Because recessive genes are still passed along to the next generation with the same frequency that dominant genes are passed along. The only way in which natural selection will select against a particular allele is if carrying it decreases the odds of that individual reproducing and passing along their genes. In other words, natural selection only cares if a recessive gene is harmful even when the dominant gene is present, and the harm has to happen before the age of reproduction.

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the recessive gene is still within the person and can be passed down to the offspring. if both parents carry the recessive gene (ie, blonde hair) and a copy of the dominant gene, (ie, brown hair) then both parents will have brown hair. however, they can both pass on the blonde hair gene to the offspring resulting in a child with blond hair.

the recessive gene would only die out if we had, say a predator that found it easier to find and eat blonde haired people. then those people would get plucked out of the gene pool while brown haired people were more likely to reproduce.

this happens a lot with goldfish. goldfish can be gold or brown. birds find it easier to see and eat the gold ones, leaving the brown ones behind. a pond full of mixed colour goldfish will eventually become all dark colours after time as the gold ones get eaten off by birds more easily.

Recessive genes are, in fact, tougher for natural selection to completely eliminate than dominant ones.

If you have only 1% having the gene, only 1% of carriers will express the gene and be selected against. Meanwhile, any predators exploiting that gene would be getting almost no benefit, pushing them away from that behaviour.

Dominance & recessivity of alleles simply have nothing to do with their inheritance. Neither is inherently tied to selection advantage.

in the case of the goldfish though, if only 1% carry the gene, then there's almost no bright gold ones and when there are, the birds can see them and eat them easier. the bird is unlikely to push away from the behaviour of 'being able to see brightly coloured things more than dark things.'

if the other 99% aren't carrying the gene at all then they carry on reproducing dark colours with absolutely no chance of their offspring being gold coloured.

also, any gene that's only carried by 1% of a population will almost surely disappear anyways. even if it's not detrimental to survival. if you have a pond of 100 fish and only 1% carry the gene, then a bird comes and eats 20 x fish, there's a random chance the 1 fish with the gene will be removed from the gene pool entirely. whereas the other 99% will always have numbers among the survivors.

For the same reason that Covid and HIV/AIDS were successful- it can be passed on by asymptomatic carriers. You can have one copy of a recessive gene and be unaffected by it.

Tay Sachs is a disease that is caused by having two copies of a recessive gene. It’s pretty much always fatal if you have it, and it kills people before they are old enough to have kids. But you can have one copy of the Tay Sachs gene and not even know it. If the other parent of your children doesn’t have the gene, it doesn’t affect you or your children at all. The problem only happens when two carriers have children together. Even then, only 1 in 4 of their children will have Tay Sachs.

It gets more interesting if having one copy of a recessive gene is beneficial. There are several recessive genes where, if you get one copy of the gene, you have more resistance to malaria. Sickle cell anemia is the most famous example. If malaria is a significant problem where you live, someone with one copy of the recessive gene might be more likely to survive and reproduce than someone with no copies would.

It’s even more complicated with humans, because having as many children as you possibly can isn’t necessarily the best reproductive strategy for humans. We’re K strategists, which means we tend to have fewer children and put more parental resources into the ones we do have, rather than just having as many children as possible. If two carriers of a recessive gene have children together, but only have a few children, there’s a decent chance that none of their children will inherit two copies of the gene.

The environment doesn’t stay the same, and who’s fittest can change. New diseases happen, as we’ve all seen in the past few years. There are also a lot of non-genetic factors that determine which humans survive and reproduce.

It wouldn’t necessarily die out even in that case, because there are lots of other factors than that predator involved in who survives and reproduces. There are going to be brown haired people who die for other reasons, and blonde haired people who aren’t killed by the predator.

Even if the recessive gene is always fatal if you get two copies of it, and the gene is the only factor in survival, it’s still going to take a long time for it to disappear, because humans take so long to get to reproductive age.

A recessive gene doesn't mean it doesn't exist, just that it doesn't get presented. Someone can carry the gene without expressing it.

For example, if we have a parent with brown eyes, and let's say they have 2 copies of that dominant gene, we will call that BB, and a parent with blue eyes, they kust have 2 copies of that recessive gene we will call by. The child will have one of each copy, to they will have brown eyes, but will have the genes Bb. If they have a child with another person with Bb, then they each pass one copy on at random. We can get B from both, resulting in BB brown eyes, or B from 1 and b from the other, resulting in Bb brown eyes, or they could get blue eyes and have bb.

Recessive genes just mean it's less likely to be expressed, but it's just as likely to be passed on.

Dwarfism is a dominant gene, but it's not taking over the genepool. Although is mostly due to the fact that having two copies of the gene is fatal.

It’s called a hardy weinberg equilibrium. Unless there’s some catastrophic event, like we kill all the redheads, or something less sadistic like some kind of geographic change like a flood and a new River happening to divide people, random chance plus the continued passing of recessive genes keep things where they’re at.

We’re talking about absolutely massive sample sizes so it works out that you might end up reproducing with someone who has recessive genes you didn’t know about.

That's true if there is 100 fish - but not if there is 1 000 000 fish.

If there's a huge number of fish, the chance that a given gene is removed randomly is very low. Unless it provides a disadvantage, it's entirely random. There have been experiments, both in digital environments and in sealed live environments, tracking genetic drift (the change in gene representation in a population over time); and pretty consistently there are cases of genetic variations that end up spreading by chance that don't do anything.

The classic example of real-life variations are two different genes that code for the same amino acid chain using different base pairs - there's no advantage one way or another. In such a case, the most common result over time is that both versions of the gene persist; even if you start with one variation is less common than the other.

The exception is if you simulate bottlenecks - like the "100 fish" scenario you posted. With such small numbers getting through, it becomes a lot more likely for some genes to be lost forever by random chance - including useful genes that happen to get unlucky.

That does not help op find his answer at all. You clearly know the right answer but gotta try doing a better job at explaining it to them, like they are 5

Theres nothing about recessive genes that make them inherently less likely or less desirable to pass on, and they're still there even if they aren't expressed.

If you imagine the genes as red and blue cards, red cards always go on top of blue cards. If you lay those out in every possible variation (RR, RB, BR, BB) the blue card is only ever visible in 1/4 of those combinations, the one with two blue cards. But there is an equal number of red and blue cards, if you shuffle them together and pick a card at random you have an equal chance (1/2) of drawing either color. So they don't die out, you just only actually see it 1/4 times

If a recessive gene exists in the population, why would you expect it to die out without knowing anything about it? By default, you should expect a gene to stay at the same frequency in the population unless it's helping or harming the organisms that carry it.

OP can ask a followup question if they want. Otherwise, I'd be writing an essay explaining a bunch of things that aren't really related just to make the point they aren't related.

"Recessive alleles aren't selected out because there's nothing about recessivity that inherently carries selection disadvantage" is a perfectly fine point to start with.

That’s not a recessive gene though. If 1% of the population carries a recessive gene (and assuming it’s equally in male and female) then 0.025% of the population or 1 in 40000 offspring will actually have the recessive trait. This is why it’s almost impossible to eliminate a recessive gene from a gene pool.

yeah i was using the figures supplied by the person above. in most real situations, we usually have a broader dispersion of genetics than 99% vs 1%.

Whoops I totally misread your comment my bad!

Two reasons. Either they offer some benefit, or they cause no harm.

My mom had dark brown eyes. My dad had light brown eyes and siblings with blue or green eyes. I have brown eyes, my wife has green eyes.

We have a kid with brown eyes and one with blue eyes.

So...I inherited brown from my mom, and my dad must have been a carrier for blue and passed that on to me. It was recessive, but did neither me nor my dad any harm. My wife must have blue and green, with blue being recessive. It caused her no harm.

One kid inherited brown from me, which was dominant over whatever my wife gave her. The other must have inherited recessive blue from both of us. It has caused him no harm. His child has blue eyes.

So recessive blue has passed from one of my dad's parents down to my son and grandson, even though my dad and l had brown eyes, because there was no reason for it to die out - it caused no harm.