Genes and Family Resemblance

Factual Questions
1

Hello, and welcome to another edition of “When Ingorance Attacks: The Dangers of Letting People Who Don’t Know Anything About Science Think About Science,” hosted by me.

So my first cousin just became a father and once again, both our family and his wife’s family gathered around and said “wow, no one’s ever going to question who that baby’s father is!” because the little tyke looks like a “Mini Me” version of my cousin (and also because we do tacky things like pretend to speculate on the parentage of newborn infants). This is one of those family lore things – as an extended family, we seem to have a strong “look.” This has been observed by other, non-family members, oftentimes in-laws who are perplexed as to why their grandchildren resemble our side of the family so strongly. It’s the sort of thing where “little Suzy is the spitting image of her mother … but I guess you can see a little of her dad around her eyes” so I’m not claiming that anyone looks like an actual clone of one parent, but more of a predominating family resemblance.

In an other example, my brother and I recently took our father’s cousin’s five year old son … is that our second cousin? … on an outing. Two people (like random passers by) commented to my brother that “his son” was a chip off the old block, because they looked so much alike. The kid’s actual father wasn’t with us, and if he was, I think people would have been much more likely to guess that he, not my brother, was the father, although he and my brother do look more like siblings than cousins.

The source of “the look” is my paternal grandmother and her siblings, who all strongly resembled each other. So, if my calculations are correct, this little boy and my brother only share two of fourteen great-grandparents.

So I’m pondering why this is. Here is what I came up with. Could you Dopers who actually know how human genetics work shed some light on what’s going on?

  1. The possibility that we’re vain, self-centered people who like to think we all look alike, and impose this view on the way we see each other. We see “the look” because we’re expecting to see it. Strong possibility here.

  2. We’re secretly inbreding. :eek: I would like to think this is not the case, but I suppose I have to add it to the list for the sake of thoroughness.

  3. Even though we’re a somewhat large family, however in terms of the general population, we’re not talking about a statistically significant sample. The roll of dice in whether one takes after mom or dad is what’s at play, and even though it might be somewhat unusual in the small view, the larger picture isn’t really unusual at all.

  4. I don’t know enough about genetics to explain this one well, but another factor here is that (as of yet) no one has partnered with people who are from very different ethnic backgrounds as our own. So I’m wondering if maybe the elements that make up “the look” are fairly common within this ethnic background, and that “the look” isn’t really our family’s look, but more of an amalgam of features typical of our background. For example, going away to college is a fairly new thing in my family, so I think in the future people would be more likely to meet and date and marry and reproduce with partners from a greater variety of backgrounds.

In a larger sense, I guess I’m asking if some genes are more likely to show themselves than other genes. I feel a little creepy saying there might be “strong genes” because that sounds a little … well, master race-ish, and I don’t intend it to be that at all (and as far as I know, no one would design a master race with the big, horsey teeth that we’ve got going on). I know the high school bio class basics about recessive genes, like how two blue-eyed people won’t have a child with brown eyes, but that’s not quite what I’m talking about. By the way, the blue eye thing is always the example of a recessive gene – are there other genes in people that will always trump their counterparts? I’m thinking about genes for characteristics that are harmless, like curly v. straight hair, as opposed to a recessive gene linked to an inherited disease, for example.

2

I didn’t want to slice and dice your post in my reply, so I’ll just get right into it.

Why children resemble their parents was one of the great questions of the ages for many centuries.

The nut finally began to develop some cracks in the 19th century, with the work of Mendel, a religious personage who did very controlled breeding experiments that confirmed the very point you shy away from at the end of your OP: some traits are simply stronger than others.

The "somethings’ governing each trait he called genes, and he figured that genes are somehow passed on from the parents. Since some of the offspring of his mated pairs showed traits that neither of the parents had (but some of their grandparents had had), he eventually surmised that for each trait gene, we get one “allele” from each of our parents, and the dominant one wins out. It can get a lot more complex than that depending on what trait you’re talking about (not to even mention mutations and sports), but that’s the basic gist of it.

Mendel had no way of researching into biochemistry to the point where he could describe just how genes create traits in a concrete, physical way, so his work went somewhat neglected for a number of decades until the rest of biology caught up. Eventually the location of the genes was narrowed to the chromosomes in the nucleus of the cell. The work or Watson, Crick and others in the mid-part of the 20th Century led to our current understanding of the situation:

Chromosomes are made of DNA, which consists of molecules of 4 different chemicals strung together in long, helix-shaped polymer chains. As far as genetics goes, the chemicals in DNA work in groups of three. With four chemicals, there are 64 possible triplets. Watson and Crick found that there is one triplet that signals the start of a gene, antoher that signals the end. Of the rest, many triplets are associated (through a complex chemical interaction I won’t even begin to go into here) with amino acids, chemicals whose molecules get strung together in a cell to form the very important chemicals, proteins.

Each triplet that is associated with an amino acid is only associated with one.
Some amino acids are associated with more than one triplet.
Some triplets are not associated with any amino acids, and have acquired the reputation of being “genetic junk”. (More on this later)

What happens, basically, is that a “gene reading system” (really a complex series of chemical interactions) finds the start triplet somewhere on a chromosome, and reads along the DNA molecule, triplet by triplet, with the amino acids from each triplet being added to a protein in progress. the protein keeps getting strung together until the “stop” triplet is reached, even if some of the triplets in between have ben “junk”. The completed protein then goes off to do whatever it does.

In the late 1980s/early 1990s, it came to be realized that the technology existed to try and map “The Human Genome”, that is, to find the locations of all the “start” and “stop” triplets on all the chromosomes, and catalogue all the triplets in between. Funding was acquired, the project was attacked, and great leaps in technological advancement during the 1990s meant that we have a First Draft many years earlier than originally planned.

The science community now knows that we have several tens of thousands of genes. However, the percentage of those where we actually know what proteins they code for is rather small. There are mountains of data, and the emerging biology-computer science hybrid field, bioinformatics, is beginning to tackle it. Compounding the difficulty is this: while it is known that the presence of “junk” in the middle of a gene will not stop its expression, what came as a complete surprise to the Human Genome Project (HGP) folks was this: if the gene is re-engineered to omit the “junk”, the protein will not appear. the “junk” does SOMETHING, and the field of biochemistry is at absolute square one in figuring out just what. The HGP, meant to be the pinnacle achievement of biology, has turned out to be the first baby step.

Which is all a roundabout way of explaining that there is a hell of a long way to go between where we are now and knowing precisely, chemically, why the specific traits of your family’s “look” are so dominant.

Some thoughts:

  1. It is thought by some that your traits are largely governed by how your specific body chemistry reacts with its surroundings during development. Genetics sets the chemical stage, and the rest is more or less pre-determined by physics.

  2. Studies I have read about in the last 10-15 years looking into the nature of pair-bonding (AKA falling in love). Across species, pair-bonded couples share a very surprising correlation is physical traits, very subtle things like proportional distances between features on the face (distance between the pupils relative to the distance from the pupils from the prominence of the cheeks, wierd crap like that). In other words, couples who may not look too much alike often do in very hard-to-notice ways that they are apparently picking up on sub-consciously.

So your relative’s tendency not to marry too far afield ethnically could have a lot to do with how widespread the proportions of your “look” are. The alleles for many traits of the look may be present in both parents, and may be domininant in your family. Hence the look perpetuates itslelf.

Hope this explained some things.

3

It may have as much to do with the way the human brain processes information when recognizing human faces as it does with any genetic relatedness. There is a part of the brain that specifically deals with recognizing faces and it probably weights certain facial features differently when determining likeness. It may also be less sensitive in perceiving subtle differences in other features.

4

i think i have an odd family. I’ve been told ever since i can remember that i look exactly like my dad. and know that i am older that i act like him and have the same mannerisms. Pictures of us look the same, the only difference are a lot of his when he was younger are in black and white. ANd my aunt looks just like my grandmother. its really quite scary when you look at it

5

Thank you very much for the thoughtful insights! I am intrigued by this idea that we can pick up on the less obvious facial characteristics, and that this might factor in to our choice of partners. I am now tempted to carry around a pair of calipers and try to measure people when I met their new boyfriend or girlfriend. That wouldn’t be too socially awkward, would it? :wink:

6

It’s important to notice, however, that the mixture of genes from the two parents and their ancestry is likely to have several effects:

  1. It may tend to make a given child look like a younger carbon copy of one parent.

  2. It will mix-and-match genes from both parents to produce a unique individual with elements, “good” and “bad,” from the combined ancestry.

  3. It may express a recessive gene having nothing to do with either parent, and possibly going far back in the ancestry to discover a parallel.

Of my “son’s”* three children:

a) The oldest is a girl who very closely resembles her mother in a wide variety of characteristics, hair color being the most obvious, but who as she reaches puberty is displaying her father’s height and body build, and who has always had his “sharper” features – her mother having a beautiful, expressive face but one that has since she was little been described as “like a cocker spaniel” while the daughter has sharper, model-like features.

b) The older boy looks precisely as his father did in childhood pictures, and is even going through the “pre-puberty pudgies” as his father did – gaining a touch of excess weight on what was a lithe slender boy body through most of childhood. The one touch he has from his mother is her most beautiful feature, IMO – she and two of her brothers have enormous, beautiful eyes – the real-life equivalent of the Big-Eyed Waif school of kitsch art. This does not appear to have been an expressed family characteristic before them, but the three of them, and at least four kids from the next generation, have it.

c) The younger boy looks very much like his father as well, but with characters randomly assembled from both ancestries, and appears to be headed for a body build like his youngest maternal uncle – slender almost to emaciation, but nonetheless muscular, what’s often referred to as a “wiry” build but without the obvious muscular definition associated with that term. He is also extremely cute, and knows it, and has a charisma that enables him to pull off what would be conceit in another child.

Needless to say, I’m prejudiced about my “grandkids”* – but this may serve to illustrate how characters can come out as “spittin’ image,” mixture of parents, and recessive-expression in a real-life instance.

  • Note: My “son” was my ward, and is of no blood relationship to me. The only genetic relationship is something we discovered through genealogy years after forming the relationships – his wife and I share a great-great-great-great-grandmother, adding two -greats- on her side.
7

I think the easiest way to test whether your perceptions are due to bias is to simply take age-matched photos of parents and children and show them to individuals who do not know them. Mix in some photos of unrelated children, or related children you do not think resemble a given family member, and ask the observer if they see the same similarities/differences you do. Chances are, it’s not your imagination. I’ve seen plenty of people grouped together who I knew almost immediately were related, even though I was meeting them for the first time. Human brains are good at pattern recognition, and it’s likely the ability to recognize likenesses of the kind you describe is a highly-selected for trait, as it has obvious repercussions in breeding.

Some of the genes and linkages dictating things like hair color and eye color have been well-described. Some physically-obvious traits have not been described at all. The thing is, simple Mendelian models of gene distribution among offspring do not take into full account of the linkages between some traits; and actually, it’s because there are these deviations from naive predictions based upon simple crosses that we can predict quite accurately that certain traits are determined by genes that like physically close on a particular chromosome to other known genes, even if we know nothing else about that gene.

Gametogenesis involves the process of meiosis, and before the pairs of chromosomes are divided in half among the gametes, the chromosomes are sort of stuck together, and their DNA can be swapped from one member of a chromosome pair to the other. This process is imaginatively named “crossing over” and leads to genetic recombination, and further adds to the variability of the genetic mixmaster that is sexual reproduction. Some genes that are physically close together (tightly linked) tend to swap together, so they don’t get as jumbled-up as one might expect. Hence, they can show up in better than 50% of the offspring even when simple crosses would predict a more even distribution. Add to that the issues of dominance (the ability of one allele to phenotypically mask the presence of another allele) and penetrance (the extent to which the trait is genetically, versus environmentally determined), and it’s easy to see how some genetic traits can show up very strongly in familial lines. It sounds like there are some closely-linked, dominant genes derived from your paternal grandmother that give rise to a short list of easily-recognizable, if not obviously associated, physical traits.

8

ok this is getting a little interesting. I have a question. Let’s relate this to mandelian genetics and breeding. Now i know with plants, that when you cross two plants you are going to end up with lets say for this arguments sake four different phenotypical offspring. Ones like parent x, ones like parent y and crosses of the two. But people have a much larger genetic makeup, that i would imagine is rather intercrossed, so wouldnt you have typically more numbers of phenotipic children, lets say that you had 10 kids, there should be wide variety between the two of them, as oppsed to planting 10 seeds that will produce 4 of one phenotype, 2 of another and 1 of the other two. I forgot where i was going with this :smack: