The Maternal Inheritance of a Mitochondria

For those of you who do not know, I am a Genetics major (also an English major, hence I have this blog). For me, writing things down is how I process them and how I clarify all of the thoughts whirling around in my head at any given time. This blog post is basically me detailing this Genetic concept that clicked for me this week. It was a time when I knew that I am right where I need to be and that I have been validated in the field I have chosen.

To begin, the mitochondria is the powerhouse of the cell. Its main job is to create ATP which is used for energy in the body. Genetically, the mitochondrion has its own genome called mtDNA and in zygote formation the mitochondrion is always inherited from the maternal side. This is why there are such diseases as Leber's disease, which is where a mutation in the mitochondrial DNA causes blindness to be passed from the mother down to all the children. It had always puzzled me how this disease was passed just from the mother and that the father in all cases, had nothing to do with the phenotype of the children, they all had Leber's.

This week in my Genetics class we were learning about the different modes of inheritance and one that stood out to me was cytoplasmic inheritance. In this form of inheritance, it is not random like normal Mendelian inheritance, but just depends on how the cell splits apart in telophase II of meiosis. It means that the gametes produced will get different organelles and amount of cytoplasm. In females at the end of meiosis, an egg is formed along with three polar bodies which are not usable. An egg is a cell obviously, but the important fact to note here is that the egg has cytoplasm which has organelles in it, and thus mitochondria. Sperm however, have very little cytoplasm because the most important part of sperm is the DNA and the speed in getting to the egg. When the sperm and egg come together, the egg is what contributes cytoplasm and what organelles are in the cytoplasm, which includes mitochondria. 

From here, the now zygote will divide and keep creating new cells. The mitochondria that are passed on to each cell are going to be from that original mitochondrion since mitochondria have their own DNA. This explains why in Leber's disease, the mother passes the disease on to all her children. It is not an X-linked or autosomal disease, but rather one where the mutation is on the mtDNA. If a mother is affected, her mitochondria are mutated. When she has children, the egg that she contributes to reproduction will have those affected mitochondria in it. As the cell begins to duplicate itself and the zygote begins to grow, all of those other cells have the same affected mitochondria since the original mtDNA was affected. Because of this, every child of that mother will have the disease and the females will pass that down to their kids, but not the males, since they don't contribute cytoplasm in the coming together of the sperm and egg. 

It was a great feeling to have a hypothesis, research it, and find out that your hypothesis was indeed true, and that you were correct in your thinking. Genetics is where I am supposed to be and it's a great feeling!

--GeneticGinger