Sex chromosomes

• human body cells have 23 pairs of chromosomes in the nucleus.
• 22 pairs are known as autosomes, and control characteristics, but one pair carries genes that determine sex- whether offspring are male or female.
• males have two different sex chromosomes, XY. the Y chromosome is shorter than the X chromosome, so it has less alleles.
• Females have two chromosomes, XX.
• the gene on the Y chromosome determines maleness (testes).
• if only one copy of the gene is present, then ovaries will develop instead.
• sex determination occurs during gamete formation.
• A genetic cross, like a Punnett square, shows how the alleles inherited from the parents may combine in a zygote.
• during fertilisation, if the egg is fertilised by an X bearing sperm cell, the baby will be female. If the egg is fertilised by a Y bearing sperm cell, the baby will be male.