All language subtitles for 27031x_MH_Inferring_Patterns_02_Autosomal_Dom_v1-en

MICHAEL HEMANN: So let's look at a number of pedigrees.

So here is a pedigree of a family that

has brachydactyly polydactyly.

In this case, you see actually the hand of one

of these individuals.

And they have five fingers and a thumb

on that hand, which is unusual.

Not a devastating condition, but certainly an interesting one

developmentally.

And so we see a number of affected individuals

and unaffected individuals.

So just looking at this pedigree,

what do we think the mode of inheritance is?

We've got some mixed answers, the majority of which

suggest that this is dominant.

And in fact, it is a dominant condition.

And we'll talk about what that means

to be dominant in a minute.

But among other things, you actually

see the appearance of this phenotype in every generation.

So all of the generations here, we see affected individuals.

And this is a really common characteristic

of a dominant condition.

We don't know, based on this pedigree,

whether it is autosomal dominant or X-linked dominant.

Autosomal means any chromosome that's not the X chromosome.

So for most conditions that are dominant conditions,

they are autosomal, just because the vast majority of our genes

are not located on the X chromosome.

But there's nothing per se here that would

exclude the possibility that this is an X-linked condition.

But clearly, it is a dominant condition,

again, because we're seeing it really in every generation.

We're also seeing that, on average,

if you have this condition, about half of your children

have this condition, which is also

characteristic of a dominant condition.

So if you have one allele that's conferring this phenotype,

then you'd expect there's a 50% chance that your child is

going to inherit this allele.

And if they inherit this allele, then they're

going to be affected by, in general,

this dominant condition.

So here's another interesting situation.

This is another inheritance pattern

that looks to be a dominant inheritance pattern.

And this is actually myostatin deficiency,

which is very rare in people.

So this pedigree actually was from a paper that included

the picture on the top.

And those arrows are pointing out

areas of substantial musculature in basically a neonate,

a newborn baby and a young child.

So really hypergrowth of musculature.

The inheritance here is actually curious.

I won't go into it significantly,

but this condition actually in this family

appears in homozygotes and heterozygotes.

So it appears to be dominant, and the severity is probably--

increased severity means the magnitude of the phenotype

is increased in homozygotes, like this person

at the bottom here.

All right.

So in terms of autosomal dominant or dominant

conditions-- we'll just say autosomal dominant--

affected individuals generally have affected parents.

And so there are a couple of conditions and situations

in which this doesn't apply.

One is that if there is what we say incomplete penetrance,

meaning you don't always see the phenotype.

So in general, when we talk about pedigrees,

we'll say is there complete penetrance or not,

meaning do you always see the phenotype

if the person has that allele.

But sometimes the phenotypes are difficult to see,

and so you don't always characterize

in a family history, whether somebody is affected or not.

And also, this is unless there is a new mutation.

And so if there's not a new mutation,

generally you see every generation has this phenotype.

There are conditions that are characterized by new mutations.

For example, the condition achondroplasia,

achondroplasia is the most common etiology of

or manifestation of short stature.

So for little people, little people in general

have alterations in long bone growth

that we call achondroplasia, caused by a dominant mutation.

This dominant mutation generally is a new mutation.

So there's really a high frequency

of new mutations appearing in populations.

So more cases of achondroplasia occur by new mutations

than they do via inheritance from parents,

although that inheritance clearly happens.

And if a parent is affected, generally one half

of the children are affected.