Source: Video and Images Created by Amanda Soderlind
Welcome to this lesson today on X-linked traits and disorders. Today we are going to be discussing what an X-linked trait is and examples of various X-linked traits and disorders. So our sex chromosomes are chromosomes that are related to our sex and gender.
So the females, their sex chromosomes are composed of two X chromosomes. And a male sex chromosome contains one X chromosome and one Y chromosome. So this is the difference of chromosomes between males and females. Females possess two X's and males possess an X and a Y chromosome. So X-linked traits are disorders are related to a person's X chromosomes, hence the name X-linked.
So you can see, as we mentioned, females have two X's and males have one X. So both males and females can be affected X-linked traits, because they both possess X chromosomes. However, males are generally at more of a risk for inheriting or an X-linked trait because females have two X's. So if one of their X's is affected, the other X chromosome will generally mask that affect.
However, because males only have one X chromosome, if this X chromosome is affected, they have nothing to mask it. So males therefore are at a higher risk for inheriting these X-linked traits or disorders.
So X-linked traits or disorders can be caused by a recessive allele on an X chromosome or a dominant mutant allele on an X chromosome. So again, both are affecting the X chromosomes.
So some of these disorders right here are X-linked disorders. So hemophilia, red-green color blindness, and Duchenne's muscular dystrophy are examples of X-linked recessive disorders. So it's caused by a recessive allele on the X chromosome.
Hemophilia is a bleeding disorder in which blood doesn't properly clot. Red-green color blindness is a disorder where a person can't distinguish between the colors red and green. And Duchenne's muscular dystrophy is a disorder in which muscles begin to degenerate over time.
And then faulty enamel trait is a disorder caused by a dominant mutant allele on the X chromosome. So these are actually much less common. But there are a few disorders that are affected in this manner. So faulty enamel trait, basically what happens is that enamel that protects your teeth doesn't properly develop with people with this disorder, so their teeth will rot very, very easily because they don't have this protective enamel on their teeth.
So let's take a look at an example of a Punnett square. And we're going to use hemophilia here as our example. So we're going to kind of show you how these disorders can be passed from parent to children, these X-linked disorders.
So here are the alleles that can be donated from a father. So these are the sperm cells. So he can donate an X or a Y allele. And a mother can donate-- these are the egg cells-- an X or an X.
Now I've drawn the second acts in another color. And so what this is going to indicate is that this X allele of the mother is a recessive-- there's a recessive allele on this X chromosome. So this X chromosome is affected.
Now, the mother probably will not be showing this disorder of hemophilia. So hemophilia is our example. So because the mother has two X chromosomes, this normal X chromosome can mask this X chromosome. So the mother therefore is like a carrier for hemophilia. She doesn't display the characteristics. But she can pass that trait off to her offspring.
So if we go ahead and do this cross, we can see what our outcome could possibly be. So if we take a look at this, first of all we know that 50% of the offspring are going to be male and 50% are going to be female. So these right here are going to be male. And these right here are going to be female.
And now we can also see how those affected offspring are affected by this faulty X chromosome. So if you take a look here, first of all we have a normal daughter. So they have a 25% chance of having a normal daughter, and a 25% chance of having a normal son.
Now you can see that this one here has that recessive allele on the X chromosome. So in this case, they could have a 25% chance of having a daughter who is a carrier for hemophilia. So again, because she has this normal X chromosome, she won't display the characteristics of hemophilia, but could possibly pass this X chromosome off to her offspring.
And then they have a 25% chance of having an affected son. So because this son, because boys only have one X chromosome, and the son inherited this X chromosome from his mother, he will therefore be affected. And he will inherit hemophilia. So we have a carrier daughter and an affected son.
So this is kind of how these X-linked traits can be passed on from parents to offspring, and again why males are generally more susceptible to inheriting these disorders because they only have one X chromosome. So if they inherit that X chromosome from their mother, they will automatically get that disease.
So geneticists can study patterns of inheritance to determine sources of such disorders. So many X-linked disorders follow common patterns of inheritance. So for example, only daughters can inherit recessive alleles from their affected father, because the sons will get the Y chromosome. So if a father is affected, he only has one X chromosome to give. His daughters therefore will be affected. But his sons won't, because his sons will inherit his Y chromosome, not his X.
So there's a lot of different patterns of inheritance like this that geneticists can study to understand these diseases and understand the patterns in which these diseases are passed through generations. And pedigrees are also useful tools in tracking these disorders as well, because you can kind of follow the family history, see who in the family has inherited this disease and the likelihood of other family members to inherit this disease.
So this lesson has been an overview on X-linked traits and disorders.