POPULATIONS AND ENVIRONMENT

Genetic variation and Geographic Isolation – Multiple Alleles

Genetic variation and Geographic Isolation – Multiple Alleles

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
  • IA: this represents A and is co-dominant
  • IB: this represents B and is also co-dominant

The possible genotypes are:

  • A: IA IA or IA i
  • B: IB IB or IB i
  • AB: IA IB
  • 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.

X X
X XX XX
Y XY XY

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