Along the segments of our DNA, genes are neatly packaged within structures called chromosomes. An allosome is a sex chromosome that differs from an ordinary autosome in form, size, and behavior.
DISCOVERY OF SEX CHROMOSOMES:
In most of the animals a pair of chromosomes is responsible for the determination of sex. These two chromosomes are called sex chromosomes or allosomes. The chromosomes other than the sex chromosomes are called autosomes. H. Henking in 1891 discovered the X-body. In 1902 these observations were verified by C.E. McClung and observed that somatic cells of female grasshopper carry a different Sex determination 2 chromosomal number than do the same cells in the male grasshopper. He associated the X-body to the sex determination, but erroneously considered it only to males. In the later investigations this X-body is designated as X-chromosome.
During the first decade of the twentieth century it was established that the sex of almost all many-celled biological organisms is determined at the moment of fertilization by the combination of two kinds of microscopic entities, the X and Y chromosomes. This discovery was the culmination of more than two thousand years of speculation and experiment on how an animal, plant, or human becomes male or female; at the same time it provided an important confirmation for the recently revived Mendelian genetics that was to become central part of modern biology.
According to the most biologists and historians who have written on the subject, the crucial step in the discovery of chromosomal sex determination was taken 1905 by Nettie M. Stevens and Edmund B. Wilson. But the scientific and chronological relation between their contributions has rarely been specified. (Stephen G. Brush, 1978)
Prior to Stevens's work, two theories emerged to explain sex determination. The first attributed the effect to environmental conditions, which according to some researchers caused the developmental fates of the embryo. The second ascribed sex determination to the internal factors of organisms, such as the cytoplasm or nucleus of the egg. Stevens's research was part of the internalist approach, and she focused on chromosomal differences in sperm cells. Stevens's research on the chromosome contained in sperm cells stemmed from research on accessory chromosomes conducted a few years earlier by Clarence Erwin McClung, a former teacher of Sutton, at the University of Kansas in Lawrence, Kansas.
Nettie studied Tenebrio molitor beetles and found that unfertilized eggs in female beetles always contain an X chromosome. Sperm from male beetles contain either an X chromosome or a Y chromosome. She found that eggs fertilized by sperm carrying the X chromosome produce female beetles. The combination of egg and Y-chromosome sperm produce male beetles.
Edmund Beecher Wilson, a biologist from Columbia University in New York City, made this same discovery at about the same time as Nettie. Nettie also established that chromosomes exist as paired structures in body cells.
Nettie Stevens was not credited very well for her discovery.
Studies in beetles provides one of the first observations of XY sex-determination. Stevens's findings allowed researchers to locate the material of Mendelian inheritance that passed specific traits, in this case the sex of the organism Tenebrio molitor, through a distinct chromosomal element.
DISCOVERY OF SEX CHROMOSOMES:
In most of the animals a pair of chromosomes is responsible for the determination of sex. These two chromosomes are called sex chromosomes or allosomes. The chromosomes other than the sex chromosomes are called autosomes. H. Henking in 1891 discovered the X-body. In 1902 these observations were verified by C.E. McClung and observed that somatic cells of female grasshopper carry a different Sex determination 2 chromosomal number than do the same cells in the male grasshopper. He associated the X-body to the sex determination, but erroneously considered it only to males. In the later investigations this X-body is designated as X-chromosome.
During the first decade of the twentieth century it was established that the sex of almost all many-celled biological organisms is determined at the moment of fertilization by the combination of two kinds of microscopic entities, the X and Y chromosomes. This discovery was the culmination of more than two thousand years of speculation and experiment on how an animal, plant, or human becomes male or female; at the same time it provided an important confirmation for the recently revived Mendelian genetics that was to become central part of modern biology.
According to the most biologists and historians who have written on the subject, the crucial step in the discovery of chromosomal sex determination was taken 1905 by Nettie M. Stevens and Edmund B. Wilson. But the scientific and chronological relation between their contributions has rarely been specified. (Stephen G. Brush, 1978)
Prior to Stevens's work, two theories emerged to explain sex determination. The first attributed the effect to environmental conditions, which according to some researchers caused the developmental fates of the embryo. The second ascribed sex determination to the internal factors of organisms, such as the cytoplasm or nucleus of the egg. Stevens's research was part of the internalist approach, and she focused on chromosomal differences in sperm cells. Stevens's research on the chromosome contained in sperm cells stemmed from research on accessory chromosomes conducted a few years earlier by Clarence Erwin McClung, a former teacher of Sutton, at the University of Kansas in Lawrence, Kansas.
Nettie studied Tenebrio molitor beetles and found that unfertilized eggs in female beetles always contain an X chromosome. Sperm from male beetles contain either an X chromosome or a Y chromosome. She found that eggs fertilized by sperm carrying the X chromosome produce female beetles. The combination of egg and Y-chromosome sperm produce male beetles.
Edmund Beecher Wilson, a biologist from Columbia University in New York City, made this same discovery at about the same time as Nettie. Nettie also established that chromosomes exist as paired structures in body cells.
Nettie Stevens was not credited very well for her discovery.
Studies in beetles provides one of the first observations of XY sex-determination. Stevens's findings allowed researchers to locate the material of Mendelian inheritance that passed specific traits, in this case the sex of the organism Tenebrio molitor, through a distinct chromosomal element.
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