Welcome back to Introduction to Genetics and Evolution.

In the previous video we were looking at

this dichotomy between genetics and environment.

And what we did in that context was trying to figure out whether or

not there's a genetic component to particular trait differences you see, or

whether there's an environmental component to trait differences we see.

Now what we want to really so is to actually get this part.

We want to say how much do each contribute to particular traits of interest.

We want to quantify how much they contribute.

And again, when we're inferring whether genetics or environments contribute,

typically the answer is very often both are contributing.

In that regard, it's not really that satisfying to just say, yes,

there's a genetic component, or yes, there's an environmental component.

It's much more satisfying to actually infer their relative contributions.

To say for a particular trait, this is something that 90% of the variation you

see is genetic, or 90% of the variation you see is environmental.

Well, this comes to the concept that we'll refer to as heritability.

I'll define that in just a moment but first let me go through a couple of quick

mathematical concepts that will be relevant to this lecture.

Let's talk about mean and variance, please.

And I apologize for

those of you who already have a lot of statistics background,

I just need to go over this.

Now most traits you look at tend to be variable,

that not everybody has exactly the same trait value.

So you can use height as an example there.

Now, if a trait is continuously variable, all right?

Meaning that you have all these different steps, so something like height.

You can be 5'9" tall, 5'10" tall, 5'11" tall, 5'11.5" tall.

Then we can calculate a mean, and we can calculate a variance.

Now, many of you are familiar with variance in the context of just thinking

of it as the spread of values.

And I can depict this.

Let's say you're looking at height in the classroom.

So let's say this is the height in the classroom.

This is saying 5'0" to 6'2", this is 5'0" to 6'2".

What is the mean?