Gene Function and Interaction

This section covers some basic material on how traits (phenotypes) can be determined by the interaction of multiple genes as well as how to analyze these traits. The most important thing to keep in mind is that gene interaction does not influence genotype frequencies. Genotype frequencies are determined by the processes of segregation and fertilization which are not affected by gene interaction. Instead, gene interaction influences what phenotype develops from the genotype and so you want to understand the ways in which genes can interact and how these interactions affect the phenotype. The topics that you should be familiar with for this section are linked below.

Before we look at gene interaction, though, we should cover a few general types of alleles that we will start to refer to. These are:

• Loss-of-function alleles


• Gain-of-function alleles


• Dominant negative alleles

Make sure that you are familiar with these terms before starting on the material below.

• Functional pathways


• Complementation


• The Complementation Test


• Recessive epistasis


• Dominant epistasis


• Modifiers

Gene interaction and genetic disorders. It is important to always keep in mind the main point of this section: all biological/cellular functions involve the coordinated actions of the products of many different genes. This has important implications for understanding genetic disorders. Consider a patient with hemophilia. The medical condition arises because of a problem with the blood clotting process. This process is a complex pathway involving the products of many genes. While a patient might have a genetic mutation that affects the function of one of these genes, what is important medically is that the process does not function. This is what results in the phenotype we observe and attempt to treat. This is not to say that what gene is affected is not important at all, particularly since it can help us understand the entire process and how to treat the disorder. It is just that the biological process is the immediate medical issue.

One implication of this is that there are typically many different genes and/or alleles associated with a single genetic disorder: this is because a mutation (i.e. mutant allele) in any one of a number of different genes can affect the same process such as the clotting pathway. Additionally, some of these alleles might be dominant alleles while others could be recessive. Therefore, you will often read about both recessive and dominant forms of the same disorder and, of course, that there are many genes that are implicated in the same genetic disorder.

This is also one reason why we often observe a lot of variation amongst individuals with the same disorder: different patients may suffer from a problem in the same biological function but they are affected in slightly different ways because they have different mutant alleles. This is why we distinguish between Hemophilia A and Hemophilia B. They are actually due to mutations in different genes in the clotting pathway. The complexity of gene interactions presents a difficult challenge to understanding and treating genetic disorders.