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Question : What is heterogametic ? Explain by giving proper examples ?

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Solution :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 :
=
= 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. 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 X or 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 Y-chromosomes determines the maleness.
= (ii) XX-XO Type : This type is present in a large number of insects, e.g., grasshopper etc.
=Male have only one X-chromosome and produce two types of sperms. i.e. 50% with Xchromosome and 50% without X-chromosome and female have a pair of X-chromosomes and produce homo-gametic eggs.

Determination of sex in Grasshopper : Eggs fertilised by sperms having an X-chromosome become females and those fertilised by sperm that do not have X-chromosome become males.
= Female heterogametry : In this CASE, the total number of chromosomes are same in both males and females. But two different types of gametes in terms of sex chromosomes are produced by females.
=(i) ZZ - ZW type : This method of sex determination is seen in birds fowls and fishes.
= Females have one Z and one W-chromosome along with autosomes whereas males have a pair of Z-chromosomes (i.e. homogametic).
= Thus the sex of an organism is determined by the type of ovum that is fertilised to produce an offsprings.
=
=(ii) ZZ-Zo type : In this method of sex determination the female is heterogametic [ZO] and male is homogametic [ZZ].
= It occursin certainbutterfiesand moths .


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