Source: Video and Images Created by Amanda Soderlind
Welcome to this lesson today on autosomal recessive traits and disorders. Today we're going to be discussing characteristics and examples of autosomal recessive traits and disorders. So first of all, these types of traits or disorders are caused by the inheritance of recessive traits on autosomes. And autosomes are the chromosomes in your body that are all the chromosomes in your body excluding the sex chromosomes.
So in order for a person to inherit an autosomal recessive trait or disorder, both parents must contribute recessive alleles. So the only type of genotype that a person can have to display an autosomal recessive trait or disorder is the genotype with two recessive alleles. So I guess in order for the normal genotype, or what is considered normal, a person has to have at least one dominant allele.
So these would code for just the normal trait. But in order for somebody to have an autosomal recessive trait or disorder, they would have to have the genotype of homozygous recessive. So this would be the genotype of a person with an autosomal trait or disorder.
So we're going to take a look at a couple Punnett squares here just to see how the alleles of the parent can have an impact on the possible outcomes of their offspring. So let's pretend that we have two parents that are heterozygous. So both parents display normal characteristics because they have at least one dominant allele. But they are considered carriers. Both of them are considered carriers because they possess this recessive allele. So they don't show any effects of this recessive allele because the recessive allele is masked by their dominant allele. But because they do possess the recessive allele, they do have the chance of passing this recessive allele on to their offspring.
So if we were to do this Punnett square of this cross, we would end up with this. So basically, 75% of their children, three out of four, have at least one dominant allele, meaning that they're going to show the normal characteristics. But they have a 25% chance that their offspring is going to inherit this autosomal recessive trait. So if the mom gave her recessive allele and the dad gave his recessive allele, they would have a 25% chance that their offspring would have this autosomal recessive trait or disorder.
So let's see what would happen if one parent was heterozygous and one parent was homozygous recessive. So one parent already has this autosomal recessive trait, and the other parent is normal, but is a carrier. So in this case, they have a 50/50 chance. 50% of their children will be normal, but will be carriers. And 50% of their children, or two out of four, are going to inherit that autosomal recessive trait or disorder. So here, 25% could end up with that disorder, and here, 50% could.
And let's say both parents are homozygous recessive. So both parents have this autosomal recessive disorder, in which case, the only alleles that they can give are recessive alleles, so that 100% of their children would end up inheriting that disorder. So Punnett squares can be kind of a useful tool. You can see how they can be used in order to determine the probability that offspring would inherit a particular trait or disorder.
So cystic fibrosis and PKU are examples of autosomal recessive traits. Cystic fibrosis is a type of disorder in which mucus buildup in the lungs-- it has many other different symptoms and things that go along with it. But one of the big characteristics of cystic fibrosis is that mucus buildup in the lungs.
And PKU is another autosomal recessive disorder in which the buildup of a certain amino acid will get too high in a person's body. And if they get too much of this certain amino acid built up in their body, it can cause mental retardation. So diet can actually help lower the specific amino acid and help prevent symptoms of this disorder. So this lesson has been an overview on autosomal recessive traits and disorders.
A trait or disorder caused by the inheritance of two recessive alleles on an autosome.
An autosomal recessive disorder that results in build-up of mucus in the lungs.
An autosomal recessive disorder that results in the build-up of phenylalanine (an amino acid) in the body that can lead to mental retardation if levels exceed a certain point.