1. IB2.26.3 The inheritance of sex © Oxford University Press 2011 The inheritance of sex

2. IB2.26.3 The inheritance of sex © Oxford University Press 2011 There are 23 pairs of chromosomes in the nucleus of all human body cells. One of these pairs of chromosomes determines sex. If, in that pair, you have two large X chromosomes, you are female. If you have one large X chromosome and a smaller Y chromosome, you are male.

3. IB2.26.3 The inheritance of sex © Oxford University Press 2011 Not all living organisms have the same mechanism as humans for determining sex. Whilst most male mammals are XY, male birds, butterflies and snakes are XX (usually called ZZ) and females are XY (or ZW).

4. IB2.26.3 The inheritance of sex © Oxford University Press 2011 In many species of reptile, the eggs develop into male or female depending on the incubation temperature.

5. IB2.26.3 The inheritance of sex © Oxford University Press 2011 Some organisms, such as many plants, tapeworms, earthworms and snails are hermaphrodite – they have both male and female organs.

6. IB2.26.3 The inheritance of sex © Oxford University Press 2011 Some organisms, such as the hornbeam maple tree, oysters and some fish, change sex throughout their life cycle.

7. IB2.26.3 The inheritance of sex © Oxford University Press 2011 Single-celled organisms, including bacteria and amoeba, do not have a sex.