DNA testing question

Say a son impregnates his mother (yeah, I like to start my posts with a bang!). The pregnant woman’s husband (the son’s father) finds out about his wife’s condition, she convinces him the child is his, but he is skeptical. She finally convinces him by agreeing to a DNA test, figuring that since the baby’s real father (her son) has the DNA of her and her husband, the baby will show up as being a product of her and her husband, as that would be the only DNA showing in there.

Would this actually work in real life? Or would the son (assuming that he is the product of the husband and wife and no other chicanery was going on with his conception) have some kind of identifiable marking on his DNA marking it as his that would show up in his baby that is growing in his own mother?

If Baby is a girl, then the paternity test will easily show that Son fathered the child instead of Dad, since that means Son contributed an X chromosome which could only have come from Mother.

I’ll have to think about it if Baby is a boy, but it seems to me that Baby would have an abnormally high proportion of autosomal chromosomes that could be linked to Mother and not Dad, thereby meaning they must have come from Son.
Been a long time since I’ve studied genetics!

Should be able to tell the difference easily. Baby will have too much Mom in him.

Yep. The baby in either case will inherit half of its genetic material from its mother. The remaining half comes from its father. If the son is the father, then its genetic material will be three quarter mother’s and one quarter (grand)father’s. Look at enough autosomal markers and this should be obvious. Also, in this case there will be tell-tale patterns, such as particular loci where both chromosomes contain markers inherited from the mother.

So then why wouldn’t the baby be a clone of Mom, as one X came from Mom, and the other X from Dad’s Mom - which is the same person as Mom (the first X contributor)?

Because the X chromosome is just one of 23 total chromosome pairs.

Because of meiosis. Something called Chromosomal crossover happens during the prophase I of meiosis so that the genetic material on the chromosomes end up being a mish mash of the pairs in genetic information. And the X/Y chromosomes are the chromosomes that determine sex. There are 22 other pairs that determine genetic structure of a lot of other things.

46 cromosomes, 23 pairs. (IIRC). For each pair, son gets 1 from dad, 1 from mom. A father passes on one of each pair randomly to offspring. (Sperm contains a 23 piece half-set.)

So genetic testing would find husband is related, but not the father.

Sex is determined by the one mismatched pair, a woman has XX and a man, XY

For example, let’s say it’s a girl. If husband was father, his little princess would have his X (which is ne of husband’s mother’s 2 X’s) and one of mother’s 2 X’s. A son would have his Y and one of mother’s 2 X’s. Offspring of son and mother would have:
Boy - Father/son’s Y chromosome, one of mother’s 2 X’s.
Girl - One of mother’s X’s and son’s X - but son’s X is one of mother’s. So Girl may have X1X1, or X1X2 of mother, if X1 is what the son got.

Statistically, a child would be 1/4 father’s chromosomes and 3/4 mother’s… so about 11 or 12 father, 34 or 35 mother. A DNA test looks for certain sites on various chromosomes to match. Likely, a basic paternity test would fail. (The odds of being that lucky to get the right set of chromosomes when you are talking “all of 23” are in the neighbourhood of 2^23, or one in millions) I don’t know which sites on which chromosomes are checked, IIRC them mention a 16-site match, so odds are probably about 1 in 65,000 to get it right if that’s 16 different chromosomes. It might reveal a close enough match that the husband would beat the crap of all eligible male relatives just to be safe.

A more advanced test, including the mother’s DNA, will make the identity of the culprit obvious. The cheap DNA tests do not do this, they only mention the paternity values - "this allele was 10 for the father, 10 for the child.This was 6 vs. 5; this was 3 vs 3; etc. If there is any mismatch, the paternity is not correct.

You can get relative tests (i.e. sibling, grandparent, etc.) from these too. They tell you whether enough of a match exists to support the suspected relationship. Read the FAQ from various testing lab sites.

Of course, proper medical/forensic DNA testing will probably get more detailed and specific.


There are different types of DNA testing, but full-fledged paternity tests may check both alleles at each tested locus in all three subjects. At that point it isn’t even statistical guessing; you can clearly “see” which allele came from which parent. In your example, instead of seeing “father-mother” allele pairs in every case you’ll see “(putative) father-mother” at only half, and “mother-mother” pairs at half the loci.

I won’t speculate whether this happens often enough for the software or technician to grasp the significance immediately.

If the mother’s genes for a particular chromosome pair are M1-M2, father’s F1-F2, you will see various Fn-Mn pairs, about 1/4 of them, and also M1-M1 pairs, M1-M2 pairs, and M2-M2 pairs.

BTW, DNA testing that yields incorrect(?) results is often referred to as a paternity discrepancy. The estimate is that between 3% and 10% of children have this problem. (Depending on who you believe and how they did the study).

A follow up question, and forgive my lack of knowledge of genetics but

Suppose Father and Mother have two children, Girl and Boy, and Girl and Boy procreate and have Baby. Father and Mother decided to raise Baby as their own, to protect the family name, etc etc.

Genetically is Baby different than Boy or Girl? Would doing a genetic profile on the family show that Baby’s parentage was not Father and Mother?

Again, a child inherits one of each pair from eac parent of the 23 pairs of chromosomes. SO for each random chromosome pair (excluding the XX / XY pairs) a sibling may have the opposite of the other sibling, the same, or one different frm the mother or one different from the father.

Statistically, each child should have about half their chromosomes from each parent the same and half different. They randomly pass on one half-set of chromosomes. Their offspring, then, would have a half of its pairs MF (one from mother, one from father) a quarter MM (both from mother via each sibling) and a quarter from father (FF) via both siblings. So a proper genetic analysis of DNA would show that the (grand)father is related, but not the genetic father of the the baby. Certain alleles would not match the father since both chromosomes of that pair match the mother. Ditto, certain alleles would not match the mother. Compared to bro or sis, though, you would get the expected result - each allele, one number each would match the actual paternal/maternal sibling.

Further example - dad has XdY for the sex pair, mom has X1X2; son would be either X1Y or X2Y; daughter XdX1 or XdX2. So daughter and son may or may not have the same X from mom.
If they do not - their child (son) X1Y or X2Y; if they do, say it’s X1, their son is X1Y
For daughter, the combination if the siblings have the same mother’s X (say X1) will be XdX1 or X1X1; for different, say son gets X2, the child is XdX2 or X1X2

As you can see, the combinations become complex. The number of possible combinatoins become combinatorially large, so the odds of statistically hitting the right jackpot so the baby appears to be offspring of the (grand)parents are still in the realm of “unlikely, pretty much impossible”.

Thank you. I kind of knew that, but lacked the background to explain it except in the very basic brown eye blue eye examples you get in high school science. It was quite a few years ago now (8 or 9 maybe) I got in an argument with someone about this insisting that while the Baby might LOOK like the child of Mother and Father, genetically they would not be the same as Boy or Girl, or any other siblings… ie demonstrably NOT the genetic child of of M and F.

I don’t even know that person anymore, but nyah, Im right. (sort of)

Some of these combinations will also be apparent by having the wrong number of heads.

Be thinking anything over 1 (one) head is going to be a tip off that something hinky is afoot.


To put that a bit more tactfully and accurately, offspring of siblings will very often have expressed genetic disorders, much more so than their parents. That fact alone should be enough to convince one that such a child of incest could be distinguished genetically.

I was very tempted to put in my question “Do NOT need answer fast” but I thought GQ was a little more serious than that.

I was also going to call the family “the Sixtoes” but refrained.

Yes, the child of siblings will have identical alleles at 25% of its loci (or rather about 29% if, say, 5% matching by “chance” is normal). Marriage to cousin or niece is much less damaging genetically but several instances in a pedigree can add up.

As a genealogical hobbyist, I’ve come across interesting royal pedigrees. Among the Spanish King Carlos II’s 8 great grandparents, no less than 6 were Habsburgs. His nickname was El Hechizado; from Wikipedia:

Shortly after the time of Carlos II, the House of Habsburg went extinct.

The 19th-century Bourbon King of Spain, Alfonso XII, also has an official pedigree making him as homozygous as the child of siblings. (The good news is, that’s only his “official” pedigree. :cool: )

The Austrian fellow who had 6 children by his daughter - it seems except for 1 who died at birth, and perhaps 1 that had some medical problem (epilepsy? She had seizures) the rest were somewhat normal. A father-daughter match would make as many chromosome matches statistically as a sibling match.

After all, most purebred animals are created by intense interbreeding of a narrow range of stock. The only disadvantage to this process is that often deleterious traits are ignored to produce better results in the desired traits. One article I saw on modern pig breeding suggested that pigs used to be about as smart as dogs, but modern breeding has produced huge meat-factory animals too dumb to avoid rolling over on their own offspring. Similarly IIRC many dog breeds are notorious for hip problems, if heavy running is not in their raison d’etre.

The big danger of inbreeding, as Septimus points out, is when you are starting with a relatively closed collection genes - like in an isolated village (or royalty) that has been mostly inbreeding for generations. Add to that the problem that agricultural life, and now modern medicine, tends to preserve genes that would likely die off in a more subsistence environment.

Many genetic problems come from the inability of a defective gene to produce a necessary chemical; hormones and other essentials of life are created usually in a step by step process from more basic building blocks; miss any step and the process fails. Usually this is masked if one of the two genes is capable working correctly, so a defective gene may pass through many generations without creating a problem. When it is paired against a similarly defective gene - easiest through inbreeding - then neither gene produces the right product, and the process fails.