What type of inheritance includes several genes that effect a single trait?

Some phenotypes are due to the additive effects of multiple genes rather than on simply a single gene pair. This type of inheritance is known as polygenic inheritance. 

Height and skin color are inherited in this fashion and are hard to predict by simply looking at the two parents' phenotypes. That is because each parent contributes several genes to produce the characteristic of height or skin color. And, the combination of which alleles and subsequently their additive effects, contribute to the phenotype of the individual. Based on the combination of alleles that are inherited, there will be a wide range of phenotypes that follow a bell-shaped distribution with most individuals clustered around the intermediate phenotype. On either end, will be the extreme phenotypes for that trait.

For example, if there are three gene pairs for skin color which all combine to produce a phenotype, and if each pair has one light allele and one darker allele the following would occur. If an uppercase letter represents the darker gene and the lower case letter represents the lighter gene and there is incomplete dominance, consider the following gene pairs to determine skin complexion. AaBbCc  are the six possible alleles that can be passed down to offspring.

If someone inherits  the combination of AABBCC, this person would be an individual with a very dark complexion  and if someone inherits aabbcc, this person would have a very light complexion. The number of these individuals in a population are not very high.  However, usually, the majority of people will have a complexion which is intermediate between the two. For instance, a person may inherit Aa Bb cc where they only have two darker genes A and B and the rest are for the lighter skin tones. Remember it is the combination of these alleles and their additive effects which produces the polygenic trait. The more dominant genes a person inherits will have a greater effect on the phenotype that is produced.

Also, it is important to note that environmental factors such as how much sunlight a person is exposed to will affect the trait of skin color and for the trait of height-- exercise, diet and being a non-smoker all have an effect on the person's phenotype.