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Sex Determination

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Sex Determination - Lesson Summary

Sex determination in human beings and several other animals is done with the help of specific chromosomes. These sex-determining chromosomes are called sex chromosomes, while the rest of the chromosomes are called autosomes.
In human beings and several other mammals and insects, the sex-determination system is of the XX-XY type. Human beings have twenty three pairs of chromosomes. Of these, the twenty two autosomes are identical in both females and males while the sex chromosomes are different. The sex chromosomes in females are a pair of X-chromosomes while those in males are an X-chromosome and a Y-chromosome. Of the total sperms produced during spermatogenesis, fifty per cent carry X-chromosomes while the remaining fifty per cent carry Y-chromosomes, it is obvious that the sex of the offspring is determined by the genetic make-up of the sperm.
Another mechanism of sex determination is the XX-XO type which is applicable to several insects such as grasshoppers, cockroaches and bugs. In this type of sex determination, females have two identical homologous sex chromosomes designated as XX while the males have just one sex chromosome designated as XO. Here, the ‘O’ denotes the absence of a second sex chromosome. When an ovum carrying the X-chromosome unites with a sperm carrying the X-chromosome, the resultant XX zygote develops into a female offspring. Conversely, when an ovum carrying the X-chromosome unites with a sperm carrying no chromosome, the resultant XO zygote develops into a male offspring. Therefore, sex determination is known as male heterogamety.
However, there are also several organisms in which the mechanism of sex determination is female heterogamety. That is, two different types of gametes pertaining to the sex chromosome are produced by the female. An example of such a sex-determination system is the ZW-ZZ type and it occurs in birds, reptiles, fish and some insects like butterflies and moths. In this type of sex determination, females have two different types of sex chromosomes designated as ZW, apart from the autosomes. Males, on the other hand, have identical sex chromosomes designated as ZZ, apart from the autosomes.
Another example of female heterogamety is the ZO-ZZ type of sex determination seen in some butterflies and moths. In this type of sex determination, females have only one sex chromosome designated as ZO. Here, the ‘O’ denotes the absence of a second sex chromosome. Males, on the other hand, have two identical sex chromosomes designated as ZZ.

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