Before the discovery of sex
chromosomes, the mechanism behind sex determination was not well understood. It
was not clear how the differences between males and females were inherited
genetically. The discovery of sex chromosomes was a significant breakthrough in
understanding the genetic basis of sex determination. It revealed that there
are specific chromosomes responsible for determining gender, and that these
chromosomes have distinct structures and inheritance patterns. This knowledge
allowed researchers to further investigate the genetic mechanisms that underlie
sex determination, which has important implications for understanding human
biology and development.
The genetic basis of sex
determination was introduced after the discovery of sex chromosomes.
Discovery of Sex Chromosomes
In 1910 T. H. Morgan was
working on the small fruit fly Drosophila melanogaster, which means black
bellied dew lover. The normal eye color of this fly is red. Morgan raised
thousands of red eye flies in bottles, supplying banana as food. It was found
that one fly was white eyed instead of normal red. The white eyed fly was male.
It was mated with red female. In F1 generation, flies were red. The F1 red eyed
flies were mated to produce an F2 generation. Among the offspring 1/4 flies was
white eyed male and 3/4 flies were red eyed females.
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| Chromosomes in Drosophila |
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| Chromosomes in Man |
Chromosomes in Drosophila
Just before the discovery of
white eyed males by Morgan, the research on the chromosomes of Drosophila had
been done. There are four pairs i.e. 8 chromosomes in Drosophila. Three pairs
of chromosomes are identical in male and female, which are called autosomes.
The fourth pair of chromosome is different in male and the female, and
determine gender are called sex chromosomes. In females both the sex
chromosomes are rod shaped and identical, so these are called XX chromosomes.
In males one is rod shaped and is like the sex chromosome of the female so it
is also called X-chromosome, while the other chromosome is I or hooked shaped
and it is called Y chromosome.
In humans there are 23 pairs
of chromosomes, 22 pairs of which are autosome while 1(one) pair is sex
chromosome. In female it is XX and XY in males. Human females have two copies
of the X chromosome. All eggs cells produced by a woman contain one X
chromosome. In males half of the sperms contain an X chromosome and half
contain a Y chromosome. The sex chromosome carried by the sperm therefore
determines the gender of the child. If a sperm carrying an X-chromosome
fertilizes the egg, the child will be a girl, but if a sperm carrying a Y
chromosome fertilizes the egg, the child will be a boy.
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| Sex determination in human |
SRY is the male determining
gene which is located at the tip of short arm of Y chromosome. SRY stands for
“Sex determining region of Y”.
