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Most organisms have two kinds of sex chromosomes, namely X and Y, which determine the sex of the organism. The XX/XY sex determination system is the well-known sex determination system in most of the mammals, including human beings and in some insects like drosophilid fruit flies. In this system, females have two of the same kind of sex chromosome (XX) whereas males have two of the different kinds of sex chromosomes (XY). A similar system exists in birds and some nsects like moths and butterflies, with different sex chromosomes, namely Z and W. In contrast to the XX/XY system, the males in ZZ/ZW system have two of the same kind of sex chromosome (ZZ) while the females possess two of the different kinds of sex chromosomes (ZW). Despite these differences, both sexes have two sex chromosomes in XX/XY and ZZ/ZW systems so that both males and females have equal numbers of total chromosomes. But some other insects like cockroaches, crickets and grasshoppers have only one kind of sex chromosome, namely X. Males have only one sex chromosome (X0) whereas females have two of the same kind of chromosome (XX). The zero in X0 implies the absence of the second X chromosome in males. Hence, the males and females have different numbers of total chromosomes in cockroaches. Some of the human females may get X0 genotype, which is a chromosomal abnormality and medically termed as `Turner's syndrome'.