A great many genes are inherited according to the laws of dominance, independent assortment, and segregation. Situations that follow Mendel’s laws are known as Mendelian Inheritance. However, not all genes and traits are transmitted from parents to offspring in Mendelian patterns.

Incomplete Dominance and Codominance
Dominance may be incomplete or nonexistent between some alleles. In some cases both alleles in a heterozygous individual express themselves.

Linked Genes
Genes on the same chromosome don’t assort independently but instead tend to be inherited together. The closer that the genes are on the chromosome, the more likely it is that they will be inherited together.

Multiple Alleles
An allele is all the variations of the same gene (alleles) that control a particular trait. A few genes have more than two possible alleles.

Polygenic Inheritance
Some traits are polygenic, determined not by one gene but by many. A good example is skin color which is determined by the combined action of at least five different genes.

Pleiotrophy
Some genes contribute to multiple traits. As a result, a single gene may have many different phenotypic effects.