ABO blood groups: multiple alleles
The ABO blood group is controlled by one gene with three alleles: IA, IB and i. IA and IB are codominant (both expressed together), while i is recessive. This gives four phenotypes: group A (IAIA or IAi), group B (IBIB or IBi), group AB (IAIB) and group O (ii). Although a person has only two alleles, three exist in the population, so this is an example of multiple alleles.
Sex determination
Humans have 23 pairs of chromosomes, including one pair of sex chromosomes. Females are XX and males are XY. Eggs always carry an X, while sperm carry either X or Y, so the sperm determines the sex of the child. The chance of a boy or a girl is 1:1 at each fertilisation.
Example
A cross of parents XX × XY, using a Punnett square, gives XX, XX, XY, XY — a ratio of 1 female : 1 male, or a 50% chance of each sex.
Sex-linked traits
Some genes lie on the X chromosome and are said to be sex-linked. Red-green colour blindness and haemophilia are caused by recessive alleles on the X chromosome. Because males have only one X, a single recessive allele is enough for them to show the condition, so these disorders are far more common in males. Females need two recessive alleles to be affected but can be unaffected carriers with one.
Key idea
ABO blood group: multiple alleles with codominance. Sex: XX female, XY male, decided by the sperm. Sex-linked recessive disorders (colour blindness, haemophilia) affect males more often.
Remember
A carrier female (XHXh) is not affected but can pass the recessive allele to her children.