The example above looked at two alleles for one gene. However, a gene can have more than two alleles. Blood, for example, has three possible alleles: A, B and O. In this case, they are written slightly differently again:

• i: this represents O which is recessive
• I^A: this represents A and is co-dominant
• I^B: this represents B and is also co-dominant

The possible genotypes are:

• A: I^A I^A or I^A i
• B: I^B I^B or I^B i
• AB: I^A I^B
• O: i i

Sex-linked genes

In most animals gender is determined by sex chromosomes. Humans use the XY system.

• females are XX
• males are XY

As you can see, in females the chromosomes are the same while in the males they’re different.

The chance of a couple having a boy or a girl is always