Sunday, July 26, 2020

Principles of Inheritance and Variation - Notes | Class 12 | Part 6: Sex Determination



- The chromosomes that are involved in sex determination are called sex chromosomes (allosomes). They include X & Y chromosomes.

-  Autosomes are chromosomes other than sex chromosomes. Number of autosomes is same in males and females.

-  Henking (1891) studied spermatogenesis in some insects and observed that 50 % of sperm received a nuclear structure after spermatogenesis, and other 50 % sperm did not receive it. Henking called this structure as the X body (now it is called as X-chromosome).

Mechanism of sex determination

a. XX-XO mechanism: Here, male is heterogametic, i.e. XO (Gametes with X and gametes without X) and female is homogametic, i.e. XX (all gametes are with X-chromosomes). E.g. Many insects such as grasshopper.

b. XX-XY mechanism: Male is heterogametic (X & Y) and female is homogametic (X only). E.g. Human & Drosophila.

c.  ZZ-ZW mechanism:  Male is homogametic (ZZ) and female is heterogametic (Z & W). E.g. Birds.                                  

XX-XO & XX-XY mechanisms show male heterogamety. ZZ-ZW mechanism shows female heterogamety.

Sex Determination in Humans (XX-XY type)

·   Human has 23 pairs of chromosomes (22 pairs of autosomes and 1 pair of sex chromosomes).

·   A pair of X-chromosomes (XX) is present in the female, whereas X and Y chromosomes are present in male.

·    During spermatogenesis, males produce 2 types of gametes: 50 % with X-chromosome and 50 % with Y-chromosome.

·   Females produce only ovum with an X-chromosome.

·   There is an equal probability of fertilization of the ovum with the sperm carrying either X or Y chromosome.

The sperm determines whether the offspring male or female.

Sex determination in honeybee

-   It is based on the number of sets of chromosomes an individual receives.

-   Fertilised egg develops as a female (queen or worker).

-   An unfertilised egg develops as a male (drone). It is called parthenogenesis.

-   Therefore, the females are diploid (32 chromosomes) and males are haploid (16 chromosomes). This is called as haplodiploid sex determination system.

-   In this system, the males produce sperms by mitosis. They do not have father and thus cannot have sons, but have a grandfather and can have grandsons.

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