Genes and Inheritance [8]: Huntingdons (A Level Biology)


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Transcript:
A Level Biology: Genes and Inheritance 8 - Huntingdon’s
Hi! Welcome to my eight video on the series about Genes. Today, we are looking at Huntingdon’s
disease. Huntingdon’s disease affects thousands of people in the UK and it is caused by a
dominant allele, which unfortunately means that you only need one of those alleles for
the disease to be expressed. Technically, there is no such thing as a carrier with Huntingdon’s
disease. You either are a sufferer or you’re healthy.
Some of the symptoms of Huntingdon’s disease, because it is a disease that affects the brain,
it’s related to things like loss of muscle control, forgetfulness, loss of understanding.
Something else with Huntingdon’s disease, it is usually later onset. You don’t start
to notice the symptoms of the disease until you reach roughly around 30 years old; in
some cases, sooner and in some cases, later. But generally speaking, Huntingdon’s disease
tends to be late onset.
We have here a family tree diagram and the family tree diagram shows whether the individuals
have the disease or do not have the disease. If we look at the top line first, the parents
there. Karl is a man with Huntingdon’s and Sarah is a woman without. I would just give
you a second to think about what you think the genotype of Karl and Sarah might be.
Sarah should be quite straightforward because Sarah is non-sufferer and because the disease
is caused by a dominant allele, then the only possible permutation that she could be is
hh. Karl is a bit trickier to establish. Karl could be either heterozygous or homozygous.
He must obviously have the Huntingdon’s disorder, so he must have at least one allele.
But deciding whether he is HH or Hh, then you need to look at their offspring. If you
look at the three offspring with Liam, Kate and Susan, then Kate is the one that gives
away what Karl’s genotype is. Because they can give birth to an offspring that doesn’t
have the disease, it therefore means that Karl must at least have one healthy allele.
Therefore, Karl must be Hh and that’s his genotype.
So we can probably establish all of the other offspring. We can actually look at what their
genotypes are and their partner’s genotypes as well. So we can see that Lisa, because
she’s healthy, that means she must have both healthy alleles. Liam, because he is
a sufferer and because Sarah is his mother, he must have at least one healthy allele.
Kate is healthy. Susan must be a sufferer, but also must heterozygous and Sean, obviously
is healthy has both recessive alleles. On the bottom row, you have John, Alex, Jo and
Paul, who are all healthy and who do not have the disease and you have Emma who is one sufferer
and again her genotype, because her parents are Susan and Sean, she must have received
the recessive h from Sean and because she actually has the disease, she must have received
the dominant Huntingdon’s allele from Susan.
In summary, Huntingdon’s affects the brain so it affects things like memory and it can
affect things like movement, as well.
Why there are no such things as carrier with Huntingdon’s?
Because it is caused by a dominant allele, it means that you either have it or you do
not. You do not carry the allele because you either have the disease or you won’t.
So if a heterozygous father with Huntingdon’s breeds with a healthy mother, what is the
probability that the child is a sufferer? Draw a diagram to help.
So heterozygous father, heterozygous means he’s got two different alleles in his genotype
so Hh. Half of the sperm, he has H which is causing Huntingdon’s and h allele which
means that it is healthy. The mother has this genotype so hh. All of her eggs, she carries
h. So you can see there, that the offspring, these two here would be completely healthy
and these two would be sufferers. So it is 2/4 and therefore it is 50%
[end of audio – 04:43] A Level Biology: Genes and Inheritance 8 – Huntingdon’s
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