Welcome back. As you’ve probably already ascertained, most genetic disorders are multifactorial
in nature, having both an environmental and perhaps multiple genetic components or multiple
environmental components. In this lecture, we’ll talk about ways that we start to sort out genes
from variants and environment. There are definitely genetic components where we see complex
interactions among a number of different genetic variations and even polygenic types of inheritance.
Clearly, there’s also an environmental component or many environmental components.
Each of us has a different experience in both realms including quite chance events that could happen
even during development of a fetus in utero. Moving into looking at these different traits,
we have two distinct types of traits that we can measure. First of all, we have discrete or qualitative
traits in which you either express in edition or you do not. Phenylketonuria would be a great example.
Either someone has it or they don’t, so qualitative, yes or no. Then we could have a quantitative trait.
Usually, quantitative traits are something we could measure. For example in this figure, we have systolic
blood pressure. It occurs over a continuum. We have people on the lower end and then a bell-shaped curve.
We’re all quite familiar with this Gaussian type of distribution. We can compare people with certain
mutations and those without the mutations and see some variants in the average blood pressure.
Quantitative versus qualitative, we have either a spectrum or we have a yes or a no.
There are different ways that we try to tease apart the genetics from the environment in order to
establish the relative contributions of each. We know, sure, we know that relatives share more
alleles than non-relatives. Briefly, we’ll cover that siblings generally share about 50% of their alleles.
That’s average because it’s fairly random which parts that each got from a mother or a father.
It could be quite a lot. So, you have one sibling that looks particularly like the mother or both siblings
look particularly like the mother or the father. So we get a random mix. On average, 50% is shared
which is definitely more than two unrelated individuals. Because of these familial similarities,
we can use familial aggregation studies as well as correlation studies. Now, don’t worry too much about what
those are yet. I will walk you through each of those. We can use those to determine the genetic contribution
versus environmental contribution to some of these sort of mysteriously genetically predisposed conditions.