Good Laboratory Practices for Molecular Genetic Testing for Heritable Diseases and Conditions
In humans, as in all mammals, the normal female has two X chromosomes and the normal male has one X chromosome and one Y chromosome. The female is thus the homogametic sex, as all her gametes normally have one X chromosome. The male is heterogametic, as he produces two types of gametes—one type containing an X chromosome and the other containing a Y chromosome.
There is good evidence that the Y chromosome in humans, unlike that in Drosophila , is necessary but not sufficient for maleness. A human individual arises through the union of two cells, an egg from the mother and a sperm from the father. Human egg cells are barely visible to the naked eye. They are shed, usually one at a time, from the ovary into the oviducts fallopian tubes , through which they pass into the uterus.
Fertilization, the penetration of an egg by a sperm, occurs in the oviducts. This is the main event of sexual reproduction and determines the genetic constitution of the new individual. Human sex determination is a genetic process that depends basically on the presence of the Y chromosome in the fertilized egg. This chromosome stimulates a change in the undifferentiated gonad into that of the male a testicle.
The gonadal action of the Y chromosome is mediated by a gene located near the centromere; this gene codes for the production of a cell surface molecule called the H-Y antigen. Further development of the anatomic structures, both internal and external, that are associated with maleness is controlled by hormones produced by the testicle. The sex of an individual can be thought of in three different contexts: chromosomal sex, gonadal sex, and anatomic sex. Discrepancies between these, especially the latter two, result in the development of individuals with ambiguous sex, often called hermaphrodites.
The phenomenon of homosexuality is of uncertain cause and is unrelated to the above sex-determining factors. It is of interest that in the absence of a male gonad testicle the internal and external sex anatomy is always female, even in the absence of a female ovary. A female without ovaries will, of course, be infertile and will not experience any of the female developmental changes normally associated with puberty. If X-containing and Y-containing sperm are produced in equal numbers, then according to simple chance one would expect the sex ratio at conception fertilization to be half boys and half girls, or 1 : 1.
Direct observation of sex ratios among newly fertilized human eggs is not yet feasible , and sex-ratio data are usually collected at the time of birth. In almost all human populations of newborns, there is a slight excess of males; about boys are born for every girls. Throughout life, however, there is a slightly greater mortality of males; this slowly alters the sex ratio until, beyond the age of about 50 years, there is an excess of females.
Studies indicate that male embryos suffer a relatively greater degree of prenatal mortality, so the sex ratio at conception might be expected to favour males even more than the : ratio observed at birth would suggest.
Firm explanations for the apparent excess of male conceptions have not been established; it is possible that Y-containing sperm survive better within the female reproductive tract, or they may be a little more successful in reaching the egg in order to fertilize it. In any case, the sex differences are small, the statistical expectation for a boy or girl at any single birth still being close to one out of two. During gestation—the period of nine months between fertilization and the birth of the infant—a remarkable series of developmental changes occur.
In addition, these cells differentiate into hundreds of different types with specific functions liver cells, nerve cells, muscle cells, etc. A multitude of regulatory processes, both genetically and environmentally controlled, accomplish this differentiation. Elucidation of the exquisite timing of these processes remains one of the great challenges of human biology.
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