The next piece that we need to move into is mitochondrial inheritance. As I’m sure you know,
mitochondria have DNA and so we have to talk about how it’s inherited. You probably already know
that the mitochondria are inherited from the female parent. On mitochondrial DNA, there are 37
different genes that can encode enzymes that are involved in oxidative phosphorylation.
We know that the mitochondria are the powerhouse, right? Glycolysis, Krebs cycle, electron transport
chain, all of that happens around the mitochondria as we produce ATP. Oxidative phosphorylation
was that electron transport chain piece that happens within the mitochondria. We have RNA in there
that is necessary for translating and making all of the enzymes involved in that process.
Mitochondria then come from the egg. You can see here a nice graphic showing mitochondria,
the little red dots inside our egg cell and the sperm swimming along. You probably recall that mitochondria
are not included inside the head of the sperm. But he does need lots of energy to do his job
and swim to his target. He does have mitochondria but they’re all sort of outside of the nuclear region
and don’t make their way into the egg. Mitochondria come from an internal line of inheritance.
We will not see any passage of mitochondrial genes from a male to his offspring. Here’s a pedigree
showing mitochondrial inheritance of a disorder you should probably know about also
which is Leber hereditary optic neuropathy which is early onset degenerative eye disorder, so loss of sight.
It is clearly inherited by mitochondrial DNA. You can recognize this in the pedigree when you see the father
that’s affected does not pass it on to any of his offspring, whereas a mother that is affected
passes it on to most offspring. Now, why not all? It could be some. It could be none, as it turns out.
Here is the answer to why not all. Because when cells divide, the mitochondria are divided somewhat
randomly between the daughter cells. You can see that in the case all the way over on the left here,
there could be all mutant alleles versus all the way over on the right side, you could have
very few mutant alleles. Now obviously, there’s going to be some progressive expressivity
or a continuum of expressivity for mitochondrial disorders because some cells are going to contain
many disease-causing alleles on the mitochondria. Some will contain mitochondria
that have virtually none. The less that exists, the less expression there will be of say,
Leber optic myopathy. Anyway, on that note, in this lecture, we’ve covered the expansion repeats
of Huntington’s disease as well as Fragile X syndrome. I challenge you to sit down really quickly
and take a few notes. Close your books. Pause the video. Make sure that you understand
the distinctions between Huntington’s and Fragile X syndrome as well as understanding how
mitochondrial inheritance works and recognizing those pedigrees. Anyway, thanks so much for listening.
I look forward to seeing you in the next lecture.