What is heterogametic ? Explain by giving proper examples ?
 

$=$ The chromosomal theory of sex determination was worked out by $E$.$B$. $Wilson$ and Stevens.

They named $XY$ chromosomes as allosome or sex chromosome and other chromosomes which have no relation with the sex and contain genes which determine the character as autosomes [$AA$]. Thus on the basis of the type of allosome present in the gamete, the parents can be of two types :

$(i)$ Homogametic i.e. Producing similar gametes.

$(ii)$ Heterogametic i.e. Producing different gametes.

These are various types of sex determination mechanism observed in various organisms as follow :

Mechanism of sex Determination

Male Heterogamety : The mechanism in which male produces two different types of gametes : $(i)$ Either with or without $X$-chromosome. $(ii)$Some gametes with $X$-chromosome and some without $Y$-chromosome.

$(i)$ $XX-\;XY$ Type : This type is present in certain insects like Drosophila melanogaster and mammals including human.

In males an $X$-chromosome is present but its other part is very small called as $ Y$-chromosome whereas females have a pair of only $X$-chromosome i.e. $\mathrm{XY}$.

Both males and females bear same number of chromosomes.

The males have autosomes plus $XY$-chromosomes and females have autosomes plus $XX$chromosomes.

The males produce two types of gametes containing $\mathrm{X}$ or $\mathrm{Y}$ sex chromosome and females produce only one type of gametes with an $X$-chromosome (homogametic).

Determination of sex in Drosophila : Thus in this type of sex determination the presence of Ychromosomes determines the maleness.

$(ii)$ $XX$-$XO$ Type : This type is present in a large number of insects, e.g., grasshopper etc.