Sex determination refers to the mechanism by which an individual develops as a male or female. In many species of organisms, males and females differ in the types of sex chromosomes; that is the rod-like structures which contain genetic material which they have. These chromosomes can determine sex because they contain genes; units of hereditary material that direct development of reproductive structures of one sex while suppressing development of the structures of the other sex. Scientific research has shown that, the sex with two chromosomes of different kind is the heterogametic sex and the other with two of the same sex chromosomes is the homogametic sex. In all mammals, the male is the heterogametic sex whiles females in birds, moths and butterflies are the heterogametic. Sex determination in humans is completely different from the fruit fly.
H von Winiwarter was the first scientist to have made a significant attempt to count the chromosome number of human species. This was done in 1912 when he counted forty-seven chromosomes in a spermatogonial metaphase; one of the stages during cell division. At that time, geneticists believed that sex determination in humans was based on the presence of an extra chromosome in females, that is, females were thought to have forty-eight chromosomes.
In the 1920's, Theophilus Painter observed forty-five and forty-eight chromosomes in testicular tissue and also discovered the small Y-chromosome which is now known to occur in males. For over thirty years, it was believed that human beings had forty-eight chromosomes until in 1956, when Joe Hin Tjio and Albert Levan introduced improvements in chromosome preparation techniques.These techniques led to the discovery that the correct number of chromosomes in humans was forty-six;twenty-three pairs and C.E Ford and John L. Hamerton confirmed this later that year when they were working with testicular tissue.
Within the twenty-three pairs of human chromosomes, one pair was shown to vary in configuration in males and females. The two chromosomes were designated sex chromosomes. The human female has two X-chromosomes whiles the male has one X and one Y-chromosome. The Y-chromosome is responsible for showing maleness in humans. There is a special region which forms a small part about half a percent of the Y-chromosome, which includes a gene called SRY that specifies maleness. This gene produces a protein that switches on other genes that direct the embryo to develop male structures. It also activates a gene that encodes a protein that destroys rudimentary female structures.
However, cytological evidence reveals that male Drosophila is heterogametic (XY) sex. In this case, the Y-chromosome, which is responsible for determining maleness in humans, does not do it in Drosophila. Here, maleness is determined by the ratio of one X chromosome to the paired autosomes; the other pairs of chromosomes other than the pair of sex chromosomes.
Studies conducted by Calvin Bridges in 1916 defined a term called nondisjunction which simply means the failure of paired chromosomes to segregate or separate during the anaphase stage of the first and second meiotic divisions; one type of cell division. He studied fruit flies that were the products of gametes formed by nondisjunction of the X-chromosome. In addition to the normal complement of six autosomes, these individuals had either an XXY or an X0 chromosome present. The zero signifies that the second chromosome is absent. The XXY flies were normal females while the X0 flies were sterile males. The presence of the Y-chromosome in the XXY flies did not cause maleness and its absence in the X0 flies did not produce femaleness. From the data, Bridges concluded that the Y-chromosome in Drosophila lacks male determining factors but apparently contains genetic information essential for male fertility since the X0 males were sterile.
Further studies by Bridges pointed out that, females that have a ratio of X-chromosomes to number of autosomes present to be equal to one to be fertile. As the ratio exceeds unity, a super female with rather infertile and weak characteristics is produced. Since it has lowered viability, this type is now more appropriately called metafemale. He further found out that male determining factors are not localized on the sex chromosomes but are instead found on autosomes. Female determining factors, however, are localized on the X-chromosome. On the other hand, in humans, XXY individuals are males but they have female-like breast and poor development of testes and these individuals are sterile.
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