Did she or didn't she? Genetic testing and virgin birth

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Sandra Porter

i-e3899186ef576991098fed4ff62eab0f-small_dice.gif'Tis the holiday season and, according to ancient lore, the time when miraculous events are most likely to take place.

One of those well-known and miraculous events of ancient days was the birth of a son to a young girl, who, although she was married (Okay, I'm not sure about this part of the story) she was said to be a virgin and the birth to be a miracle.

Hmmm.

How do you think the news would be received if that sort of thing happened today?

Certainly, if the young girl were to produce a grilled cheese sandwich with a burn spot that vaguely resembled a woman in a robe, someone might be able to sell it on Ebay and all kinds of people would consider it some kind of miracle (for many different reasons). But if the girl produced a baby, let's face it, most of us would question her ability to do this on her own without some kind of help. Some people might even consider using a bit of science to test her claim and find out if she was right.

Lizards do it

We do know, out there in wild, wild, kingdom of Omaha, that the virgins of some species can and do give birth. This phenomenon, called "parthenogenesis," has been documented in frogs, sharks, birds, mice, Komodo dragons, and Daphnia (1, 2). Sometimes, this occurs naturally, and sometimes parthenogenesis occurs when biologists mess around with eggs and test tubes. Whatever the cause, parthenogenesis involves activating an egg so that it develops in the absense of sperm. By knowing what's involved we can make some predictions that would help us test whether or not parthenogenesis has really happened (or not).


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Girls, girls, girls

The first prediction we can make about virgin birth concerns the sex of the offspring. Human females are female because we have two X chromosomes. If we had even a single Y chromosome, we'd be male. Since a virgin mother only has X chromsomes, she only has the genes to make girls.  So all children from a virgin birth, in humans, would have to be female (Uh oh!  Keep in mind, I'm writing about science, not supernatural happenings).

 

Going farther

Okay, so virgin females can only give birth to girls. Lots of women who give birth to girls are not virgins. What other tests could we use to figure out if a girl is telling the truth?

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We could use one of many different methods to look at the child's genotype. If the child is female, and the mother was a virgin, then most if not all of the child's DNA should be identical to DNA from the mother and every genotype that we examine in the child should be homozygous.

There are several methods that we can use to test this.  The essential point is that every site we examine in the child's DNA should have two identical copies of the same sequence. Genotyping is a term that we use to describe looking at those similar positions. If we consider the DNA in our own chromosomes, sometimes we find that sequence is identical in both chromosomes and sometimes it's different.

One way to examine genotypes is by looking directly at the DNA sequence. In this DNA trace graph, you can see that most of the molecules have the DNA sequence
i-c944e0871f512c5a140079c31e60ccd5-common_seq.png.

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When we look at the next base though, at position 181, we see a green peak and a red peak on top of each other. This means that approximately half of the molecules had an "a" at position 181 and the other half had a "t." (We see this same pattern at position 184, too). 

From these results, we can say that one chromosome has the genotype A and the other has the genotype T. Now, I don't which genotype came from the mother and which one came from the dad, but I do know that there are two different genotypes in this DNA sample.

If these data came from a sample of human DNA, we'd simpy have to grit our teeth and say "sorry kid, you may be miraculous, but your birth was not.."

References:

1. Edwards RG. 2007 "The significance of parthenogenetic virgin mothers in bonnethead sharks and mice." Reprod Biomed Online. Jul;15(1):12-5. Review.

2. Chapman DD, Shivji MS, Louis E, Sommer J, Fletcher H, Prodöhl PA. 2007. "Virgin birth in a hammerhead shark." Biol Lett. 2007 Aug 22;3(4):425-7.