In earlier days the inheritance of sex was thought to be form factors unrelated to genes. The sex determination was attributed to phases of moon, time of days during fertilization, wind direction, whether, right or left testis was involved and other such causes. By the end of 17th century a French writer had recorded 262 such theories. With the discovery of Mendelism in 1900, the search for mechanism of sexual inheritance shifted to chromosomes and cytological studies. The discovery of sex chromosomes and their presence or absence shifted the attention to chromosome mechanism of determining sex.
MECHANISM OF SEX DETERMINATION
Basically four types of chromosomal sex determining mechanisms exist: the XO, XY, ZW and compound chromosome mechanism.
(i) The XO Mechanism: This system is sometimes referred to as an XO-XX system. It occurs in many species of injects, e.g. in grasshopper and Bug Protenor. There is no pairing partner in males, therefore heterogametic and termed XO. The females are homogametic and called XX. The sex is determined by males. Males produce gametes that contain either X chromosome or no sex chromosome, whereas all the gametes form a female which contains X chromosome.
(ii) The XY Mechanism: The XY situation occurs in human beings and Drosophila.
The XY mechanism in Humans: In human females there are 46 chromosomes arranged in 23 homologous pairs of chromosomes. Males have same number of chromosomes but 22 pairs of chromosomes are homologous where as 23rd is heterologous. One pairing partner is smaller in size as compared to the other. It is called Y chromosome. The larger chromosome is X chromosome. In female 23rd pair carries both X chromosomes. Male is heterogametic and produces sperms with other X or Y chromosomes. All the female eggs contain X chromosome, fertilization of egg by Y sperm results in male and by X sperm in a female, therefore the sex is determined by males. Thus father contributes his X to daughters and Y to sons, where as mother contributes her X to sons as well as daughters.
The XY mechanism in Drosophila: In Drosophila system is same but Y chromosome is J-shaped and X rod shaped. Male and female both possess two sets of autosomes, chromosomes II III and IV in their body cells. They differ in sex chromosomes. Male carries 1X + 1Y, where as the female carries 2X’s also called chromosome
(iii) The ZW Mechanism: This system is found in moths, butterflies and birds. It is similar to XY system except in that the males are homogametic and females heterogametic and the sex is determined by females. The females carry heterologous sex chromosomes ZW and males homologous sex chromosomes WW. The eggs contain either Z or W where as all sperms contain W chromosomes.
SEX DETERMINATION IN DROSOPHILA
Calvin bridges suggested in 1922 that genes for sex in Drosophila are determined by balance (ratio) of autosomal alleles that favour maleness and alleles on X chromosomes that favour femaleness. The genes on autosomes have more tendencies towards maleness and genes on X chromosome towards femaleness. He crossed triploid (3x) Drosophila female with a normal male and observed many combinations of autosomes and sex chromosomes in the offspring. Autosomal set (A) in Drosophila consist of three chromosomes. Normally two sets of autosomes plus two X’s results in a female (2A + 2X), two sets of autosomes plus one X produce a male (2A + X). He calculated a ratio of X chromosomes a male (2A + X. He calculated a ratio of X chromosomes to autosomal sets in order to sec if this ratio would predict the sex of a fly. The results suggested that if X | A ratio is 1.00, the organisms is a female and when this is 0.50, the organism is a male. At 0.67 the organism is inter sex. Similarly if the ratio is greater than 1.00 the organisms are meta females (1.50) and if less than 0.50 they are meta males (0.33). The meta females and meta males are very weak and sterile. The meta females usually do not even emerge from their pupal case.
SEX DETERMINATION IN HUMAN BEINGS
Genic balance certainly plays an important role in Humans but there are distinct differences from Drosophila. The Y chromosome in Drosophjila is unnecessary for life and for male secondary sex characteristics. Thus X0 individual is male and XXY is female. Such is not the case in humans. The Y chromosome is essential for male attributes. X0 and XXY persons are found in about 1 in 3000 female births is usually of short stature, possesses a webbed neck, as undeveloped ovaries and an immature uterus plus cardio vascular defects and other somatic aberrations. Similarly K line felters, syndrome-an XXY male arises in about 1 out of 600 male births. These males typically show some breast development, small testes, sparse body hair and some mental deficiency.
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