All language subtitles for 37031x_MH_Comp_Inheritance_Patterns_split_03_Criss_-en

MICHAEL HEMANN: Let's think about another phenotype

and another cross.

And this is a cross between flies

that have different eye colors.

So in flies, the wild type color is red.

And this is actually one of the first phenotypes that was

characterized in fly studies.

So this was done the turn of the 19th or the 20th century

by Muller and other geneticists and one

of the initial genes they characterized

and the first gene that was really ever mapped,

that they can actually localize it

and they could place it on a particular chromosome.

So flies that are mutant for this gene have white eyes.

And the gene is actually called the white gene.

So, typically, we call genes by the names

of the mutant phenotype.

And so we're calling this a white gene

because the mutants are white, not because the wild type

color is white.

So let's do a cross here.

So we have a cross between females that have white eyes,

and we're going to cross with males that have red eyes.

And we'll look at the F1 generation.

Now, of course, these are both true breeding,

so they're homozygous at all loci,

and we get an F1 generation.

And in the F1 generation, we have females that have red eyes

and we have males that have white eyes.

Well, this is weird.

So what's going on here?

Why do we see this pattern of inheritance?

We're doing essentially complementation studies

and we're getting an answer that's totally weird.

Absolutely.

So I think some of you have seen this before.

This is a sex-linked trait.

It is an X chromosome linked trait.

And that's essentially how they map the location of this gene.

They could localize the gene to a sex chromosome

to the X chromosome.

So let's draw out how this works.

So you have a female.

And a female has two Xs.

The males have one X or have XY.

And so we'll indicate this as an X chromosome

that has a white allele.

And another X chromosome that has a white allele.

So true breeding homozygous will cross

with a male that has the wild type allele and a Y chromosome.

When we cross, we get resulting females.

And these females get one X chromosome from the mother.

So that's a white allele.

And then have to get the one X chromosome from the father.

And that's a wild type allele.

And because red is dominant to white,

we have they red eye female.

And the resulting male obtains the one X chromosome

from the mother.

So that's a white allele, and then

the Y chromosome from the father.

So the males inherit only the mutant copy, only

the white copy.

And so therefore, have white eyes.

So this is characteristic of X-linked genes.

We call this criss-cross inheritance.

And it's a characteristic of X-linked genes.

So it's a kind of complementation study.

But in essence, we're only seeing this complementation

with a single X chromosome inherited by the male.

So, essentially, we're doing a dominant-recessive study

looking at an X chromosome gene.