Sunday, July 26, 2020

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

5. PRINCIPLES OF INHERITANCE AND VARIATION

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 
  1. 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.
  2. XX-XY mechanism: Male is heterogametic (X & Y) and female is homogametic (X only). E.g. Human & Drosophila.
  3. 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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