Every living being has a definite number of chromosomes. In human beings, there are 23 pairs of chromosome. Among them 22 pairs are similar in both male and female, these are Autosomes, AA. Members of the remaining pair of the chromosome are different in male and female. This pair of chromosome determines the sex of human beings. As they determine the sex, so they are called X and y sex chromosome or Sex Determining Chromosome.
Fig: Sex Determining Chromosome
One of the sex chromosomes is X chromosome and the other is Y chromosome. If the sex chromosome pair is XY, the child will be male (son), and if it is XX, the child will be female (daughter). In Drosophila (a fly) also XX indicate female and XY male. So in case of determining the sex, the role of X and Y chromosome is vital.
The XX/XY sex-determination system is the most familiar, as it is found in humans. In the system, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). The XY sex chromosomes are different in shape and size from each other, unlike the autosomes, and are termed allosomes.