A pair of chromosomes carry the same genes in the same place, on each chromosome within the pair. However, there are different versions of a gene called alleles. These alleles may be the same (homozygous) on each pair of chromosomes, or different (heterozygous), for example, to give blue eyes or brown eyes.
Determination of Gender
When sex cells form, the pair of sex chromosomes (XX and XY) are seperated. Females carry XX and males carry XY. This means: all normal egg cells produced by a human ovary have an X chromosome and half the sperm carry an X chromosome and the other half carry the Y chromosome.
So a human baby's gender is determined by the sperm that fertilises the egg cell. The baby will be a girl if it carries an X chromosome. It will be a boy if the…