Siblings are the only people who share 100 percent of your lineage. You of course are 50 percent different from both your parents and kids in that regard, but identical to your siblings. Does that mean that genetically they are closer to you than your parents and even your own kids are?
People aren’t usually 50% mum and 50% dad. The percentages can be all over the place. That’s why, with some siblings, one resembles the mother and another resembles the father.
IANA geneticist but I have seen the outcome of a number of DNA tests designed to determine whether or not relationships exist between alleged family members. Alleged family-child relationships usually come back with either (paraphrasing here) “yes, they are almost certainly parent and child” or “no, they are almost certainly not”. Alleged sibling relationships are often a lot more equivocal.
Yes they are. Ignoring a minor few freak events, a person is exactly 50% mum and 50% dad.
No, that’s just flat wrong. Genes interact in funny ways, so depending which copy of each genes you get from each parent one or the other may dominate, but children will always get 50% of their genes from each parent.
Anyway, to address the OP, while siblings share one average 50% of their genes, the amount varies between each set. It is possible to have non-twin siblings that share 100% of there genes and it is possible to have siblings that share absolutely no genes at all.
So on average you are precisely as closely related to a sibling as to your parents. However while you always share 50% of your genes with each parent, you may share 100% of your genes with your sibling or you may be completely unrelated or anything in between. It’s totally random.
This is technically correct, but there is a distribution to consider. Random in this case doesn’t mean “you are just as likely to have 53% of your genes in common with a sibling as you are to have 2%.” Actually, the former is much more likely than the latter.
If you inherited a rare allele, there is a 50% probability you inherited it from your father, 50% from mother, 50% you’ll pass it to your child, and 50% chance you share it with your sibling. Thus parent, sibling, child all have the same 50% consanguinity as shown in this chart.
If you flip a coin 10 times, you could easily get 70% heads, but if you flip it 150 times, heads will be close to 50%. With 23 chromosomes participating in human meiosis, times two parents, times about three crossovers per chromosome, you will have very close to exactly 50% of the rare alleles that your sibling has.
Of course, IIRC, because for every single gene you take one allele from your mother, and one from your father.
So straightaway, if a candidate father has genotype DD for a gene, and the child has dd, he’s either not the father, or a mutation has taken place. A few differences like this and we can be confident he’s not the father.
Whereas if two potential siblings have dd and DD, that, in itself, just tells you the parents would have to both be Dd (or a mutation has taken place).
So I would expect potential sibling relationships to be more equivocal.
(Of course, if you also have the parents’ DNA then you can remove that uncertainty, but your observation implies we’re talking about situations where we only have the siblings’ DNA, since it would be very strange if we could easily prove two people have the same parents, but could not prove that they were siblings :))
Of course, in cultures which historically practiced cousin marriage and other forms of inbreeding, siblings are going to share more than 50% of their genes, on average. I think among some South Indian cultures in the 1960s it was closer to 52%, although that has probably (hopefully) declined with increasing westernization.
Blake has the best answer so far. I just have one wrinkle to add. If a woman had two babies, in different years, from different fathers, then those two babies would share less DNA with each other than they do with their mother. Each of them would share ~50% with their mother, but it wouldn’t be the same 50%, so (assuming the fathers are unrelated), they’d only share 25% with each other. Given the fact that we almost always know who the mother is (and that’s kinda our definition of “sibling”) but we don’t always know who the father is, I’d say that siblings, on average, would be slightly less related to each other than they are to their mother. Put it another way, if you inherited a rare gene, the chance that your mother also has it are 50% but the chance that your sibling has it is between 50% and 25% depending on whether you have the same father or not.
True, but IIRC rates of false paternity are lower than we thought a few years ago, closer to 2% than 10%.
If false paternity rates were 10%, average relatedness of siblings would be around 45% I guess.
Just an anecdote, I grew up with two brothers born about 2 years apart that had a remarkable physical resemblance. I first met them when they were about 11 and 13 and looked very similar but not unusually so for brothers. When I last saw them they would have been about 18 and 20 and had for several years been taken for twins. They were more similar in appearance than some identical twins I’d seen at that age. They seemed to be identical in many small details, the one they pointed out was a funny whorl in their hair, they also had patches of freckles around their eyes that were very similar, altogether even for brothers the resemblance was uncanny. They must have had a very close set of genes that affected their physical appearance though I assume that might be a small part of the whole set. There was some general resemblance to their father I recall he was somewhat taller then they were. I suppose in theory a pair of siblings born apart could have identical genes, even if that is very rare large overlaps must occur frequently, it’s often easy to tell a pair of siblings just by physical resemblance.
Coin flips are IIRC a binomial distribution. The same might be said for the distribution of 23*3 chromosome arrangements. It’s a coin flip - mother or father?
Using an online binomial calculator, the odds of 50-50 (35) are about 0.095, of 36/69 being shared from one parent, 0.09, and 37/69 0.08 - it starts to fall off very quickly. the odds of 23/69 is 0.002 and the cumulative odds (ie. 23 or less in common, or 46 or more in common) is 0.0038 - basically, the odds that a pair or random siblings only share one third or less (or to flip, share two thirds or more) genetic material is 4 in 1000.
So in other words, my brother and I are each 50% mom and 50% dad, but that 50% may not overlap very much- i.e. he and I may have 10% of the genes from Dad in common, and 90% of Mom, even if in total, each parent accounts for 50% of our genes.
Personally, I don’t regard being male as a minor freak event.
A male human has 3,079,843,747 base pairs: 2,867,188,341 from pairs 1-22 (equally from each parent), 154,913,754 from the X chromosome (exclusively from the mother) and 57,741,652 from the Y chromosome (exclusively from the father.)
So male humans get about 51.5% of their base pairs from their mothers and 48.5% of their base pairs from their fathers.
The answer for the OP: No. If you’re male, odds are you share the most genetic information with your mother, then with brothers if any, then with your sisters if any, and finally with your father. (A 50% chance of your sister having received the same X chromosome from your mother is more pairs on average than a guaranteed match of the Y chromosome to your father.)
If you’re female, you share the same amount from both parents and sisters, and less than 50% with any brothers. (However, from the opposite perspective, you have more than 50% of your father’s genetic information.)
Wait, I think there’s one instance where a sibling would on average share more base pairs - when a male has a son and a sister, I think the son’s aunt would on average have more pairs in common, as the aunt’s 50% chance at having the same X that dad got from grandma is more average pairs than the guaranteed Y from dad and no chance at an X from grandma.
Oh, and an uncle would be even closer to dad than that.
But as I said, the odds that you would even have only 1/3 or less in common (instead of the average of 1/2) is fairly remote… about 4 in 1000. To pick up on Septimus’ point, the odds that you’d flip a coin 69 times and get only 23 or less Heads, is about 4 in 1000. (23 chromosomes, 3 crossovers per, Heads= bro got that one same as me, tails he got opposite)
The binomial distribution is Bell-curve, the edges beyond the center probability drop off very quickly.
I don’t think this works. 3 crossovers on average per chromosome is not the same thing as saying there are 3 possible arrangements for each chromosome.
Calculating what the odds are when we allow random-sized portions of DNA to come from either parent is beyond my abilities, but I’d be happy to wager the chance of getting less than 33% commonality between siblings, or greater than 66%, is far less likely than 4 in 1000.
Siblings being closer related than parents or children has nothing to do with genetic makeup. It’s about familial relationships. With the exception of each other, you and your full sibling have the exact same relationship with everyone you’re related to. Bob’s your uncle? He’s your sister’s uncle too. Zaphod’s your semi-half-cousin? He’s your sister’s semi-half-cousin too.
Yeah, these would often be cases when one or both of the parents were missing or dead.
I had a really interesting case recently which involved testing an aunt’s DNA against two nieces and two nephews, all of whom claimed to be siblings of each other. When the test result came back, it found that two of the children probably were her niece and nephew, one probably wasn’t and the fourth simply couldn’t be determined one way or the other. We then asked for the siblings’ DNA to be tested against each other. Result: all four probably were siblings, although for one of them the probability was somewhat lower than for the others.
You can’t judge a thread by the title alone. The content of the OP makes it clear this about genetic relatedness.