1. The union of 23 chromosomes of the male with that of the female is the cause for conception. Of these, 22 pairs of chromosomes are the same in men and women. These are known as autosomes. 
2. The 23rd chromosome decides the sex of the child and is known as sex chromosome. 
3. In females this sex chromosome is similar, but in men it is dissimilar. 
4. It is important to understand that there are two types of sex chromosomes, i.e., X and Y, of which X is a long and Y is a small chromosome. 
5. The female has only X chromosome while the male has both. When the X chromosome of the female combines with the X chromosome of the male, a female baby is conceived. 
6. When the X chromosome of the female combines with the Y chromosome of the male, it results in a male baby. 
7. Chromosomes are made of genetic matter. Each gene determines a different characteristic and we inherit them from our forefathers.
8. Some people have more X and Y chromosomes than normal. As we know, the normal combination is XY and XX. But sometimes abnormalities set in to create chromosomal combinations such as XXY or XYY or XO. 
9. In XXY abnormality, a person will look like a man but will be sterile (Klinefelter’s Syndrome). 
10. In XO abnormality, person will look like a female but will be sterile (Turner’s Syndrome). 
11. In XYY abnormality, the person develops tendencies of delinquency.