the penetrance of a gene in one sex is zero, the trait will be sex-limited.
Sex Reversal and Sexual Phenomena in Plants
Female chickens (ZW) that have laid eggs have been known to undergo not only a reversal of the secondary sexual characteristics such as development of cock-feathering, spurs, and crowing, but also the development of testes and even the production of sperm cells (primary sexual characteristics). This may occur when, for example, disease destroys the ovarian tissue, and in the absence of the female sex hormones the rudimentary testicular tissue present in the center of the ovary is allowed to proliferate. In solving problems involving sex reversals, it must be remembered that the functional male derived through sex reversal will still remain genetically female (ZW).
Sexual Phenomena in Plants
Most flowering plants are monoecious and therefore do not have sex chromosomes. Indeed, the ability of mitotically produced cells with exactly the same genetic endowment to produce tissues with different sexual functions in a perfect flower speaks clearly for the bipotentiality of such plant cells. Well-known examples of dioecism usually are under the genetic control of a single-gene locus. However, at least one well-documented case of chromosomal sexuality is known in plants, i.e., in the genus Melandrium (a member of the pink family). Here, the Y chromosome determines a tendency to maleness just as it does in humans. Pistillate plants are XX and staminate plants are XY.
The ability of gametes produced by the same individual to unite and produce viable and fertile offspring is common among many families of flowering plants. Self-fertilization is also known to occur in a few of the lower animal groups. The perfect flowers of some monoecious plants fail to open (cleistogamy) until after the pollen has matured and accomplished self-fertilization. Self-fertilization is obligatory in barley, beans, oats, peas, soybeans, tobacco, tomato, wheat, and many other crops. In some species, self-fertilization as well as cross-fertilization may occur to varying degrees. For example, cotton and sorghum commonly experience more than 10% crossfertilization. Still other monoecious species have developed genetic mechanisms that prevent self-fertilization or the development of zygotes produced by the union of identical gametes, making cross-fertilization obligatory. Self-incompatibility in monoecious species can become as efficient in enforcing cross-fertilization as the system exhibited under a dioecious mechanism of sex determination.
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