When we assess the risk of a qualitative trait, so these are yes or no traits, either it’s there or it’s not,
we can use familial aggregation studies. What on earth is a familial aggregation study? It’s probably
a little bit more familiar than it even sounds. But we presume that if an allele increases the chance
of developing a disease, we’re going to expect that an affected individual to have a greater than
expected number of affected relatives. So, does that make sense? The more that the condition exists
in relatives, if it exists more in relatives than it does in the general population then we would have a number
greater than one. We call this measure of relative risk ratio λr. When we look at λr
and it equals one, we have little or no genetic impact. That means the numerator and denominator
are similar and probably, mostly just a result of no genetics. If it’s greater than one, we mean
there’s a possible genetic predisposition. It’s more likely in relatives than it is in the general population,
lambda greater than one. Here is a chart of a number of different lambdas. By no means, you need to
remember what the lambdas are, the relative risk ratios or even which ones have particularly
high risk ratios. However, I think it’s pretty interesting to look at these lambda values and so do people
in genetic counseling for sure. All of these are conditions that are said to have a genetic component.
We will dig into a couple of these in a little bit more detail later in a future lecture. Case control studies
are another way that we can assess the genetic contribution. We compare these two unrelated individuals.
So, married couples or couples in general are a great way to do this. Generally, they’re not going to share
much genetic information. However, they have shared lots of environmental conditions. We can use
these controls in order to make comparisons of family versus non-family. So using this, we can say
for example, in the instance of multiple sclerosis, we’re recognizing that perhaps there’s a hereditary
component to that. That first degree relatives that have multiple sclerosis are likely to have relatives.
So, 3.5% of people that have multiple sclerosis are likely to have relatives that also have multiple sclerosis,
first degree relatives, so siblings. Only 0.2% in the matched controls, so the married couple's kind of pool.
These are case control studies in the married couples versus familial studies in families.
Those two means are used to assess the contribution of genetics when we’re considering yes or no variables.
In the case of multiple sclerosis using those two statistics of 3.5% and 0.2%, we can say that the incidents
of multiple sclerosis would be 18 times greater among siblings or first degree relatives than it would be
among unrelated persons. So 18% more likely or 18% more risk of having it if you have a sibling that has it.