Sex determination

The genetic mechanisms by which sex is determined in all living organisms. The nature of the genetic basis of sex determination varies a great deal among the various forms of life.

There are two aspects of sexuality: the primary form involves the gametes, and the secondary aspect is gender. In its broadest usage the term “sex” refers to the processes that enable species to exchange materials between homologous chromosomes, that is, to effect recombination. Generally, recombination is essential to their mechanism for reproduction. For most organisms this involves, either exclusively or as one stage in the life cycle, the formation of special cells, known as gametes, by meiosis. See Gametogenesis

Most sexually reproducing species produce two different kinds of gametes. The relatively large and sessile form, an ovum or egg, usually accumulates nutriments in its cytoplasm for the early development of the offspring. The relatively mobile form, a sperm (or pollen grain in many plants), contributes little beyond a haploid chromosome set. Thus the primary form of sex differentiation determines which kind of gamete will be produced. The formation of gametes usually involves the concomitant differentiation of specialized organs, the gonads, to produce each kind of gamete. The ova-producing gonad is usually known as an archegonium or ovary (in flowering plants it is part of a larger organ, the pistil or carpel); the gonad producing the more mobile gametes is usually known as a testis in animals and an antheridium or stamen in plants. See Ovary, Ovum, Sperm cell, Testis

In most animals and many plants, individuals become specialized to produce only one kind of gamete. These individuals usually differ not only in which kind of gonad they possess but also in a number of other morphological and physiological differences, or secondary sex characteristics. The latter may define a phenotypic sex when present, even if the typical gonad for that sex is absent or nonfunctional. The form that usually produces ova is known as female; the one that usually produces sperm or pollen is known as male. Since some sexual processes do not involve gametes, the more universal application of the term “gender” refers to any donor of genetic material as male and the recipient as female.

In plants, sexual reproduction is not always accompanied by the kinds of differentiation described above. The majority of plant species are monoecious, with both kinds of gonads on the same plant. Plants that bear male and female gonads on separate plants are dioecious. They occur in about 60 of the 400 or so families of flowering plants, 20 of which are thought to contain exclusively dioecious species. See Reproduction (plant)

Although the sexes are distinct in most animals, many species are hermaphroditic; that is, the same individual is capable of producing both eggs and sperm. This condition is particularly common among sessile or sluggish, slowly moving forms. Some hermaphroditic and monoecious species are homothallic; that is, the eggs and sperm of the same individual can combine successfully; but most are heterothallic, the gametes being capable only of cross-fertilization, often evolving special mechanisms to ensure its occurrence.

Sex differentiations are often accompanied by consistent chromosomal dimorphisms, leading to the presumption that the chromosomal differences are related to, and possibly responsible for, the sex differences. Indeed, the chromosomes that are not alike in the two sexes were given the name sex chromosomes. Some workers use the term “heterosomes” to distinguish them from the autosomes, which are the chromosomes that are morphologically identical in the two sexes.

In most species, one of the sex chromosomes, the X chromosome, normally occurs as a pair in one gender but only singly in the other. The gender with two X chromosomes is known as the homogametic sex, because each of its gametes normally receives an X chromosome after meiosis. The gender with only one X chromosome generally also has a morphologically different sex chromosome, the Y chromosome. The X and Y chromosomes usually pair to some extent at meiosis, with the result that the XY is the heterogametic sex, with half its gametes containing an X and half containing a Y. Geneticists noted that the fundamental dimorphism of X and Y chromosomes lies in their genic contents: X chromosomes of the species share homologous loci, just as do pairs of autosomes, whereas the Y chromosome usually has few, if any, loci that are also represented on the X. Thus X and Y chromosomes are sometimes very similar in shape or size but are almost always very different in genetic materials.

The major factor in sex differentiation in humans is a locus on the short arm of the Y chromosome designated SRY or SrY (for sex-determining region of the Y). This comparatively small gene contains no introns and encodes for a protein with only 204 amino acids. The protein appears to be a deoxyribonucleic acid (DNA)-binding type that causes somatic cells of the developing gonad to become Sertoli cells that secrete a hormone, Müllerian inhibiting substance (MIS), that eliminates the Müllerian duct system (the part that would produce major female reproductive organs). The gonad is now a testis, and certain cells in it become the Leydig cells that produce testosterone, which causes the primordial Wolffian duct system of the embryo to develop the major male reproductive organs. If no MIS is produced, further development of the Müllerian duct structures occurs, and in the absence of testosterone the Wolffian ducts disappear, producing the normal female structures. Embryos lacking SRY or having mutated forms of it normally become females even if they are XY. This system of sex determination is called Y-dominant. It appears to be characteristic of almost all mammals, even marsupials, plus a few other forms. While SRY is the primary gene, many other genes, both autosomal and X-chromosomal, are involved in the course of developing the two sexes in mammals. See Human genetics, Mutation

sex determination