Divvying Up Baby

I recently bought my baby new pajamas with a decal that says, “50% Dad + 50% Mom = 100% Me!” I couldn’t resist an outfit that doubles as both math and biology lessons. But on further reflection, I’ve realized that this simple formula is wrong in more ways than one.

To begin with, my baby doesn’t look like she’s 50% Mom. At best, she looks about 10% Mom. I’ve written before about how our daughter would be a mixture of traits from European and Indian peoples, reflecting her mom and dad’s respective heritages. Yet she arrived looking like a wholly Indian baby. This is fine, of course. I think she’s absolutely perfect with her caramel skin and jet black eyes and hair. But it’s hard to keep a straight face when friends politely ask us who we think she resembles. And when I’m out with her in public I’m aware that I look like her nanny, if not someone who’s stolen a baby. She truly doesn’t look like she’s mine.

How else is the formula wrong? Genetically. Sure, our daughter’s nuclear genes are comprised of DNA sequences from both my husband and me. But she has another sort of DNA in her body, one that literally outweighs the conventional type. This DNA lives in her mitochondria, the bacteria-like structures that populate our every cell. Mitochondria are like tiny internal combustion engines, generating all of our energy through respiration and releasing heat that makes us warm-blooded animals. Although mitochondria don’t have many actual genes, they each carry several copies of those genes. Multiply that by the 10 million billion or so mitochondria in our bodies and you’ll find that we each contain more DNA by weight for mitochondria than humans. And these mitochondrial genes are inherited entirely from the mother.

Mitochondrial genes can’t claim credit for your eye color, jaw shape, or intrinsic disposition. Their reach is mostly limited to details of your metabolism and your susceptibility to certain diseases. But mitochondrial DNA is significant for another reason: scientists use it to trace human lineages across the globe. After all, they don’t get reshuffled in each generation as our nuclear genes are. Mitochondrial inheritance can be traced back hundreds of thousands of years, following the maternal lineage at every generation. Unlike the historian’s genealogy, which often follows surnames passed down from fathers, the scientist’s genealogy is a tree built of mothers alone.

So it is through our mothers that our heritages can be traced into the distant past. In every one of her cells, my baby carries a map leading back through me and my mother and her mother and beyond . . . unbroken all the way back to our earliest origins as modern humans. And since my baby is a girl, she can continue that line. So long as she has a daughter and she has a daughter and so on, I will remain a part of that ongoing chain.

My condolences to all you men out there. Same to all you women who only had sons. You’ve passed on your nuclear genes and your child may be the spitting image of you, but your mitochondrial chain has been broken and you will be left out of the biologist’s tree. Although my daughter looks classically Indian, her mitochondrial DNA reveal only her European lineage. Despite the hair, eyes, and skin she inherited from her daddy, my baby’s mitochondria are mine all mine. She and I are links in a traceable chain of human life while my husband is nowhere to be found.

That’s something I can remember the next time I’m mistaken for the nanny.

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