Autosomal dominant traits or disorders are caused by the inheritance of at least one dominant allele on a person's autosomes. As you learned in a previous lesson, autosomes are the chromosomes found in your cells, excluding the sex chromosomes.
For each trait that you have, you possess two alleles for that trait, one from each parent. For a person to express an autosomal dominant trait, he or she needs to have at least one dominant allele.
This means the person's genotypes could be either big A/little a (Aa) or big A/big A (AA). You only need one dominant allele to possess an autosomal dominant trait; you can be homozygous dominant, or heterozygous, and still have that particular autosomal dominant trait.
Some traits are controlled by recessive alleles, and some traits are controlled by dominant alleles. Punnett squares are thus one way to see how the genotypes of parents can affect the outcome of their children.
If a parent possesses at least one dominant allele, that parent will have that trait. The only way that 100% of the children would be normal is if both parents were homozygous recessive. In the Punnett square below, you can see that if both parents are homozygous recessive, meaning neither possesses a single dominant allele, then they have a 100% chance of having all normal children.
In this case, the normal trait would be homozygous recessive; however you only need one dominant allele to possess a dominant trait.
Say both parents are heterozygous, meaning they have an autosomal trait or disorder because they both have at least one dominant allele. If you look at the Punnett Square below, 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.
Now say one parent is heterozygous, and the other parent is homozygous recessive. The first parent contains one allele for that autosomal dominant trait -- that parent possesses the trait -- while the second parent is completely normal. As the Punnett square shows, this combination of parent alleles means the parents have a 50% chance of having totally normal children.
a. Huntington's Disease
Huntington's disease is a disease that can be inherited, but the symptoms don't show up until adulthood. Oftentimes, people don't realize that they have Huntington's disease until they've already reproduced and passed it on to their children.
b. Marfan Syndrome
Marfan syndrome is another autosomal dominant trait. This disorder causes a weakening of the aorta, meaning that the aorta can rupture over time.
Generally, people with this disorder are very, very tall and lanky, as those are the phenotypes associated with this disorder. However, the weakening of the aorta is the most serious aspect of Marfan syndrome since rupturing can occur with intense physical activity.
Achondroplasia is another type of autosomal dominant trait or disorder that results in a person who has short arms and legs, and who is short overall.
Most people with this type of disorder only get to be about 4 to 4.5 feet tall, so achondroplasia affects a person's height, and then -- as a result -- causes shortness of the arms and legs.
d. Familial Hypercholesterolemia
The last autosomal dominant trait or disorder that we'll discuss is familial hypercholesterolemia, which leads to high blood cholesterol.
This is because the cholesterol in the blood of people with this disorder won't bind to LDLs, which is the first step in removing the cholesterol from the body. Therefore, most people with familial hypercholesterolemia often don't have as long of a lifespan as other people as a result of their very high cholesterol.
In this lesson, you learned that the cause of autosomal dominant traits or disorders is the inheritance of at least one dominant allele on an autosome. In other words, if one parent has one dominant allele, there is a chance that the children will have the dominant trait as well, even though the other parent does not have that dominant allele.
You now understand that some common examples of autosomal dominant traits are Huntington's disease, Marfan syndrome, achondroplasia, and familial hypercholesterolemia.
Source: Adapted from Sophia tutorial by Amanda Soderlind.
An autosomal dominant disorder in which the person is abnormally short in stature with short arms and legs.
A trait or disorder caused by the inheritance of at least one dominant allele on an autosome.
An autosomal dominant disorder in which a person has chronic high cholesterol.
An autosomal dominant disorder which does not show up until later in life often after the gene has been passed onto offspring. It is a neurodegenerative disorder that causes motor and cognitive impairment and eventually becomes fatal.
A genetic disorder of connective tissue that causes people to have a certain appearance: being abnormally tall with long limbs and digits. It can also affect other connective tissues such as heart valves and be fatal.