Opposite Siblings?
July 27, 2006 2:18 AM   Subscribe

What are the odds of a sibling inheriting the absolute opposite complement of chromosomes from their parents compared to another sibling? It has to be a pretty slim but not zero chance, right?
posted by geekhorde to Science & Nature (13 answers total) 2 users marked this as a favorite
When you're talking just about the 23 chromosome pairs, it would be 0.523, or approximately one in ten million. But there's things like mitochondrial DNA, though I'm not sure to what extent that manifests in the phenotype. I'm sure some biologist MeFite can say more about that.
posted by fvw at 2:31 AM on July 27, 2006

23 pairs of chromosomes, 50% chance of each - one in 8.4 million. But crossing over can happen so you're not ending up with the exact same chromosomes as are found in the parents' cells.
posted by edd at 2:36 AM on July 27, 2006

I'm pretty sure it's zero. Crossing over is random, so it's got to be very near impossible for all of the chromosomes to randomly exchange in such a way as to generate two completely opposite sets. Plus since exchange always happens, two hypothetically opposite chromosomes would still contain a mixture of maternal and paternal genes, so the children would still be a "mixture" of both parents rather than identical to one.

I'm not sure what happens if this process catastrophically fails, but I doubt the child would survive to term.
posted by Sianion at 6:11 AM on July 27, 2006

Remember, we're talking about 23 pairs of chromosomes -- meaning that (without getting into crossovers and mitochondrial DNA we're talking about 46 independent inheritance events, or 0.546, over 1 in 70 trillion. Low odds. But with mitochondrial DNA, some of those 70+ trillion combinations are impossible to achieve, and with crossovers, there are a countless number of additional possibilities that are unaccounted for, so it's more a pie-in-the-sky question than anything else.
posted by delfuego at 6:16 AM on July 27, 2006

Mitochondrial DNA is inherited only from the mother, so it doesn't really factor in here.
posted by needs more cowbell at 6:22 AM on July 27, 2006

edd, crossing over not just "can" happen, it does. The generally accepted measure in genetics is centiMorgans, a conceptual (rather than actual) distance related to the probability of a crossover occurring in meiosis. A lot of these are expected to occur over a genome length of 33 Morgans.

Let's say your complete opposite could be born to your parents (I dont' happen to think it could, but let's assume.) On the practical level, it would be difficult to test. At a particular marker your father will have two copies of an allele, one he inherited from his father and one from his mother. Let's say those happen to be the same, which is fairly usual (after all, there are only a few alleles across all humans at that marker). He must pass one or the other on to his offspring, so let's also say one sibling gets the allele he inherited from his mother and the other gets the one he inherited from his father. But they're identical alleles, so how would you know the difference?

Disclaimer, IANABiologist (but I am a genetic epidemiologist, albeit one who knows very little about genetics).
posted by methylsalicylate at 6:23 AM on July 27, 2006

Man, I know nothing about this stuff but it's fascinating...
*grabs popcorn, waits for debate to continue*
posted by sprocket87 at 6:36 AM on July 27, 2006

[smacks head] delfuego - totally got confused over all those pairs floating about. Quite right.

Meth: yeah, when I said 'can' it came across sounding like I thought it only happens rarely but I meant it to be taken as 'this happens, it'll screw the chances'.
posted by edd at 7:01 AM on July 27, 2006

although not exactly opposite (its hard to tell about things like height, personality, and a billion other things), here's a story of two twins who were born two two half-black/half-white parents... one twin is white, the other is black.
posted by ruwan at 7:08 AM on July 27, 2006

I've read that some points on the genome are more likely to be crossover points than others. Is that true, and would it shift the odds down again?
posted by Leon at 8:04 AM on July 27, 2006

There are indeed crossover hotspots. The mechanism is not completely understood. Repetition of combinations of base pairs (microsatellites) can discourage crossover, certain suppressing alleles at particular markers... these have mostly been examined on a case-by-case basis. Interestingly for people who like Dawkins's Selfish Gene theories this seems very of a piece with his ideas.
posted by methylsalicylate at 8:35 AM on July 27, 2006

Heinlein touched on this idea in Time Enough for Love. The siblings in the section called "Tale of the Twins who Weren't" required some relatively lightweight genetic engineering to get this done. If memory serves he also at least outlines the reasons such a thing is very unlikely if not impossible to have happen in nature.

I know this only sort of answers the question. The bottom line is that Mr. Heinlein outlines the odds, whys and wherefores in a more readable fashion than most of us will be able to.
posted by ilsa at 8:38 AM on July 27, 2006

During mitosis and meiosis, the chromosomes are torn apart and the DNA is duplicated. But during that process, it's temporarily broken into pieces. Eventually you've got twice as much finished DNA as you started with, and the cell reassembles it into twice as many chromosomes as it started with.

But none of the resulting chromosomes will be identical to any of the original ones. Every resulting chromosome will contain DNA from both original ones of that type.

Fact is, no two cells in your body have identical chromosomes.

Mitosis is normal cell division. Meiosis is the process of creating germ cells. A parent cell undergoes chromosome duplication, and then divides into two daughter cells which have 46 chromosomes each, but each of those immediately divides again to create granddaughter cells with 23 chromosomes.

In females, one of those four gets nearly all the cell mass and becomes an egg. The other three are discarded. In males, all four become sperm cells. Females only do this once a month (if they're not pregnant) but males are doing this constantly and producing millions of sperm cells -- and among those millions of sperm cells, there are no analogous chromosomes which are identical EXCEPT the X chromosomes.

Oddly enough, the Y chromosomes also are varied. It was recently discovered that the unique genes on the Y chromosome appear redundantly on the opposite wings of it, and during cell division exactly the same kind of chaotic reassembly takes place for the Y, mixing DNA from the wings of the parent (father?) chromosome for the child (son?) Y chromosomes.

So the chance that two kids would have no parental DNA in common isn't zero, really, but the odds against that occurring are so long that we can be all but certain it will never happen.
posted by Steven C. Den Beste at 11:31 AM on July 27, 2006

« Older Improving Creativty Video?   |   What charities are working in the Lebanon and... Newer »
This thread is closed to new comments.