Sex Determination

Heredity And Evolution of Class 10

The mechanism, by which the sex of an individual is determined as it begins life, is called sex determination.

In diploid organisms having separate sexes, a specific pair of chromosomes in each diploid cell, determines the sex of the individual. They are called sex chromosomes. All other chromosomes are termed autosomes as these have genes, which control the somatic (body) characters. The two members of each pair of homologous autosomes are similar in size and shape, but this may not be true with sex chromosomes. For instance, In human beings, there are 46 chromosomes. Of these, 44 (22 pairs) are autosomes and 2 (one pair) are sex chromosomes. The sex chromosomes in human beings and also in fruit-fly (Drosophila), are of two types – X chromosome and Y chromosome.

(i) A male individual contains one X chromosome and one Y chromosome i.e. XY (heteromorphic sex chromosomes having different size and shape). Therefore male produces two different kinds of gametes (sperms); half of the gametes having X chromosome and other half having Y chromosomes. Therefore, male is called heterogametic.

(ii) A female individual contains two similar X chromosomes i.e., XX (homomorphic sex chromosomes of same size and shape). Female, therefore, produces same type of all gametes (ova or eggs). So, female is called homogametic.

The sex of the child is determined at the time of fertilization when male and female gametes fuse to form zygote.

If a sperm (male gamete) carrying X chromosome fertilizes an egg or ovum (female gamete) carrying X chromosome, then the offspring will be a girl (female). This is because the offspring will have XX combination of sex chromosomes.

If a sperm (male gamete) carrying Y chromosome fertilize an egg or ovum (female gamete) which has X chromosome, then the offspring will be a boy (male). This is because the offspring will have XY combination of sex chromosomes.

              Sex Determination

So, we can conclude that the sex of the child (offspring) is determined by the type of sperm that fuses with ovum at the time of fertilization. Therefore, there is 50% chance of a male child being born and a 50% chance of a female child being born.

sex determination

Sex Determination

This mechanism of sex determination in human beings and also in fruit-fly (Drosophila) is called XX - XY mechanism. In grasshoppers and some other insects, the male has only one sex chromosome (XO) whereas the female has two homomorphic (of same size and shape) sex chromosomes (XX). This type of sex determination mechanism is called XX – XO mechanism.

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