All language subtitles for 37031x_MH_Gene_Function_06_Hemoglobin_Example_v1-en

MICHAEL HEMANN: Let's think about alleles of hemoglobin,

which, of course, is a protein in blood that carries oxygen,

so it's encoded by the Hb gene.

And there are different alleles of this hemoglobin gene.

So, for example, the HbS allele is the sickle cell allele

of hemoglobin.

And people that have two of these HbS alleles

have a condition called sickle cell anemia, so anemia

essentially meaning low blood count and, of course,

an inability to properly carry oxygen. This

can be a really debilitating condition that affects

a large number of people.

So if you are a heterozygote at this locus--

so say you are HbS, and you have another allele, which

is the more common allele, which is HbA,

you do not have sickle cell anemia, right?

So, in this case, what kind of a trait is sickle cell anemia?

It is recessive because it is a trait that

is present in the homozygote, but it

is not present in the heterozygote,

so it's recessive.

But let's think about another condition.

So this other condition will be malaria resistance.

So in malaria resistance or sensitivity,

if you have two of the common alleles of hemoglobin,

so two of the HbA alleles, you are sensitive to malaria.

If you are a heterozygote, so you have HbA, and you have HbS,

you are now resistant to malaria.

So here what is the trait?

It is-- absolutely, it's dominant

because it's a trait that's present in the heterozygote as

well as the homozygote.

If you were HbS, HbS, you'd have sickle cell anemia,

but you'd have malaria resistance.

So this idea of dominant and recessive

is entirely phenotype dependent.

It really matters what we're talking about in terms

of the biological outcome, and it's

very specific in terms of that biological outcome.

And this biology is actually an incredibly interesting one--

so that you actually have a condition or an allele

that's maintained in a group of people.

Even though, for homozygotes, it causes this debilitating

condition, it's kept in the population

because in areas where there's high malaria--

so in sub-Saharan Africa, there is a net benefit

in heterozygotes for having the preservation of this allele.

So you can actually directly correlate here on the left side

the allele frequency of that HbS allele

with these locations in Africa, where malaria is endemic.

And so later in the class, we're going

to talk about essentially allelic populations

and the pressures that are occurring

in individual alleles in populations

to try to make a little bit more sense of how this actually

works at a population level.

But here, again, I think it's important just

to remember that these ideas that we're putting out

of recessive and dominant-- these

are experimental results that are

dependent upon particular phenotypes

that we're looking at.