Your Uncle Oscar speaks in grunts and always smells of spoiled herring. Your cousin Nellie never stops smiling and reads nothing but religious tracts. Your 80-year-old paternal grandfather has an eyepatch, leers at your mother, and keeps the mummified hand of an SS officer in a trunk in the attic.
For years, you prayed you were adopted.
You couldn't really share blood with this mixed bag of loonies and creeps your parents insisted was your family ... could you?
Years later, at a family reunion, you finally met your third cousin twice-removed Valerie from California. You hit it off almost immediately. She had a smile like a sunrise, and she was everything the rest of your family was not. In short, she was hot.
That night, you couldn't purge the impure Valerie-thoughts from your mind, hard as you tried. She's your cousin for pity's sake ... if the two of you had kids, they'd be sure to have 14 fingers or two heads or an extra eye or something.
Or would they?
Unzip those genes and show us what you got, Baybee
People have argued Nature vs. Nurture since Crick annd Watson figured out that DNA is responsible for our heredity. Your genes are the recipe for you. The food you eat and the environment you grow up in can improve or wreck that genetic recipe, much as using unripe cherries and moldy flour would wreck a prize-winning pie recipie, but using fine shortenings and flour and just a little extra sugar can make an ordinary cake sublime.
A baby with a pro NFL player's genes isn't going to be any kind of athlete at all if he or she gets exposed to thalidomide in the womb at just the wrong time. If you separate two identical twins at birth, and send one to live with a kindly adoptive family in Marin County, CA and the other to an angry alcoholic foster mother in Stripmine, WV and check back in 30 years, you'll probably find that the California twin tests as having a notably higher IQ than the West Virginia twin. But at the same time, you'll probably find that the pair have truly eerie similarities when it comes to personal likes and dislikes.
Nurture counts for a lot, and being in the same family with someone often means you've gotten the same kind of nurture (or lack thereof). But the genes are always a powerful factor in who we are; one gene can mean the difference between having blue eyes or brown, getting cancer or not.
And many evolutionary biologists such as Richard Dawkins (author of The Selfish Gene) think our genes are the root of why we care for family more than our neighbors, and care for our neighbors more than random strangers. The Game of Life is all about getting your genes into future generations, and so you have some stake in ensuring the welfare of those who share your genes: your immediate family, or failing that, your tribe.
Tell me about your mother ....
When a person makes eggs or sperm, half his or her genes get parceled out in fairly random fashion into each gamete. And every child gets half his genes from his mom, and half from his dad.
So, you share 50% of your genes with each of your parents, right?
Well, probably. It depends on whether or not your parents have genes in common; if they're from the same ethnic group (for instance, if they're both blonde, blue-eyed Swedes), they almost certainly have some DNA in common (aside from the DNA that all human beings share as a matter of course1). If a tribe of people settles in an isolated valley, after a few generations everyone in that tribe is going to be related to everyone else, even if everyone's careful not to accidentally marry a first cousin or uncle; many ethnic groups developed in this kind of genetic environment. If your parents have some genetic relation to each other, then you could have more than half your genes in common with one or both of your parents.
But, for the purposes of this article, we'll go with the 50% figure.
Now, let's talk about your brothers and sisters. Full brothers and sisters share the same parents, and because of the random dispensing of genes, it's generally thought that siblings share about 50% of their genes with each other.
Things, of course, aren't that simple.
If your mom is unusually homozygous -- in other words, she got mostly the same genes from both your grandparents due to accident or inbreeding -- then she will be passing out pretty much the same genetic set to you and your siblings. So no matter how varied your father's genes are, in that case you and your sibs would have somewhat more than half your genes in common.
If both your parents are largely homozygous, then you and your sibs would have lots of genes in common, even if your parents are from very different ethnic groups. Picture a scenario where a petite, buxom lass from a long-isolated village in Ireland meets and marries a tall thin man from a long-isolated village in Nigeria; you'd have two very different people who'd have kids who all looked a whole lot like each other.
Conversely, it's possible through the magic of random assortment for two siblings to get different genetic halves from each parent. It's rare, but it can happen; in such a case you'd share relatively few genes with your brother and sister. Got a brother or sister you seemingly have nothing in common with? Don't blame the postman; it could just be one of those things.3
And if you have a half-brother or half-sister whose father shares genes with your biological father, the two of you will have more in common than most half-sibs.
If you're half of a pair of fraternal twins, it's possible (though unlikely) to have different fathers; however, you two would likely resemble each other more than other half-brothers or half-sisters because you shared the highly-influental prenatal environment.
And to complicate matters further, there's always the chance of mutation to give you genes that none of your relatives have.
I Lost My Number ... Can I Have Yours Instead?
With the previous caveats in mind, here's the breakdown of the percentage of specific gene variants you probably share with various relatives (as opposed to genes you might share with any random stranger of your ethnic type on the street) (with possible deviance in parenthesis):
- Great-grandparent: 12.5% (possibly a bit more, possibly a lot less)
- Grandparent: 25% (possibly more, possibly less)
- Parent: 50% (possibly more if your parents share genes, but not less ... unless you're a mutant)
- Identical Twin: 100% (but maybe less if one of you mutated after your embryos split apart in the womb)
- Fraternal Twin/Full Sibling: 50% (possibly more, possibly less)
- Half-Sibling: 25% (up to 50% or so, possibly less)
- Aunt/Uncle: 25% (maybe a bit more, maybe less)
- Niece/Nephew: 25% (maybe a bit more, maybe a lot less)
- First Cousin (child of your aunt/uncle): 12.5% (maybe a bit more, maybe a lot less)
- First Cousin Once Removed (your first cousin's kid)2: 6.25% (maybe a bit more, maybe a lot less)
As you can see, unless your ancestors have significantly shared genes (and provided your more recent relatives haven't been going nutty with cousin marriages), once you're at the level of second and third cousins there's not much blood relation there at all. (Scroll down for a chart explaining cousin relationships)
So, if your grandfather really freaks you out, you can sleep safe in the knowledge that you really might not share many of his specific genes at all. Unless he's your father's father and you're a guy; then you're definitely stuck with his Y chromosome. Alas.
But Wait! There's More: The Seven Brides For Seven Brothers Family Effect
So what happens when the girls of the Walters family marry the boys of the Ruiz family? Obviously, this results in cousins who are far more related to each other than usual, and inbreeding isn't a factor.
In such a case, first cousins would share 25% or even more of their genes, though aunts and uncles would still be only 25% related to nieces and nephews and vice versa (however, the immediate in-laws would also all be blood relatives to the nieces and nephews). First cousins in this family would would share a genetic similarity much closer to that of brothers or sisters. Second cousins would be more like first cousins genetically, and so on down the line.
1: Genetic disclaimer: all human beings share the same genome. You've probably heard the figure that chimpanzees share 98% of the genes humans have, right? That's at the genome level. This article is talking about gene variants and simplifies a good number of things; large texts have been written on human genetics and this article should not replace the information gained from said textbooks, nor is it intended as a replacement for advice from professional genetics counselors. If there are genetic diseases in your family history, and if there's no reliable test for them, having kids with anyone you think you're even slightly related to is a bad idea. If illnesses like Tay-Sachs disease or sickle cell anemia run in the family, you should probably get genetic testing if possible no matter who you marry if there's any chance you'd have children.
2: On Cousin Relationships: I earlier had this level of relatedness listed as second cousins; my error, since my first cousins once removed had always been presented to me by my mother as second cousins, and I've never had much extended family to keep track of. Below is a handy chart I grabbed from Wikipedia, with slight text alteration. The original colors remain, presumably for contrast; I don't think this was intended as a "green means go!" type thing. Marrying first cousins is illegal in about half of all U.S. states (see this page for a state-by-state list), and is socially discouraged in most others.
However, some scientists argue that first cousin marriages are perfectly okay; see "First cousins face lower risk of having children with genetic conditions than is widely perceived" from the University of Washington for details. And it turns out that third and fourth cousins who marry (remember Valerie?) may be more fertile than unrelated couples; see ""Why cousin lookin' fertile....?" for more. (Although increased fertility is generally equated with good health and a good match, whether or not taking advantage of it is actually a good thing in the grand scheme of things -- given how badly the booming human population is stressing the environment and natural resources -- is a matter of debate).
Anyhow, here's the cousin relations chart:
3: On Sibling Resemblance: This is supposing your brother or sister doesn't have physical traits -- like a drastically different skin color -- that don't match those of your parents and which can't be the result of a recessive gene or a mutation or birth defect etc. Bear in mind that traits like red hair, curly hair, and blue/green eyes can skip generations. Kids sometimes don't resemble their biological parents at certain ages, which is part of why paternity testing is such a big business.