X chromosome: together with the Y chromosome, carries the genetic information that determines the sex of an organism. Males have an X and a Y chromosome, females two X chromosomes.

xenophobia: a fear of strangers or strange places.

XXV syndrome: also known as Klinefelter's syndrome, affects males who are born with an extra X chromosome. Males with this condition typically have underdeveloped male genitalia and pronounced feminine characteristics, such as the development of breasts.

XYY syndrome: a chromosomal abnormality where there are three sex chromosomes. People who possess this abnormality are males who are of above average height with typically low levels of fertility. There used to be considerable interest in the possible links between this condition and aggressive behaviour, although this has never been proven.