Nettie Stevens and sex chromosomes

It was clear from Morgan's original analysis that the white-eyed allele was somehow linked to sex. So we need to address the question, how is the sex of an organism determined?  

The question is complex, as different mechanisms are used by different types of organisms. 

In most species, the ratio of males to females is very close to 1:1 at birth.   This is the same ratio seen when a heterozygote is backcrossed to the homozygous recessive parent (try it if you don't believe it using javaGenetics™.

This lead to the idea that sex was determine by a dominant genetic factor.

Nettie Stevens (1861-1912) discovered that males and females of the yellow mealworm Tenebrio molitor, an agriculture pest still studied today, differed in their chromosomes, their karyotype.

 

An adult T. molitor

 

Compared to females, males have one unequal chromosome pair. Each sperm contains one or the other version of this unequal pair. 

The pair of sex determining chromosomes in mammals is commonly known as the "X" and "Y", in birds they are "Z" and "W".

Which version of the"dimorphic" sex chromosomes delivered to the egg determines the sex of the resulting organism.

The non-sex chromosomes are known as autosomes. In Drosophila, females are XX and males are XY. 

  • Why is sex considered a dominant rather than a recessive trait?
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