Determining male or female form of living organism through sexual reproduction is called as sex determination. Sex determination on a large scale is of two types- environmental, genetic and chromosome based.
A. Chromosomal Sex Determination
Chromosomes are always found in a pairs and their number remains fixed in individuals of different species.
Table: Number of chromosomes in different organisms
S. No. Name of organism Number of Chromosomes
1 Cherry 32
2 Frog 26
3 Man 46
4 Dog 78
5 Horse 60
Out of 23 pairs of chromosomes in man 22 pairs of chromosomes are similar. These are called as XX chromosomes or Autosomes. Remaining one pair is dissimilar and is called as XY chromosomes or sex chromosomes. Human sperms contain half the number of chromosomes than the chromosomes contained in normal cells. A human female contains 22pairs of autosomes and one pair of sex chromosomes. Their sex chromosomes are similar i.e.XX.
When cells of human testis divide to form sperms then the pair of sex chromosome segregates into X and Y. Half the sperms get X chromosomes and the remaining half get Y chromosomes. The ova in females contain half the number of chromosomes than the normal cells like man. Egg cells produce ova but all the ova have only X chromosomes.
During fertilization it depends only on chance whether the X chromosome will fertilize the ovum or Y chromosome. If X-chromosome fertilizes the ovum the sex of the child will be XX or female but if Y chromosome fertilizes the ovum the sex of the child will be male or the XY.
In the above conditions there is 5o-50 chance of male and female child.
B. Some other Examples
I. In birds and butterflies male contains XX and female contains XY chromosomes. So the sex determination in these occurs through eggs and not through sperms.
II. In drosophila the sex is determined through the ratio of X and autosomes. Thus 2A+2X OR 3A+3X having the ratio X/Y OR 1:0 will be females and sexually active. But if ratio increase than 1 or 3X+2A or X/Y = ratio 1.5 the result will be such females which are sexually inactive. If ratio decreases or 3A+XY or X/Y = RATIO 0.33 then the males will be infertile.
C. Other types of sex determination
1. Sea snails if they are alone they become females but if they are with females they become males.
2. In Lizards and Crocodiles temperature decides the sex.
3. Chlamydomonas has sex determining gene.
4. In maize plant there are separate sex determining genes.
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