Long before the advent of genetic testing or even complete understanding of DNA and RNA, astute observers noticed that genetic traits, including many disorders, were passed from one generation to another in somewhat predictable patterns. To understand heredity, you have to know a little about human chromosomes and how they work.
Chromosomes come in pairs in the cell's nucleus.
Humans have 46 chromosomes in each cell nucleus, which are actually 23 pairs of chromosomes. One chromosome comes from a person's mother, the other from his father. The 23rd pair is an exception and determines gender. The 23rd chromosomal pair differs according to whether you're a male or a female. Males have an X and a Y chromosome, while females have two Xs for this 23rd pair of chromosomes.