Heredity is Lawless

Here’s how Carl Zimmer defines a scientific law in his awesome book on heredity, She has her Mother’s Laugh:

“A scientific law predicts some aspect of the universe, usually with a short, sweet equation.”

By that token, he says something almost blasphemous for a book on heredity:

“The patterns (Mendel) saw wasn’t really a law.”

Huh? Is it because Mendel’s Law doesn’t have a “short, sweet equation”? Nope:

“(Other scientific laws) were as true thirteen billion years ago in the universe’s infancy as they are true today. Mendel’s law has far narrower boundaries.”

 

For example, take microbes:

“None of them follow Mendel’s Law.”

Why not? Two reasons. One, microbes reproduce via the parent splitting into two (children). Whereas Mendel’s Law describes heredity from two parents (aka “vertical inheritance”). Two, bizarre though it may sound:

“(Microbes) can also inherit genes from unrelated microbes, through horizontal inheritance.”

 

Next, here’s a reminder that life only follows Darwin’s rule of survival of the fittest. Thus, even species that follow the two-parent rule can, er, change parties if the “evolutionary benefits” of mixing genes of two parents “no longer outweigh the costs”. Yes, that means a species can stop following Mendel’s Law.

 

If you thought such deviations don’t apply to complex species (like humans), you’d be wrong. Women, as we know, have XX chromosomes, while men have XY chromosomes, right? Not always! Several women have been found to have a Y chromosome… in some parts of their body. If you’re wondering, yes, they could usually still have children. How come? Because cells in different parts of their body had different chromosomes. As long as the “male” Y chromosome wasn’t in the cells of their sex organs (but say, only in the immune system cells), they could have babies! Wait, it gets even stranger. When scientists compared the DNA of the sex organ cells v/s the immune system, they found them to be different! It was like the cells in different parts of the body were from different people. How?

 

Sometimes, the reason was that a few of an earlier born boy’s cells had remained in the womb. Those cells had entered the next embryo that grew in the womb, thereby making the second embryo a hybrid. At other times, twins growing in the womb had fused together into a single child ending up with the child having genes from both the never-born twins. In such cases, the question of who is the parent of such a fused child has no meaning. There are enough such cases that scientists even have a term for it: “chimera”. How appropriate!

 

It’s not only genes that get inherited. Remember, most species, including humans, have bacteria within us without which we couldn’t survive. They’re called endo-symbionts (symbiosis inside an organism). Keep all that in mind when you read this:

“In a few cases, bacteria get passed down so loyally from human parents to children that they can serve as rough genealogical records.”

How do such bacteria get passed on? We don’t know for sure, but “whatever route (it) takes, it’s a tremendously successful one”.

 

Then there’s the famous mitochondria within the cells of all non-microbes. At some point in history, they entered and became a part of our cells. And ever since, mitochondrial DNA been passed down without changes from mother to child without changes (The father’s copy never gets passed down). And yes, it’s different from the DNA in the rest of our body.

 

The ways of heredity are endlessly fascinating. The laws of physics can dazzle you by how a handful of them can produce the universe we see while in biology, the countless different ways of heredity prevent us from framing any universal laws.