So the genetic condition that I mentioned
that I work on, retinitis pigmentosa, it is an inherited form of eye disease and it shows
all three Mendelian modes of inheritance. In other words you see patients with autosomal
dominant form of RP, autosomal recessive form of RP as well as X-linked form of RP. Now
in the dominant form if you get one disease gene you get the full manifestation of the
disease, and that’s why it’s dominant. Whereas in the recessive form, you have to inherit
both copies of the disease gene, one from each parent. And that is why it is called
recessive RP. And in the x-linked form, the disease gene is on the human X chromosome.
And as you know, females have two X chromosomes and males have one X and one Y chromosome. So in the
case of females if they have one X chromosome with the RP gene on it, then on the other
X chromosome they have the normal copy of the gene, and therefore the normal copy is
able to rescue the mutation. And therefore the females end up being carriers. They never
manifest the full disease spectrum. Whereas in the case of the male if they inherit the
X chromosome with the RP gene on it then they are affected, whereas if they inherit the
other X chromosome, with the normal gene on it, then they are completely normal. So that
is the x-linked form of RP.