In this video, we're going to talk about X-linked recessive disorders as we discuss a specific example in hemophilia inheritance. Hemophilia is a disorder that's characterized by abnormal blood clotting, and it is an X-linked recessive disorder found in humans. By X-linked recessive disorder, the X-linked part means that the disorder is associated with the X chromosome and a gene on the X chromosome. By recessive, it means that it's only going to be expressed if the individual only has the recessive allele or the recessive alleles on the X chromosome or X chromosomes. Females must be homozygous recessive or have 2 recessive alleles to be affected by an X-linked recessive disorder. However, males require only one recessive allele to be affected because they have only one X chromosome and need only receive 1 recessive allele to be affected. This makes males much more likely to be affected by X-linked recessive disorders, a characteristic of these disorders, as they tend to affect males more than females.
If we examine our image down below, notice that we're showing you a Punnett square crossing a heterozygous mother unaffected by hemophilia with an unaffected father. The heterozygous mother will have an XH and an Xh. It's the Xh that is associated with the disorder. A mother with an XH, a female with an XH, is saved from having the disorder. Females need to have 2 Xh alleles to be affected, whereas males need only one Xh to be affected. Completing this Punnett square, we can bring down the XH to these areas. We will have the X and the X here, each with uppercase H's. Additionally, we can do the same with the Xh, bringing that down so the males, positioned here and here, will each carry a Y chromosome. What you will notice, observing the females in the top half, is that they are unaffected because they have at least one capital H, one XH. Regarding the males in the lower half, 50% of them will be unaffected, but 50% will be affected, or that's the likelihood.
Hemophilia, characterized by the inability to form blood clots and abnormal blood clotting, concludes our introduction to X-linked recessive disorders, specifically hemophilia inheritance. We'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.