In this video we'll be discussing
human genetic disorders. [Intro Music] Gregor Mendel's principles
apply to the inheritance of many human traits. Things such as having
freckles or no freckles, having a widow's peak versus a straight hairline, free earlobes versus
attached earlobes. Now, we wouldn't call these disorders, these are just variations within
the human population. But, there are genetic disorders that also follow these inheritance
patterns. Many human traits show these simple in
heritance patterns. These traits are controlled
by genes on the autosomes or non-sex chromosomes. Humans have 46 chromosomes,
found in 23 pairs of chromosomes. Autosomes, in humans, are the
22 pairs of non-sex chromosomes. Most human genetic disorders are recessive, meaning that individuals can be carriers of these diseases. A carrier of a recessive
disorder will have one of the disease alleles, but will not show that disease trait.
The carrier will always be heterozygous, and often these c
arriers will have no idea that
they are carriers for this genetic disorder. Now, genetic diseases are different than other
types of diseases in that genetic diseases are not contagious. They can't be
passed by one person to someone else, but the disease alleles may end up being received
by the offspring. And, it will be the offspring, if they receive the disease allele from both
parents, who may end up having that trait. So, here we're looking at a particular case of
genetically-caused dea
fness and it's caused by a recessive allele. Both of the parents are
heterozygous, yet they have normal hearing. When we look at the possible
offspring from this cross, it's possible that they will have a child
with normal hearing, just like the parents. But, if that child receives a recessive allele
from both parents, they then will be deaf. Now, not all human genetic disorders are recessive.
Some human genetic disorders are dominant. Achondroplasia is an example of this.
Achondroplasia i
s a form of dwarfism. Polydactylism is another
dominant genetic disorder. Polydactylism is having additional
digits, extra fingers or toes. So, more than ten. Even though both of these genetic
disorders are dominant, it doesn't mean they are common, and that's an important concept to
realize. Just because an allele is dominant doesn't mean it's common. Now, for a dominant
genetic disorder, it is impossible to be a carrier because if an individual has even one of the
disease-causing alleles
they will show that trait. And here, we see an image of a therapeutic
surgeon who has Achondroplasia and he specializes in surgeries to deal with some of
the consequences of this genetic condition. If a dominant disorder is lethal
and it is expressed early in life, that organism is not likely to survive
to pass it on to the next generation because they won't have offspring at all. The
disease will only be caused by new mutations. Only if the disorder is expressed late in life
will that in
dividual survive to have offspring and this is the case with Huntington's Disease and
certain forms of Alzheimer's, where it is a lethal autosomal dominant disease
which expresses their symptoms late in life. Now, I'd like to spend some time
talking about the sex chromosomes. They determine the inheritance of certain traits.
They are designated as X and Y in humans and most other mammals, and
a few other types of animals as well. And, they typically define an
individual's anatomical sex. A
s mentioned in the previous video,
there is a recognition of exceptions and differences and things aren't always as
simple as presented in this introductory lecture. When it comes to anatomical sex determination,
males are the heterogametic sex, in that males produce two different types of
gametes. They are both sperm cells, but they're either sperm cells with an X or a sperm cell with
a Y chromosome, and depending on which sperm cell fertilizes the egg, the offspring will either
be anatom
ically male or anatomically female. It turns out that there are several genes on
the X chromosome and many of them have nothing to do with anatomical sex. So, we call these
X-linked genes. They are any genes located on the X chromosome and as I mentioned, majority
of these have nothing to do with anatomical sex, orientation, or gender. Males have only one x chromosome, and so, if
a mother is heterozygous for an X-linked gene, then 50 percent of sons from carrier mothers are expected to be af
fected.
Females rarely get these diseases because they must receive two affected
X chromosomes, one from each parent, and this would also mean that the father would
have to be affected by this disorder and the mother would have to at least be a carrier.
So, there are a number of human conditions which result from X-linked or sex-linked genes. Red-Green colorblindness is an example.
It is characterized by a malfunctioning of light-sensitive cells in the eyes. Hemophilia
is another. Hemophil
ia is a blood-clotting disease. The mutation that causes hemophilia
occurs on the X chromosome and it is recessive in females. Here we can see a family
pedigree of Tsar Nicholas II of Russia and we see that his son Alexis had hemophilia,
and so passed away relatively early in life. Tsar Nicholas did not have hemophilia, and so,
we have to follow this path back and we can see several generations this allele was carried
within females of the family. They did not have the disease. Only once a
male inherited that
affected X chromosome did those symptoms show up. In our next module we'll be learning about
how DNA can act as the genetic material.
Comments