Human Sperm Lack mtDNA

Researchers knew mtDNA is passed to children by their mothers but not their fathers and now they think they figured out why.

Hiya!

I assumed that everything we inherit is roughly a 50/50 split between our biological parents, but it turns out that’s not totally true. I also thought we had only one set of DNA, but that’s not entirely true, either. The truth is that two sets of DNA are floating around in our cells — and one comes almost exclusively from our mothers.

About half of our main DNA, the one we typically think of, comes from our biological fathers, while the other half is inherited from our birth mothers. But a different DNA exists independently, within our cells’ mitochondria which we inherit from only our biological mothers.

While this knowledge was new to me, it’s well-known in the scientific community but not fully understood. However, the researchers of a new study discovered something that may help explain it.

Mitochondrial DNA

To better understand today’s topic, let’s go back to biology class and review what mitochondrial DNA is.

Mitochondria are bean-shaped organelles that exist within a cell but outside the cell’s nucleus and have been around for at least 1.5 billion years after evolving from free-living bacteria. They’re most known for being a cell’s power source, fueling its biochemical reactions by converting oxygen into usable energy.

Thanks to their long history, mitochondria still possess their own DNA, which is wholly distinct from the 23 chromosomes humans have in our genome — and the mitochondria’s DNA is ring-shaped.

Image Source: WikiCommons

Intriguingly, we inherit mitochondria and the mitochondrial DNA (mtDNA) along with it from our mothers but not our fathers. This is because the mitochondria we inherit come from our mother’s egg cell, but there’s still plenty of mystery around why we don’t inherit any mtDNA from our father’s sperm.

Previous Thoughts

Scientists previously thought the mother’s immune system views paternal mtDNA as intruders and destroys it when sperm fuses with the egg, or oocyte, during fertilization.

It’s a rational idea, but a recent study offers new insight into why mtDNA is passed down through the maternal line (almost) exclusively — and it has nothing to do with the mother’s immune system.