Nettie Maria Stevens was the first scientist to describe the chromosomal basis of sex. In mammals, females have identical sex chromosomes called X, and males have an X chromosome and a smaller Y chromosome.

The sex of individuals is also determined, among other things, by chromosomal characteristics. 
 For the majority of the population, it rules that a woman has two X chromosomes on the 23rd pair, and a man has an XY pair. But scientists insist that there may be individuals with two Xs who develop male characteristics, and others with one X and one Y who never do. In addition, there are also people who are XXY. 
The gene responsible for hemophilia (a disease that causes blood clotting problems) is located on the X chromosome. It is a recessive trait (h). 
The following genealogical tree represents the XX-XY sex chromosomes and indicates the alleles that the following individuals have for this trait: 
(a) Generation I individual 1. 
b) Generation II, individuals 2 and 3. 
c) Generation III, individuals 2 and 4.