Blood Groups and Possible Transfusions in Various Persons

            The sexual reproduction requires participation of two different sexes - a male and a female. Normally, these two differ from each other in many external and internal features, but the basis for determination of sex is rather simple. The mechanism of sex determination is different in different organisms. However,in most of the cases it has a genetic or chromosomal basis, e.g., humans, birds, insects, etc, but in some it way be environmental cues viz. 

               In Drosphila, sex is determined by the ratio of X chromosomes to set of auto sames (X:A). If X:A<, the fly develops as a male temperature or diet also! In some reptiles viz. turtle Chrysema picta, temperature, determines the sex of the offspring. Here, high incubation temperature leads to the development of female offsprings.

             There are two types of chromosomes in organisms - autosomes and sex chromosomes. Sex chromosomes that are important for sex determination. If the same sex chromosomoes are present in pair, the sex is said to be homogametic (e.g., XX) and if they are dissimilar. It is said to be heterogametic (e.g., XY). Sex determination in different animals is summarized in the fig. B.8.1

X:A> 1.0 the fly develops as a female and 0.5 < X :A < 1.0 the fly develops as an intersex.

Note: In 1990 , a British research team identified a gene on the Y chromosome required for the development of testes in humans. They named the gene SRY, for Sex- determining Region of Y. In the absence of SRY, the gonads into ovaries because this is the gene required for the development of tests.