Source: Video and Images Created by Amanda Soderlind
Welcome to this lesson today on autosomal dominant traits and disorders.
Today we will be discussing characteristics and examples of these types of traits and disorders.
Autosomal dominant traits or disorders are caused by the inheritance of at least one dominant allele on your autosomes. Remember, autosomes are the chromosomes found in your cells, excluding the sex chromosomes. So it's all of the chromosomes in your body, excluding sex chromosomes. For each treat that you have, you possess two alleles for that trait, one from each parent. For a person to express an autosomal dominant trait, they need to have at least one dominant allele. So their genotypes could be either big A little a (Aa) or big A big A (AA). You only need one dominant allele to possess that autosomal dominant trait.
You can be homozygous dominant, or heterozygous, and still have that particular autosomal dominant trait.
So what we're going to do is take a look at an example of a couple different Punnett squares, just to see how the genotypes of parents can affect the outcome of their children.
Let's draw a Punnett square here. We know that if a parent possesses is at least one dominant allele, they will have that trait. So the only way that 100% of the children would be normal is if both parents were homozygous recessive. So in this Punnett square, we can see that if both parents are homozygous recessive-- neither of them possesses a single dominant allele-- then they have a 100% chance to have all normal children. So it's 100% chance that all their children would be normal.
Some traits are controlled by recessive alleles, and some traits are controlled by dominant alleles. So let's take a look at another example here. In this case, the normal trait would be homozygous recessive. But it in order to possess a trait if you have at least one dominant allele, you're going to end up with this abnormal trait or disorder. So let's say both parents in this case are heterozygous. So both parents in this case have this autosomal trait or disorder because they both have at least one dominant allele. But if we do our cross here, you'll notice that because they also each possess a recessive allele for the normal condition, they actually have a 25% chance of having normal children.
Let's just do one more quick example here. Punnett squares can be a good way, in order to figure out the probability of offspring, depending on the parent's genotypes. So let's say one parent is heterozygous, and one parent is homozygous recessive. So one parent contains one allele for that dominant trait-- so they possess that trait-- but the other parent is completely normal. If we cross this, you can see in this case, with this combination of parent alleles, they have a 50% chance of having totally normal children.
Some examples of autosomal dominant traits are Huntington disease, Marfan Syndrome. Let's talk about Huntington disease first. Huntington disease is a disease that can be inherited, and the one thing about this disease is that the symptoms don't show up until adulthood. So often times, people don't realize that they have this disease until they've already reproduced and passed it on to their children. So that's one example of an autosomal dominant disease.
Marfan syndrome is another example. Basically, with this type of disease, it causes weakening of the aorta. It causes a weakening of the aorta, and over time then, the aorta can rupture. Generally, people with this disorder are very, very tall and kind of lanky. Those are kind of the phenotypes associated with this disorder. But the most serious part of this disorder is that the aorta starts to weaken over time, and can rupture with intense physical activity.
The next disorder we are going to talk about here is achondroplasia. This disorder is another type of autosomal dominant trait or disorder that basically results in a person with short arms and legs, and they're short overall. Most people with this type of disorder only get to be about 4 to 4 and 1/2 feet tall. So it's a disorder that affects a person's height, and then as a result, also they have very short arms and very short length legs.
The last autosomal dominant trait or disorder that we're going to talk about today is familial hypercholesterolemia. This disorder, basically what happens is it leads to high blood cholesterol because the cholesterol in their blood won't bind to LDLs, which is the first step in removing the cholesterol from the body. So they actually end up with very, very high cholesterol, and most people who have this disease often times don't have as long of a lifespan as other people, in addition.
This lesson has been an overview on autosomal dominant traits and disorders.