We know that some of our traits are essentially 100% due to single genes (in particular, what alleles we have of these genes). For example, we have seen that inherited diseases such as cystic fibrosis, Huntington disease, phenylketonuria, and hemophilia can be understood on the basis of inherited alleles of single genes. Cancer is an example of a more complex genetic disease, being due in the most general terms to the somatic accumulation of mutations and chromosomal alterations and, in some cases, the inheritance of mutant alleles of some specific genes.

What about "complex" traits like body shape, brain function, mental health, athletic performance, etc? We have a general sense that these are "part genetic and part environmental". How much of each for what traits? What are the genes that are involved? How do these genes work to influence the complex phenotype? These are very difficult questions. Some of them will become approachable now that we have the human genome sequence and can begin to study details of our 25,000 or so genes. We can start to see how functional genomics, the study of the expression patterns of our genes (using, for example, the new whole genome microarrays), should allow us to start investigating some of the questions posed above for some complex traits.

1. What are the expected phenotype distributions for quantitative traits determined by several (or many) genes?

Consider a trait determined by alleles of three genes. For simplicity, assume that the three genes show independent assortment, that each gene shows partial dominance, and that the allele frequencies are all 50%. The inheritance pattern is shown in Figure 18.4 and the phenotypic statistical distribution is shown in Figure 18.5.


2. How can we graphically show the rough estimates of HOW MUCH is "genetic" for a wide variety of human diseases, abnormalities, and various characteristics?

If a trait is at least partially determined by a person's genes, the heritability of the trait will lead to the trait being more common in people who have a sibling with the trait than among people in the general population. Thus, we can get a rough measure of the "heritability" of a trait by seeing where it sits on a graph of "{Percent affected among siblings of affected people} vs {Percent affected in general population}". This is shown in Figure 18.16 for over 30 diseases and abnormalities.
For a disease or condition that has NO genetic component, we would expect the numerical values on the vertical and horizontal axes to be the same. Thus, all of the actual diseases/abnormalities indicated on the graph have some level of genetic basis.


3. For a given complex trait, how can researchers search for specific genes that might be involved?

Chapter 18, and the entire textbook, ends with an example of this. See Figure 18.18 on page 786, which shows results from a 2003 study on finding one gene that seems to be involved in the complex trait of "serious depression in response to stressful life experiences".