Inheritance: Sex Determination in Humans

All humans have 23 pairs of chromosomes in most of their cells- one half inherited from the mother, one half from the father.
These include two sex chromosomes, which determine the person’s biological sex.
Females have the sex chromosomes XX while males have the sex chromosomes XY.
When male sperm cells are formed, each cell either carries an X or a Y sex chromosome.
All eggs (female cells) contain one X chromosome.
If a sperm cell carrying an X chromosome fertilises the egg, the child will be female (XX).
If a sperm cell carrying a Y chromosome fertilises the egg, the child will be male (XY).
This means that it is the male sperm that determines the sex of the child.
There is a roughly equal chance of a child being male or female, as the sperm carrying either an X or Y chromosome has an equal chance of fertilising the egg.
This process is known as sexual reproduction, it is controlled by genes on the sex chromosomes.
Sex-linked characteristics, like red-green colour blindness and haemophilia, are carried on the sex chromosomes and so are more often expressed in males. This is because males have a single copy of these genes, while females would need the defective gene to be inherited from both parents.
In addition to the simple XY system, variations exist, such as Turner Syndrome (XO) or Klinefelter Syndrome (XXY), which can affect physical development and fertility.