Spitting image equals spitting genes?
July 29, 2022 9:53 PM   Subscribe

If a child looks just like their grandma Jean and grandma Jean has something genetic like the breast cancer gene, is the child likely to also have the breast cancer gene?
posted by cda to Health & Fitness (8 answers total) 2 users marked this as a favorite
I have very minimal understanding of genetics but I do look almost exactly like my mother and my (maternal) grandmother. So much so that when I come across photos of either of them when they were younger I often think “I don’t remember being there/that other person/that dress” and it takes everyone a good few minutes to realize it’s my mom in a non-vintage 70s pantsuit or my grandma hanging out in upstate New York in a summer dress. There’s a picture of my mom when she was a little girl in a swimsuit and both of us thought it was me until my grandma said it was my mom and pointed out how her older sister wore that swimsuit in older pictures before it was handed down. And don’t even get me started on how every adult who has met my mom before me remarks on how they know who I am just by how I look. It’s tiresome, to say the least.

Anyway. As I age, I still look very very much like my mom. But health wise I’m shaping up to be just like my dad. Mental health challenges, high blood pressure, last month I had a kidney stone! My dad has some issues with his hearing and it’s becoming clear that I will also have similar issues - unlike him I’m not stubborn about assistive technology so maybe we can have dueling hearing aids in a few years. My shape and some specific stuff like weak ankle joints I get from my mom, but it’s like all my internal organs are from my dad’s side. And I’ve developed adult onset allergies like him - even to the same things, which I thought was environmental!

In my minimal understanding of genetics, some things are linked and other things aren’t. Like, genes for appearance don’t always link to genes for internal systems. Certain indicators of health are genetic but that doesn’t mean because I have teeth like my mom that I’m going to also have heart palpitations like my mom. There are some conditions that cause people to look morphologically similar and have similar health challenges, like say, dwarfism, or how very fair skinned people must be more careful about protection from the sun, but the list of benign things science has figured out indicate other health concerns is pretty small, and the list is even smaller if you just count things we can see without blood tests and similar.

This being Mefi I’m sure a genetic biologist or someone similarly qualified will come in and give a thorough cited answer. My anecdotal answer is “nope”.
posted by Mizu at 10:40 PM on July 29 [8 favorites]

Hard to prove a negative, but in all my years of reading about the topic I have never seen it posited that the genes for appearance and the genes for breast cancer are linked.

However the genes for breast cancer are very definitely linked to breast cancer, and a family history is one of the larger predictors of it. So the child should be keeping a special lookout regardless.
posted by Tell Me No Lies at 10:58 PM on July 29 [1 favorite]

I don't think this can be true at all since my brother and I are alike as two peas, we both look just like my dad and yet my brother takes after my mom medically and I don't.
posted by Frowner at 11:01 PM on July 29

Genes tend to stay together if they're on the same chromosome, but people have twenty-three pairs of them.
You get half of them from each parent, but without knowing which genes are on which one it's impossible to say what the chances are that you'll get any two traits, or that they'll tend to stay together.
Of course humans don't have much genetic variability (I believe we share more than 95% of our genes), so if you don't get a trait from one parent you may get it from the other. I also suspect that our brains are very good at spotting familial similarities, and even if you only have half the genes that influence appearance, or a quarter if they're from a grandparent, people will still very often be able to see that you're related.
Most things are controlled by a number of genes, and it's unusual to have just one that can be identified as causative for a specific thing. Quite often people with a particular condition have more or less of the associated genes, rather than having or not having a specific thing. Even if that's the case, and it's pretty rare, there are other factors, genetic and otherwise, which allow you to deal with it more or less effectively.
In any case, the best we can say about genetics is that while we understand a lot about the overall mechanisms, the details are not entirely clear.
Nonetheless, doctors usually assume that having a parent with a condition or a predisposition to a specific problem makes it likely in the next generation, and most medical exams include questions about what conditions your parents had.
posted by AugustusCrunch at 11:18 PM on July 29 [3 favorites]

No link between appearance and BRCA (I assume this is the "breast cancer gene" you mean)

More on BRCA here. Note that there are actually two BRCA genes. There are also many non BRCA genes that can influence cancer risk.
posted by basalganglia at 12:10 AM on July 30 [4 favorites]

No, the genes are not linked at all. I hope someone who does genetics comes in here at explains how it works, the right way. I read a lot about genetics when I was a teen, because of horse breeding, but I have forgotten.

But for anecdata: I have inherited the BRCA mutation from my dad, and so has one of my sisters. We both look like our (different) mothers.
posted by mumimor at 12:32 AM on July 30 [1 favorite]

Broadly, no.

Humans have 23 pairs of chromosomes. Within each of those pairs, one chromosome came from each parent. This means both that (i) half of your DNA came from each of your parents, and (ii) half of each of your parents' DNA came to you.

The incomplete version of this which most people learn is that each parent gave me one of their two copies of each chromosome. By this model, from my dad I got e.g. the copy of chromosome 1 that he got from his mother, the chr.2 from his father, chr.3 from his father, and so on at random. And the same from my mum.

However, more random mixing goes on than this. During creation of sperm and egg progenitor cells, the two copies of each chromosome in that cell line up and swap parts at random. So what I actually inherited from my dad was a chromosome 1 that's a mix of the chromosome 1s that he inherited from his parents, a chr. 2 that's a mix, chr.3 that's a mix, etc. And the same from my mum.

Now, this swapping of chunks of DNA between chromosomes is entirely literal, it's a physical process. Imagine you have two editions of a book, and randomly grab e.g pages 1-12 from book A, 13-93 from book B, 94-102 from book A, etc, compiling a new book that's a mix of both editions. Two pages that started immediately next to each other have a strong chance of still being together in the new edition, because they were probably carried along together inside a bundle. But the further apart any two pages were at the start, the more likely it is that they were in different bundles, so the chances go up that they've been separated. So if you see that page 8 is from book A it feels likely that page 9 is from the same book. But if you see page 8 is from book A, it doesn't tell you much about which book page 300 came from.

Some genes are known to be linked to each other this way: they're physically close together on the same chromosome, making their separation via this mixing process rare. So if you inherit one you probably also inherit the other that's right next to it. However, a chromosome is *very* long and quite a lot of swapping goes on, so most genes on the same chromosome are not strongly linked together in this way.

(Roughly, this is part of how genetics was studied before we had affordable DNA sequencing technology: relative distances between genes were calculated based on how likely they were to be inherited together. This kind of statistical work needs hundreds or thousands of individuals studied over many generations to see the patterns, so it was mostly in e.g. fruit flies or microorganisms. The actual maths is more complicated than I've implied, as some sections of DNA are more amenable to being swapped than others, and the swapping process is very occasionally a bit messy, introducing errors like duplications or deletions. But the advent of sequencing technologies revealed that genetic maps made this way, although definitely not perfect, were impressively close).

All that said: the genome is huge and split across 23 chromosomes, and even within a single chromosome most of the genes are far enough apart that they don't have a strong linkage effect between them. So without knowing exactly what gene(s) you're talking about and their relative locations there's virtually no predictive power to say "I inherited [x] from my (grand)parent so I probably inherited [y] as well".

As a further thing to consider: most visible, variable traits in humans come from the combined output of a bunch of different genes spread across different chromosomes. Two similar-looking people could be the product of different sets of genes which happen to combine together (and with their environment: hormone exposure in utero, childhood nutrition, etc) to produce a similar output. So even if a gene partly responsible for appearance were strongly linked with some other interesting trait, looking just like a recent ancestor doesn't necessarily mean that someone actually inherited that gene from that ancestor.
posted by metaBugs at 3:39 AM on July 30 [48 favorites]

I think metaBugs has a given you a good scientific answer.

Here's a bit of anecdata. I look a lot like my paternal grandfather (photos of us at the same age are kind of scarily similar.) I look way more like him than my father did. My father actually looked like a bunch of people in his maternal line. I also look a bit like my maternal grandmother, again more so than my mother did. However, as I have gotten older, my medical issues seem to be most similiar to my maternal aunt, and we look nothing alike at all - people who saw her, and saw me and my mother (her sister) never guessed we were so closely related. I also have some issues of my own that no one else in my family (that I know of), ever had ... no family history for those that I know of anyway.

Also, what gets complicated is that so many of the older generations smoked, and did that for years - all my relatives did going back a ways (though they all gave it up when they got old). I never did, though I was exposed to second hand smoke growing up, which was not good. In terms of illnesses and health effects it is always a complicating factor to consider in their health history that is not in mine in the same way, and it makes it harder to consider health history in older generations because of that.
posted by gudrun at 7:10 AM on July 30 [1 favorite]

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