X's and Y's

[BaalChatzaf]

On this day (July 7) in 1861 was born Nettie Stevens who became a cytlogist and discovered the X and Y chromosomes of the human genome.   She discovered that mail sperm has either X or Y  chromosomes and the female egg has only X chromosomes.   Thus it is the male that determines the gender of the child.  In her time it was widely believed that the mother determined the sex of the child or it was a random happenstance.   Nettie set the record right.

Nettie Stevens (1861 - 1912)