1. Causes of Variation
Mutation, Incomplete Dominance, Dodominance, Multiple Alleles, Linked Genes, Sex-linked Genes.

2. Incomplete Dominance.
One allele is not completely dominant over the other and the heterozygote is a mixture of two alleles.
Eg: When a red snapdragon flower is crossed with a white flower the offspring is pink in the F1 generation.
See page 177 POL

3. Codominance.
This is similar to Incomplete Dominance except that both traits show up in the heterzygote.
Eg: A black-spotted cat crossed with an orange -spotted cat would give black-and-orange spotted offspring.

4. Lethal Genes
There are certain genes that are essentail for life. Any allele that doesn’t produce the required gene will be fatal.

5. Multiple Alleles.
Many genes have more than two alleles that can fit at a locus on a chromosome.
Eg: ABO blood group in humans. There are four phenotypes, A, B, AB, O. Controlled by three alleles, A, B and O. Symbols IA, IB and i. A and B are equally dominant and O is recessive.

7. Sex-Linked Genes.
Genes located on one sex chromosome but not the other are called sex-linked genes. They are almost always carried on the X chromosome. The X chromosome is much larger than the Y, so there are genes on the X that have no matching part on the Y.
Eg: Red-green colour blindness, haemophilia, muscular dystrophy.