Sex Determination

• Sex determination is the method by which the distinction between male and female is established in a species.
• Sex chromosomes determine the sex of the individual in dioecious or unisexual organisms.
• The chromosomes other than the sex chromosomes of an individual are called autosomes.
• Sex chromosomes may be similar (homomorphic) in one sex and dissimilar (heteromorphic) in the other.
• Individuals having homomorphic sex chromosomes produce only one type of gametes (homogametic), whereas heteromorphic individuals produce two types of gametes (heterogametic).

• Henking (1891) could trace a specific nuclear structure all through spermatogenesis in a few insects, and he also observed that 50% of the sperm received this structure after spermatogenesis.
• In contrast, the other 50% of sperm did not receive it. Henking named this structure the X body, but he could not explain its significance.
• Further investigations by other scientists led to the conclusion that the ‘X body’ of Henking was a chromosome, which is why it was given the name X-chromosome.

Y-chromosome: The human Y chromosome is only 60 Mb in size with 60 functional genes whereas X chromosomes are 165 Mb in size with about 1,000 genes.

1. Heterogametic Sex Determination: In heterogametic sex determination, one of the sexes produces similar gametes, and the other sex produces dissimilar gametes. The sex of the offspring is determined at the time of fertilisation.
   
   
2. Heterogametic Males: In this method of sex determination, the males are heterogametic producing dissimilar gametes, while females are homogametic producing similar gametes. It is of two kinds XX-XO type (e.g. Bugs, cockroaches, and grasshoppers) and XX-XY type (e.g. Human beings and Drosophila).
   
   
3. Heterogametic Females: In this method of sex determination, the females are heterogametic producing dissimilar gametes, while males are homogametic producing similar gametes. To avoid confusion with the XX-XO and XX-XY types of sex determination, the alphabets ‘Z’ and ‘W’ are used here instead of X and Y, respectively. Heterogametic females are of two types, ZO-ZZ type (e.g. Moths, butterflies, and domestic chickens) and ZW-ZZ type (e.g. Gypsy moths, fishes, reptiles, and birds).

1. Sex Determination in human beings:

• The chromosomal mechanism of sex determination in human beings is XX-XY type. In human beings, the nucleus of each somatic cell contains 46 chromosomes or 23 pairs of chromosomes. Out of these, 22 pairs are autosomes and one pair of sex chromosomes.  

Thus genotype of:  

Female = 44 Autosomes + XX  

Male = 44 Autosomes + XY 

• If sperm containing X chromosome fertilizes egg (ovum), then diploid zygote is formed, that grows into a female child. If sperm containing Y chromosome fertilizes the egg, then diploid zygote is formed that grows into a male child. 

• This indicates that the sex of a child depends on the type of sperm fertilizing the egg and hence the father is responsible for determination of sex of child and not the mother. 

Sex determination in human beings 

2. Sex Determination in birds:

• In birds, the chromosomal mechanism of sex determination is ZW-ZZ type. In this type females are heterogametic and produce two types of eggs; 50% eggs carry Z- chromosome, while 50% eggs carry W- chromosome.  

• Males are homogametic and produce one type of sperms. Each sperm carries a Z chromosome. Thus sex of individual depends on the kind of egg (ova) fertilized by the sperm. 

 Sex determination in birds 

3. Sex Determination in honey bees  

• In honey bees, chromosomal mechanism of sex determination is haplo-diploid type. In this type, sex of individual is determined by the number of set of chromosomes received. Females are diploid (2n=32) and males are haploid (n=16).  

• The female produces haploid eggs (n=16) by meiosis and male produces haploid sperms (n=16) by mitosis.  

• If the egg is fertilized by sperm, the zygote develops into a diploid female (2n=32) (queen and worker) and unfertilized egg develops into haploid male (n=16) (Drone) by way of parthenogenesis. 

 Sex determination in honey bee