The question on incest and evolution brought a question to my mind that hadn’t occured to me before. What specifically makes a gene be dominant or recessive?
I’m going to guess that it is some section of the “junk” DNA that doesn’t code for a protein. I understand that some of what used to be considered junk has been found to have other purposes, but I’m not exactly sure what those purposes are. Is one of them a code to indicate that the gene is dominant, recessive, or co-dominant? Or is it something intrinsic to the gene itself?
I’m not sure exactly what causes a gene to be dominant or recessive, so I hope someone will come along with the answer for you.
I do know that there are specific on/off sequences that determine whether a gene will be expressed at a given time or not. (Operons? Regulator genes? I can’t remember)
Also, some genes are expressed if some quantity of the protein coded by that gene is present. In Sickle-cell anemia, even if there is just some of the hemoglobin S present (like in a person with one sickle-cell gene and one normal gene), some of their blood cells are sickle-shaped because the protein is present, even though not all of the hemoglobin in the body is of the S-type.
I don’t know the answer in general, but for blood types A, B, AB, and O. Let’s take only types A and O. A is dominant over O. If you have one or two genes for the A antigen, you will have type A blood because you have the code for the A antigen. O does not have the code for an antigen.
I think it is also that way (like Dr. Matrix said) for eye color. Let’s simplify to brown and blue. Brown is dominant over blue because it says send pigment to the iris. Blue doesn’t say, “send no pigment.” Blue doeasn’t say anything at all. So somone with Brown-Blue gets brown eyes, because blue is too timid to stand up for itself.
Oversimplified and anthropomorphized for your reading enjoyment.
Let’s say we have an enzyme that performs a certain critical function. Without the enzyme, you die.
One allele of the enzyme’s gene encodes for the functional protein. As long as you have at least one copy of this functional enzyme gene, you will survive. Therefore, this allele is dominant.
Another allele has a mutation in it that makes the enzyme nonfunctional. If you have two copies of the inactive enzyme gene, you will die. However, if you have one nonfunctional allele and one functional allele, you will survive because functional enzyme is being made. Therefore this other allele is recessive.
Now not every dominant gene is functional and not every recessive gene is nonfunctional. There are many ways an allele can be dominant or recessive, the above description is just the simplest example.
Maybe you can explain this to me then: Why doesn’t every person on earth have brown eyes, dark hair, and have all males be bald? I’ve always wondered this. Shouldn’t 1/4 of each generation technically have the dominant trait? After a while, 1/4 would go to 1/16, and so on, until no one has recessive traits. Since there’s 4 possible combinations. A=dominant, a= recessive: There’s AA, Aa, aA, and aa. 3 of these transmit the dominant trait…so why isn’t it like that in the real world?
Because most things aren’t simply a case of dominant vs. recessive. Most physical features, including eye color, are the result of many gene products interacting. And each of those genes can have varying levels of dominance due to complex genetic interactions.
Besides, recessive genes never go away even though the recessive traits may not be visible. People can still carry the recessive gene.
For things like hair and eye color, there’s also the selection factor: In some populations, those with recessive physical characteristics are more likely to reproduce, so the next generation isn’t going to be a pure 3-to-1 ratio.
For instance, it seems like the blue-eyed blonde has some evolutionary advantage in places like California, even though those traits are recessive.
Of course, the ratio could get a lot closer to the purely genetic prediction now that you no longer need to have the genes for these superficial appearances to get selected for reproduction. Hair dye and colored contacts could affect the evolution of our species!
Because the gene frequencies don’t change. Let’s set up a simple system, eye color. If you get a brown gene you can make pigment for the iris, if you have no brown genes you can’t and your eyes are blue. And lets say that half the genes are for brown, and half for blue. And lets say that we have random mating, there is no advantage for either eye color.
So, a simple punnett square:
B | b
B
BB
Bb
b
bB
bb
If you get a B (Brown) from your mother and a B (brown) from your father, you will be BB…you will have brown eyes since you have two copies of the brown gene.
If you get b (blue) from your mother and a B (brown) from your father, you will be bB…you will have brown eyes since you have one copy of the brown gene.
If you get B (brown) from your mother and a b (blue) from your father, you will be Bb…you will have brown eyes since you have one copy of the brown gene.
If you get b (blue) from your mother and a b (blue) from your father, you will be bb…you will have blue eyes since you don’t have a copy of the brown gene.
So we see that there are three (Bb and bB are equivalent) kinds of people. Some are brown eyed, some are brown eyed but carriers, and some are blue eyed. If the frequency of the blue gene is 50%, then 25% of the population will be brown, 50% will be brown but carriers, and 25% will be blue. But the next generation will have exactly the same ratios, since the brown gene doesn’t destroy the blue gene, it only masks it.
So, if a BB mates with a BB, all the kids will be BBs.
If a BB mates with a Bb, half the kids will be BB, half Bb.
If a BB mates with a bb, all the kids will be Bbs.
If a Bb mates with a Bb, 25% of the kids will be BB, half will be Bb, and 25% will be bb.
If a Bb mates with a bb, half the kids will be Bb, half bb.
If a bb mates with a bb, all the kids will be bb.
This means that if two blue eyed people have children they will always be blue eyed. Most of the world (asia, africa) are BBs…that’s why there are no blue eyed kids in Japan. But the frequency is pretty high in Europe, so it’s not uncommon for blue eyes to show up.
OK, now lets look at what happens if the frequency of the b allele is 10%. Then 81% of the population is BB, 18% are Bb carriers, and only 1% of the population is bb and has blue eyes.
If the frequency is 1%, then 98.01% of the population is BB, 1.98% is a Bb carrier, and only .01% is bb and has blue eyes.
So we can see that extremely rare genetic traits are actually much more common than we might think. This is called genetic load. Most people are carriers for several nasty recessive genetic diseases. But they hardly ever get expressed, because the odds are that you won’t reproduce with someone who also shares the same trait. Even if two carriers have children, only 25% on average of their kids will actually have the trait. This is why there are so many recessive genetic defects. There is typically very little selection against them, because there are very few individuals who actually express the trait.
Of course, if you reproduce with a close relative, it is much more likely that they will share the same recessive genetic defects. So it is much more likely that the kids will have problems.
Also, this is why most really bad genetic diseases are recessive. If they were dominant, then everyone who had them would die. Then the trait would be eliminated before it begins. But remember that dominance and recessivness has nothing to do with selective advantage. Dominant genes aren’t always good genes, it’s just that really bad genes are very likely to be recessive.
About the ABO blood system, the classical example: there are three alleles, A, B, and o. o simply means the lack of A or B. You can be AA, in which case you only express the A allele; Ao, which is also just the A allele; BB; Bo; AB, in which both are equally expressed; or oo, in which case neither are expressed. You can do all kinds of fun inheritance expamples with this in freshman biology, which is why so many people are familiar with it.
This is a good example of an important point: not all genes come in just dominant and recessive. There are other types of systems, like codominance, where both alleles are expressed equally. I think it’s fair to say that “dominant” and “recessive” are used less now than they used to be. As we learn more about the biochemistry of the genes involved in a given trait, it becomes much more complicated. You can get gene alleles that interfere with entire biochemical pathways, making one gene dominant to lots of others, for example. The dominant/recessive label is useful in some instances, but it’s usually more informative to figure out what’s happening on a biochemical level.
At every gene location (locus), you need a certain amount of function in order not to be a mutant. If you have enough product to get above this threshold, you are not a mutant. If one copy of the “wild type allele” (the normal copy) can get you above threshold, you are not mutant. In this case, the wild type allele is dominant and the mutation is recessive (because you need two mutations in order not to make it to the threshold). The exact opposite is true for dominant mutations.
As Alphagene said, lots of different things can happen at a locus – there are lots of ways things can be mutant. Here are a few examples of dominant mutations:
Haploinsufficiency – Opposite of above example. One copy of the wild-type gene cannot get to above the threshold, therefore only firing on one cylinder if you will causes a phenotype.
Dominant negative – The mutation actually reduces the wild-type gene’s levels. So, if you can normally get above threshold with the wild type gene, having the dominant negative will force it back down. An example of this is a mutation which causes the wild type allele to be bound up and non-functional.
New function (neomorph) – The mutation lends a new function to the cell, which makes a new phenotype.
Too much function (hypermorph) – The mutation overwhelms the system with too much product (or it works too well), and this causes a phenotype.
There are equivalent things for recessive mutations – hypomorphs and so forth. There is also codominance and incomplete dominance.
I’m a teaching assistant for a graduate school Genetics course.
Been offline for a while–thank you Verizon strike.
Another thing to consider is that expressing a gene also involves other factors besides what the actual protein is/does. A protein that causes, say black coloring in fingernails might be dominant to a protein that causes, lets say, yellow coloring in the phenotype–hypothetically similar to eye color. But it’s not just about protein function. If the gene causing black color doesn’t bind polymerase, transcription factors, coactivating proteins, enhancers,… as well as the ‘recessive protein’ gene, you might still get yellow fingernails.
And mRNA transcription doesn’t necessarily mean protein made.