Humans are diploid. We have 2 independent copies of every chromosome in each cell, one from each parent. They function as "backup" copies. If an error is on one, as long as the other one has a good copy of the effected gene, nothing happens. However, if parents are related, it becomes possible to receive two identical (or nearly identical) copies, and lose that backup. In that case, any error/mutations will cause problems.
this post was submitted on 05 Feb 2024
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Many birth defects are rare, and require 2 copies of a defective gene to show up. Most "normal" people will be carrying a few defective genes (out of thousands of pairs), but are fine because they have a good copy still working.
Family members tend to have similar genes.
The chances of you and a family member having the SAME defective gene are massively greater than you and some random stranger.
Thus any child would also have a massively greater chance of inheriting 2 identical copies of the defect.
I have a disease that is autosomal dominant (and absolutely sucks.) That means only one gene (the one I got from Dad) was needed to give me the disease.
Now I have 2 of these genes. Everybody does. You get one from your mother and one from your father. So I got the bad one from Dad, and a good one from my mother.
It's a super rare disease! If I nail somebody at random in the population, they have 2 good genes they can contribute. My one bad one and one good one flip a coin; our kids could have a bad one from me (and die young) or a good one from me (and be fine!)
But if I nail a brother who had the bad gene, there's a coin flip for both of us. Even if I give a good gene, he might give a bad one! Way more likely the bad thing happens.
(So, so grateful none of my brothers have it so I don't have to get all squicked imagining that but it makes my point.)
Not all bad things are as easy to see as my disease. Lots of them "hide" until you get two bad things. But it's more likely two siblings have similar hidden bad things, which makes it more likely the bad things will show up in their child.
I am a carrier of CFTR mutations. If I knock up a random woman theres a 1 in 80000 chance of the child having cystic fibrosis. If I knock up my sister theres a 1 in 16 chance instead.
I'm being pedantic, but if you know you're a carrier (and don't know the carrier status of your sister) then it is 1 in 8
This guy does the math before mating with sis.
You are correct, and i did realize this some time after posting I'm just too lazy to edit it.
Every person gets their genes from their mom and dad, however certain traits (defects, if you will), are present in everyone. Thats why we have mental illness and birth defects. However, you isolate a gene pool, and those bad traits get more and more focused. For example, their is a family known as the Blue Fugates who have been doing incest since the 19th century, and now the family has a blood defects that makes their skin blue.
Genes mutate.
If you have one copy of a mutated gene, it could be bad. However, one mutated gene is usually recessive or will be co-dominant with the other gene so it isn't that bad. If you have two copies of that gene, it can be a lot worse.
Two close family members are more likely to have the same mutated gene, so the odds of getting that same mutated gene twice are a lot higher.
Not enough genetic diversity.
Over time this causes mutations. Mutations aren’t always bad but when they are it’s really bad and causes noticeable defects.
You're on the right track, and others have explained it. Mutations happen regardless. The important thing is that because of redundancy between two copies, the effects of many mutations can be masked. The mutated alleles circulate throughout the population and only face selection when combined with another loss-of-function allele in the same functional unit (gene).
Those recessive mutations are more likely to be matched with an identical partner allele if you reproduce with close relatives. Even if you don't do that, the odds are higher if you're in a closed community.
Say you're doing homework and you want to compare answers with a friend. But you didn't do your own homework, you copied most of it off Dave. So you compare with Sara, and if there's any errors that Dave made or Sara made you have a chance to catch and fix them. But if Sara also copied off Dave, you're not gonna catch those mistakes.
Similarly, you have two sets of chromosomes, and for each "gene" (homework problem) you have genes (answers) that are more or less dominant. Bad genes that kill or impair you tend to be recessive, because if they are dominant the carrier doesn't survive. So it's all right if you have one copy, because it's not expressed. But if you have two copies...
When combining two genetic codes you don't have a way of predicting and selecting what's good and what's bad.
Similarly to doing a homework, if you got two copies from two different people and their solutions are not aligned it could mean that one of them is right, or they're both wrong, or they're both right, cuz there can be multiple ways to solve a problem, even a math one. You need to be able to circle back and discuss solutions with both of them to understand which one is correct. You do not have this mechanism for genetic cross-over.
Coincidentally, two people can be wrong about something together. School assignments were notorious for setting up traps that everyone would fall for.
TL;DR comparing answers and picking "only matching answers" doesn't seem like a necessarily better path.
I explained that in the second paragraph. You get genetic defects even without inbreeding, sure, and you get wrong answers on homework. But you're more likely to catch wrong answers if two people worked independently, and you're more likely to have a healthy set of dominant traits when people marry outside their community.
To reiterate: Inbreeding allows for greater expression of recessive traits. Recessive traits are more likely to be disadvantageous than dominant ones, because dominant traits that pose disadvantages are winnowed out by natural selection, whereas recessive ones can be carried unexpressed and passed to the next generation.
A good example is cystic fibrosis. We found out my wife is a carrier. If I was also a carrier, there's the risk a child could have cystic fibrosis, which would prevent them from surviving long enough to reproduce. But since I'm not, there's merely the concern that she passed on the carrier gene to our children. It doesn't affect their survivability at all, and they will live to be the chance to pass on the recessive carrier gene themselves.
Slightly more correct to say alleles are the "answers"
A gene is a spot where the DNA usually codes for something, where an allele is the particular version of a gene carried by an individual.
Lanisters up to it again..
Hello Alabama!
I'll try to keep it relatively simple - your cells contain chromosomes that contain your genes. You usually* have two sets of every chromosome.
These genes come in different variations/mutant forms called alleles. Most alleles function more or less the same, but some malfunctions result in deformities.
If a malfunctioning allele results in errant gene inactivation, it is known as recessive, which means as long as your other copy works, you're all good.
If a malfunctioning allele results in an errant gene activation, it is known as dominant, which means if you have the allele you get the deformity regardless of if your other copy works or not.
Fortunately for life, most malfunctioning alleles are recessive, so as long as you've got high genetic variance (a lot of alleles) in a population, the chance of two people meeting with the same recessive malfunction is low.
Incest can result in a drastic decrease in genetic variation, which can result in malfunctioning alleles becoming much more prevalent than they usually would be, resulting in many more cases of recessive deformities than in the wider population.
*For males this is not true of their sex chromosomes. Many genes present on the X chromosome are missing on the Y chromosome, which can lead to sex exclusive traits and diseases.
For example, it is the reason why there are almost no calico/tri-colour male cats, as the genes for it are in X but not Y chromosomes.
And so the reason for malfunctioning alleles not to be dominant is probably natural selection. E.g. you select away bad dominant alleles, but if we assume a low pct of inbreeding, the recessive ones are irrelevant and so they stay.
Pretty much...
Recessive malfunctions can hide away amongst carriers for generations before manifesting any deformities, during which time they have no effect on the carrier's survival, so there's very little selective pressure against them.
Dominant malfunctions which cause deformities simply can't hide away, so have enormous selective pressure against them.
Interestingly enough though, there are times where dominant malfunctions can survive that pressure...
For example, having Sickle cell disorder increases your resistance to Malaria, so even though the full form is rarely passed on, the single allele form (which caused partial disorder) is passed on due to a slight positive selection pressure.
For example, it is the reason why there are almost no calico/tri-colour male cats, as the genes for it are in X but not Y chromosomes
And the male calicos that exist actually have Klinefelter syndrome, where they wound up with an extra X chromosome, making them XXY instead of XY.
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