With what we've talked about for Mendel's work, there were always only two options (or two alleles) for each gene. But some genes have more than two alleles. A classic example is human blood type. In the major ABO blood type, there are three alleles - A, B, and O. Each person will still only inherit two alleles, but with more possibilities, there are more combinations and potential phenotypes.

When we learned about pea plants, tall plants were dominant to the dwarf (short) ones. In addition, in a lot of lower-level science courses, when traits like eye color are taught, they're incorrectly taught as simply one gene where one color is dominant to another. However in humans, and many other species, traits like height, skin color, and eye color and controlled by many genes working together - known as polygenic inheritance. Each contributing gene adds an effect, resulting in a wide range of possible phenotypes. These traits display something known as continuous variation rather than clear and distinct categories; the more genes involved, the more variation!

Continuous variations are also known as quantitative variation. On the other hand, traits where there are well-defined distinct categories are known as discontinuous or qualitative traits.

The opposite of polygenic inheritance is pleiotropy. In this situation, a single gene can influence multple distinct phenotypes.

Sometimes, it's not even the offspring's genotype that determines its phenotype, but the genotype of its mom! In maternal effect inheritance, mom's genotype determines the offspring's phenotype. This happens because mRNA or proteins made by mom are present in the egg cell, and can be responsible for certain phenotypes.

A commonly used example of this is the direction of shell coiling in certain snails, which is determined by the first few cell divisions, which is controlled by mom's genotype.

Some alleles can be deadly - often caused by mutations in essential genes involved in growth, development, or general survival. Most commonly recessive, these are traits that, when expressed, result in the death of the offspring and can alter offspring ratios. As an example, in some mice, the yellow fur color allele is dominant, but inheriting two of them results in the death of the offspring because it is recessively lethal.

Sometimes individuals can have a specific genotype, but not show the associated phenotype. This is known as incomplete penetrance. This can be caused by things like gene interactions, the environment, or epigenetics.

There can also be varying levels of gene expressivity - the trait may be expressed more strongly in one individual than another, despite them having the same genotype.