Tag Archives: testing

About that Paternity Test… (Part 2)

(Find Part 1 here)

Okay, so Ms. Washington’s twins have two daddies. It’s just tabloid-fodder, right?. But reading it (hey, it’s my job!) I saw a kind of grandeur in this tale: a window into our evolutionary past. After all, routine gestation of a single child is a late development; most mammals carry littermates, often with different baby-daddies. These offspring compete for scarce maternal resources during and after the pregnancy — a competition that may explain the origin of maternal and paternal imprinting of chromosomes.

Here’s the thing: in evolutionary terms, the best interests of the mother and the best interests of the father are not always aligned. During a pregnancy andpig litter over her lifetime, a mother aims to produce as many healthy offspring as possible, which means protecting her own health and distributing her resources evenly, to maximize the chances of multiple babies in multiple litters. What with monogamy being a new idea – maybe that’s why we’re not better at it! — fathers back in the day were relatively unconcerned about the long-term health of the mother. Their mandate was to promote the success of their own offspring, even if it came at the expense of the gestating, caretaking parent (why do you think they call it MAN-date, anyway?). Competition would be particularly intense among littermates – if one father could find a way to get his offspring a disproportionate amount of maternal resources, his genes would thrive at the expense of others. In evolutionary terms, a good day at the office.

Conflict Theory, a school of evolutionary thought espoused by David Haig among others, looks at the consequences of these dueling agendas. Genetic changes that increase or speed-up growth would be favored – when they came from the father. Genetic changes that restrict or delay growth would be favored – when they came from the mother. Evolution would become a see-saw affair.

Let’s take the example of IGF-1, which promotes growth in multiple tissues in utero. Dad wants to make sure his kids are not the runts of the litter; mom wants to look a bit less like a beached whale. Mutations over time alternately increase and decrease the rate of production of IGF-1. Then one day a mutation occurs affecting methylation patterns that shuts down the maternally-inherited allele entirely – imprinting. In the war between the sexes, it is the evolutionary equivalent of the discovery of gunpowder.

Davor Solter first provided evidence of something like imprinting when he discovered in 1984 that proper mouse development required one male and one female set of chromosomes. Use all paternal genes and you get an underdeveloped fetus and too much placental tissue – like a molar pregnancy, which results from an empty egg and two sperm. Molar pregnancies are dangerous because of their unrestricted, highly invasive growth – just what you might have predicted from conflict theory. Maternal-only fetuses look more like mice but are small, and lack supportive tissue.

thinkingThe same logic can be used to predict which parent’s genes are over- or under-expressed in syndromes involving imprinted alleles. Beckwith-Wiedemann, an overgrowth disorder, can be caused by a double dose of paternal genes. The diminutive Russel-Silver baby? – a double dose from mom. Recently, a novel variant was found that is associated with the development of type II diabetes, but only when the allele is inherited from…drumroll please….dad.

So if you think the superfecund Ms. Washington is a sign of the times, think again. Men may not have realized until now that such a thing was possible, but their genes have known it forever. And if you think your genes are making you fat – kids, I’m begging you – blame your Dad.

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About that paternity test…I’ve got good news and I’ve got bad news

PART 1 of 2

When Mia Washington and her boyfriend James Harrison ran a paternity test on her eleven-month-old twins, they got a result that made news at home and abroad. Harrison was the father – of only one twin. The other dad? His identity has not been released. Washington told Fox News: “Of all the people in America and of all the people in the world, it had to happen to me. I’m very shocked.”

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How rare is this? To no one’s surprise, there isn’t a lot of research on this subject (Can you imagine doing the informed consents for that study?). However, estimates suggest that as many as 12% of all fraternal twins are conceived in two separate acts of coitus, a phenomenon common enough to have its own six-syllable name: superfecundation. Parenthetically, this raises an interesting question: how many genetic counselors routinely consider the possibility that twins may differ by days or even a week in their gestational age?

And if mommy is spreading the love, there is no guarantee that the resulting children won’t have different fathers (“heteropaternal superfecundation”). In fact, a review of one database of paternity test results revealed bi-paternity in three cases, or 2.4% of all fraternal twins tested. You have to assume a major ascertainment bias in a population doing paternity testing, so it is hard to know how to generalize those numbers. One study suggests that of all naturally conceived fraternal twins born to “married, white women in America,” one in 400 sets are bi-paternal. The author adds that the number may be higher in certain populations, “like prostitutes”. Going out on a limb there, buddy.

Bi-paternity may be news but it is a pretty safe bet it isn’t new. As shocking as it is to the rest of the world, to genetic counselors it has a familiar ring to it – another cautionary tale about how treacherous it is to make assumptions about paternity. But there is a more ancient angle to this story as well; more on that in my next post.

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