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Last updated May 26, 1999
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Divison of Genetics
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Paternity
tests and amniocentesis
Spina bifida
How is blood type determined?
Paternity tests and amniocentesis
As far as I know, they haven't developed a test yet that can prove without a shadow of a doubt that a man is someone's father. However, there are a couple tests that may be able to prove that someone is NOT someone's father. The easiest one is a simple blood typing test where you find out a person's blood type (A, B, AB, or O). If you know the mother's blood type, it can eliminate certain individuals by Mendelian genetic principles (if you want more detail, write back and I'll explain further). However, if someone has a feasible blood type, they COULD be the father but not necessarily because there are only 4 blood types in the world and millions of people have each particular blood type. Confusing?
If you wanted to spend lots and lots of money, you could also get a DNA test of the child and possible father. (Remember the OJ Simpson trial?) They have the ability to do gene matching sequences now with DNA probes and a technique called PCR (Polymerase Chain Reaction). If the person really is the father, there will be a significant number of matching sequences with the child. But since half your DNA comes from your mother, it of course wouldn't be a complete match. So a lot of matches would lead you to highly suspect paternity, and no or very few matches would make paternity very unlikely.
When can a paternity test be done? Can it be done before the child is born?
Most of the time (at least I've never heard of it), you wait to do a paternity test until after the child is born, just for convenience and safety sake. Every time you try to do a test to an unborn child, there's a definite (though small) risk of damage to the baby or miscarriage. So you'd have to have a *pretty* good reason to want to do it beforehand and subject the baby to such a risk.
From my understanding, you need actual blood from the baby/fetus to determine blood type. The test for an unborn baby would be called PUBS, or percutaneous umbilical blood sampling. To do that, you use an ultrasound-guided needle to go in through the mother's abdomen and into the baby's umbilical cord to draw a sample of blood. This is the riskiest of prenatal tests and has a 1 - 2 % pregnancy loss rate. PUBS cannot be performed until the 2nd or 3rd trimester because you have to wait for the umbilical cord vessels to be big enough to go in. The blood sample obtained from PUBS could also be used to do a DNA test.
Can it be determined from the specimen that is taken during amniocentesis? At what stage in the pregnancy is amniocentesis usually performed?
Hmmm, good question. Amniocentesis is done any time after 15 weeks gestation. The main reason for amnio is to detect "cytogenetic abnormalities", saith my textbook, which means stuff like Down's Syndrome or Trisomy 13 or 18. Amnio means sticking a needle through the mother's abdomen into the amniotic sac and sucking out some fluid, which has sloughed off cells from the baby in it. They take these cells and can do a karyotype on it, which means they find a cell undergoing mitosis and take a picture of its chromosomes, then cut out all the chromosomes from the photograph and arrange them. I don't *think* you can do a DNA test on the amnio sample -- at least I've never heard of it.
Can gender be ascertained via amnio?
Yes, at least the chromosomal sex can be (does the kid have an XY = boy or an XX = girl). That may in some rare cases not be the same as the physical sex, but those situations get pretty complicated. In general, yes, you can tell the gender.
Is amniocentesis sensitive to congenital deformities such as spina bifida?
Yes, it would usually show an elevated AFP (alpha-fetoprotein) level, indicating a neural tube defect (spina bifida is one of the neural tube defects -- so are anencephaly, meningomyelocele, etc.).
Would spina bifida show up on ultrasound?
It could if the defect was very large, but it might not if it were spina bifida occulta. There are a lot of people walking around with spina bifida occulta that don't even know they have it. A larger spina bifida defect could definitely show up. Most babies with significant spina bifida are diagnosed prenatally these days.
Would drug or alcohol use during pregnancy be a cause for spina bifida?
Um, I don't know of an exact association with alcohol and spina bifida (alcohol does cause fetal alcohol syndrome, but spina bifida isn't one of the manifestations usually). Drugs? Well, there probably are some prescription drugs that have an increased risk of spina bifida, but I don't know of any off the top of my head. The major association we know of is that a lack of the vitamin folate is definitely associated with spina bifida and all neural tube defects. Not having enough folate can be due to bad nutrition (which is something alcoholics and drug users often have because they don't take care of themselves). Certain drugs can also give you folate deficiency -- I think the anti-epileptic drugs like Dilantin can do that.
Since you're asking about detection of spina bifida, you might be interested to know that the main way doctors screen pregnant women for spina bifida and other neural tube defects is through a blood test called MSAFP (maternal serum alpha-fetoprotein). You just take a blood sample from the mother and measure the level of AFP. If the AFP level is high, this suggests a neural tube defect. This test is very sensitive for spina bifida (85%) but not very specific (7%). That means that if a mother was tested and the AFP level came back high, it wouldn't necessarily mean her baby was malformed/had spina bifida. On the other hand, if a mother was pregnant with a baby with spina bifida, her test would almost certainly come back positive (high AFP). Confused yet? Basically, if the test comes back positive (bad result), there's a high chance that the baby's perfectly fine and a small chance that it has a neural tube defect. If the test comes back negative, you can be almost totally sure that your baby is okay. The MSAFP test is performed from 15 to 19 weeks gestation. This test can also detect some cases of Down's syndrome. The nice thing about this test is that there is no risk to the baby, unlike an amniocentesis.
The basics about the ABO blood type:
You have two ABO genes. Each gene can be A, B, or O. Therefore, there are quite a few combinations: AA (type A), BB (type B), OO (type O), AB (type AB), AO (type A), or BO (type B). [People who are AO are type A (because the A is dominant) and people who are BO are type B (because the B is dominant).] Your blood type is determined by which gene you get from your mom and which gene you get from your father. It is possible to have an entirely different blood type from either your mother or father. It all depends on which 2 genes each of them have and which 2 genes you ended up getting.
Besides the ABO blood type, people often speak about being blood type A/B/O "positive" or "negative". The "positive" or "negative" part refers to whether a person has or doesn't have something called the Rh factor. The basics about the Rh factor:
You have 2 Rh genes: one from your mother and one from your father. Therefore, your combination of genes can be either 1) two positive Rh genes, 2) two negative Rh genes, or 3) one positive and one negative Rh gene. The Rh factor is a dominant gene, meaning if you have either 1 *OR* 2 Rh positive genes, your blood will be Rh positive. The only way your blood can be Rh negative is for *both* your Rh genes to be negative. Most people (85%) are Rh(+), meaning that their blood is positive for the Rh factor. About 15% of people are Rh(-).
Needless to say, blood type can be a little confusing, especially when you aren't used to working with Mendelian genetic principles.