Sex-Determining Mechanisms Help
The Importance of Sex
We typically think of sex in terms of males and females . Not all organisms, however, possess only two sexes. Some of the simplest forms of plant and animal life may have several sexes. For example, in one variety of the ciliated protozoan Paramecium bursaria there are eight sexes, or "mating types," all morphologically identical. Each mating type is physiologically incapable of conjugating with its own type, but may exchange genetic material with any of the seven other types within the same variety. However, most complex organisms have only two sexes. These sexes may reside in different individuals or within the same individual. An animal with both male and female reproductive organs is usually referred to as a hermaphrodite. In plants where staminate (male) and pistillate (female) flowers occur on the same plant, the term monoecious is used. Most flowering plants have both male and female parts within the same flower. A few angiosperms are dioecious, i.e., having the male and female elements in different individuals. Among the common cultivated crops known to be dioecious are asparagus, date palm, hemp, hops, and spinach.
Whether there are two or more sexes, or whether these sexes reside in the same or different individuals, sex is still important: it is a mechanism that provides for the great amount of genetic variability that characterizes most natural populations. The evolutionary process of natural selection depends upon this genetic variability to supply the raw material from which better adapted phenotypes usually survive to reproduce. Many subsidiary mechanisms have evolved to ensure cross-fertilization in most species as a means for generating new genetic combinations in each generation.
Sex Chromosome Mechanisms
Most mechanisms for the determination of sex are under genetic control and may be classified into one of the following categories.
In most mammals, there are two different, or heteromorphic, sex chromosomes, the X and the Y chromosomes. The presence of the Y chromosome determines maleness. Normal human males have 22 pairs of autosomes and an X and a Y sex chromosome; females also have 22 pairs of autosomes, but have two X chromosomes. Since the male produces two kinds of gametes as far as the sex chromosomes are concerned, he is said to be the heterogametic sex. The female, producing only one kind of gamete, is the homogametic sex. Thus, assuming unbiased segregation and equal success of each type of gamete during fertilization, an equal number of each sex should be produced in each generation. The proportion of males to females is referred to as the sex ratio. This mode of sex determination is commonly referred to as the XY method.
A gene called SRY, for sex-determining region Y, on the short arm of the Y chromosome encodes a gene product often referred to as the testis-determining factor (TDF). SRY seems to be highly conserved in mammals. This gene, in combination with several other autosomal genes, encodes a DNA-binding protein that appears to control the expression of one or more other genes in a hierarchy or cascade of gene activation involved in testicular development and sperm production. In the absence of SRY embryonic, gonadal tissue would normally develop into an ovary. The location of SRY was aided by the discovery of rare exceptions to the rule that XX results in femaleness and XY results in maleness. Normal-appearing but sterile XX human males have at least some of the SRY gene attached to one of their X chromosomes and normal-appearing human XY females have a Y chromosome that has lost a crucial part of the SRY.
In some insects, especially those of the orders Hemiptera (true insects) and Orthoptera (grasshoppers and roaches), males are also heterogametic, but produce either X-bearing sperm or gametes without a sex chromosome. In males of these species, the X chromosome has no homologous pairing partner because there is no Y chromosome present. Thus, males exhibit an odd number in their chromosome complement. The one-X and two-X condition determines maleness and femaleness, respectively. If the single X chromosome of the male is always included in one of the two types of gametes formed, then a 1 : 1 sex ratio is predicted in the progeny. This mode of sex determination is commonly referred to as the XO method where the O symbolizes the lack of a chromosome analogous to the Y of the XY system.
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