Interactions between genes

• complete dominance – recessive allele is not expressed in the presence of dominating one
• incomplete dominance – both alleles are expressed what leads to developing a blend, an intermediate trait
• codominance – both alleles are fully expressed and as a result a mosaic of traits develop

Traits can be determined by one gene or by many genes. When many genes determine a trait it is possible that for example presence of all dominant alleles is necessary to develop the trait. Such genes are called cumulative. It is also possible that arrangement of alleles (for example the number of dominant alleles) influence the trait. Effects of expression of genes sum and form the trait. Such alleles are called complementary.

Some genes influence more than one trait – this is called pleiotropy.

If there are more than two different version of gene in the population they are called multiple alleles.