The 23 pairs of chromosomes of the normal male and female. Genetic sex is determined by the so-called sex chromosomes-the 23rd pair. There are two sorts of sex chromosomes, X chromosomes and Y chromosomes. Every fertilized egg contains one X chromosome that is contributed by the mother. The second member of the pair contributed by the father, may be either another X chromosome-in which case a female is formed-or a Y chromosome-which produces a male. Since only males have the Y chromosome, the genetic sex of a child is entirely determined by which one of the father’s two sex chromosomes the child inherits. Occasionally something goes wrong with this process and a child is produced with only one X chromosome, or two Xs and a Y, or two Ys and an X. At other times hormonal abnormalities may cause a child whose genetic sex is male to appear female and be reared as a girl or vice versa. (Such cases are discussed in later articles).
The 44 other chromosomes normally occur in matched pairs, although here again there are occasional genetic errors In the most common of these there are 3 chromosomes instead of 2 in the 21st set; the result is a child born with Down’s syndrome (formerly called ”mongolism”), which consists of mental deficiency combined with various physical abnormalities.
Although the transmission of information from one generation to the next via the genes usually works very well, on occasion something goes awry. In a process called mutation one or more genes in an ovum or sperm cell is altered. Mutations can affect the structure and development of an organism in ways that may be beneficial or harmful; or they may have no significant effect on the organism. The causes of mutations are varied and include exposure to X-rays and certain chemicals.
Although chromosomes are large enough to be visible through a good microscope, this is not the case with genes. No one knows exactly how many genes there are in a human cell (the total number has been estimated to be somewhere between 20,000 and 125,000) or in exactly what order they are arranged on the chromosomes. Most of what is known about genetic action is derived by inference from breeding experiments, such as those that Gregor Mendel performed on pea plants more than a century ago. Nowadays geneticists often use fruit flies for their experiments, partly because it is possible to produce three or four generations of fruit flies in the time it takes for one pea plant to mature.
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