Human Inheritance patterns and diseases – 8.2 – Biol 189
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Human Inheritance patterns and diseases – 8.2 – Biol 189


In this video we’re going to talk about the
inheritance patterns of certain human traits and genetic disorders. So do human traits follow the same inheritance
patterns that Gregor Mendel saw in the pea plants? The answer is, for the most part, Yes! Gregor Mendel’s principles apply to the inheritance
of many human traits. In fact, several of these traits just show
the simple dominance relationship that Gregor Mendel saw. For example, freckles is dominant to no freckles,
having a widow’s peak is dominant to having a straight hairline, having free earlobes
is dominant to having attached earlobes. In all of these cases, these traits are controlled
by a single gene and there are two common alleles, one dominant and one recessive. So many human traits show these simple inheritance
patterns. These traits are controlled by genes on the
autosomes. Remember, the autosomes in humans are the
22 pairs of non-sex chromosomes. Most human genetic disorders are recessive,
meaning that it takes two copies of these recessive alleles in order to show the trait,
and individuals can be carriers of these diseases. Now a carrier of a recessive disorder will
have one of the disease alleles, but they will not show the trait. The carrier is always heterozygous. Likely, the carrier has no idea that they
actually are a carrier for this particular trait because they do not show any of the
symptoms. Now, unlike being a carrier for a contagious
disease, genetic diseases are not contagious, meaning if someone is a carrier for a genetic
disorder, they don’t have any risk of passing it on to anyone except for their direct offspring. With genetic diseases, a carrier could possibly
pass a disease allele to the next generation. Since it is a recessive disorder, it’s only
concerning if both parents are carriers for the same disease. Here we see an example of a genetic disease
where D is the allele for normal hearing and the d allele is for a genetic form of deafness. Now in this case, both parents are heterozygous
for this particular gene, meaning both parents have normal hearing, but they are a carrier
for this recessive deafness allele. Now when we look at the Punnett square of
this cross, we see that there’s a possibility for their offspring to be homozygous dominant,
possibility for their offspring to be carriers, just like the parents, and also a possibility
for their offspring to be deaf. Keep in mind, this Punnett square is not saying
that this couple has exactly four children. It’s also not saying that the first child
will always be homozygous dominant. What this Punnett square is telling us is
that each time this couple has a child, there’s an equal probability of that child landing
in any one of these four squares, and so looking at the probability of the child being deaf,
we see that there’s a one in four chance or a 25% chance. Now some human genetic disorders are dominant. Remember, just because an allele is dominant
doesn’t mean that it’s common. Dominant genetic disorders typically are very
rare in the population. What makes it a dominant disorder is that
an individual needs only one copy of the disease allele in order to have that genetic disorder. So Achondroplasia is an example of a dominant
disorder. It is a form of dwarfism. Polydactylism is having extra fingers or toes
on your hands and feet and it is another example of a dominant genetic disorder. Now both of these disorders are caused by
a dominant allele, yet they tend to be very rare in the general population. So again, emphasizing just because an allele
is dominant, doesn’t mean that it’s common. Now for dominant genetic disorders, it is
impossible to be a carrier for that genetic disorder, because as long as someone has one
of the disease-causing alleles, they will end up showing the trait. Here’s a photo of a surgeon who has Achondroplasia. Now this particular surgeon, he dedicated
his medical career to helping others with his genetic disease. He performs therapeutic surgeries for them. Now, achondroplasia is not a disease that
can be cured through surgery, however there are secondary side-effects and complications
which can be limited or helped with therapeutic surgeries. Now what about dominant disorders which are
lethal, which are fatal? Now, if a dominant disorder is lethal and
it’s expressed early in life, that organism is not likely to survive to pass it on to
that next generation. They won’t have survived long enough to reproduce. Only if the disorder is expressed late in
life and in this case again, referring to lethal dominant disorders, only if that disorder
is expressed late in life will that individual survive to have offspring. Now examples of lethal dominant disorders
include Huntington’s disease and certain forms of Alzheimer’s. Now what’s tragic with these situations is
that often individuals do not know they have these diseases, again, until later in life. And at that point, their children are often
taking care of them and the child now knows that they have a 50-50 chance of also having
received this dominant lethal allele from their parent who is affected. This is the end of our discussion of inheritance
patterns for human genetic disorders. In the next video we’re going to talk about
pedigree analysis and also what happens when genetic disorders are found within the genes
in the sex chromosomes. See you in the next video!

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