Question
Tue January 15, 2013 By: Ankit Rao
 

IN SEX DETERMINATION OF AN UNBORN BABY IF CHROMOSOMES ARE XX THEN IT IS FEMALE AND IF XY THEN A MALE. CAN WE CONCLUDE THAT Y CHROMOSOME IS DOMINANT?

Expert Reply
Fri February 15, 2013
You can't really correctly call one chromosome dominant over another. Dominant and recessive really apply only to specific genes, not entire chromosomes.
 
Chromosomes X and Y do not truly make up a homologous pair. They act similarly in their roles, but they are not homologous. The X chromosome in humans is much longer than the Y chromosome and also contains many more genes. These genes are said to be sex linked, due to the fact they are present in one of the sex chromosomes. During fertilisation, when the opposing homologous chromosomes come together, the smaller Y chromosome offers no dominance against the extra region of the X chromosome as shown below.
 
 
 
 

The arrows indicate sex linked genes in the X chromosome. In this homologous pairing, all those genes are dominant, because there are no opposing genes in the Y chromosome to offer dominance.

Y has extremly low number of genes and most of them are related to being male. All the other genes needed get expressed due to the X chromosome. So X chromosome is essential for the survival of Y chromosome because of the number of vital genes it carries.

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