Hi ,
I’m trying to understand what is meant by dominant and recessive genes. Is a dominant gene unchanged vis-a-vis the parental genes? Is a recessive gene a changed gene vis-a-vis the parental genes? Please give me an explanation any layman can understand. I look forward to your feedback.
davidmich
You can probably get an excellent explanation in wikipedia, but think of it this way:
A dominant allele (genes come in pairs) “cancels out” a recessive allele. The classic example is “A” and “a” (where “A” and “a” are the two alleles that make up the gene, and “A” is dominant while “a” is recessive).
You may be AA, Aa or aa. AA and Aa will make the person look the same, while aa makes them look different. IOW, you only need one copy of the dominant allele to have the dominant trait, but you need both alleles to be recessive to have the recessive trait.
If parents are AA and aa, then all the kids will be Aa, and will exhibit the “A” trait. But… they can pass on the “a” trait to their kids.
If the parents are Aa and Aa, then 3/4 of the kids will exhibit the “A” trait and 1/4 will exhibit the “a” trait (they can be AA, Aa, aA, or aa). Or, rather, each kid has a 75% chance of showing the “A” trait and 25% chance of the “a” trait.
A mutation can result in a recessive trait (more common) or a dominant trait (more rare).
Recessive ones usually mean the gene has lost its function. Broken, doesn’t work. So if you have a functional copy (everyone has two copies of each gene), you still have that gene’s function provided. Functional, normal copy masks the broken recessive copy.
Dominant mutations give the gene a new, disruptive function. Think of a gear with a tooth missing. The gear is still there, but mucks up the works. Even if you have the normal, functional gene, the disruptive gene is is there, throwing a monkey wrench in the works. So, it’s dominant- you see the mutant trait even with a good copy present.
At any given gene site, you have two genes. One came from your father, and one came from your mother. The one that came from your mother came from either her mother, or her father. Likewise, the one that came from your father came from either his mother, or his father. And when you have a child, the child will get one of your two, either the one from your mother or the one from your father. Through all of this, aside from the occasional rare mutation that isn’t relevant to your question, the gene remains unchanged.
So far, this is true of all genes. Now, given that you have two genes at each location, they might say two different things. What trait results in such a case? Well, there are a few possibilities. Sometimes, the trait you get is in between the traits associated with the two genes. This is called “mixed dominance”. Sometimes, instead what happens is that you get a trait that looks just like the trait from one of the genes, as if you didn’t have the other gene at all. In this case, the gene that influences your trait is called a dominant gene, and the other one is called a recessive gene.
Note that, whatever happens, the two genes are both there, separate and unchanged. For a simple example, let’s assume an organism that has genes for two different colors, black and white. Maybe an individual with a black gene and a white gene is gray, or black-and-white-striped, if it’s a case of mixed dominance. Maybe black is dominant over white, and the individual is all black. In any event, though, the individual still has one black gene and one white gene, and can pass either to its offspring: It does not have any gray gene, or a stripe gene.
Two alleles.
Also, the OP should note that many genes do not operate in a simply dominant/recessive manner. In many cases, if you have two different alleles, you will exhibit an intermediate condition. And, of course, many physical features (like skin color) are controlled by more than one gene.
Thank you all for your very helpful replies.
davidmich
Well, there are a few genes on the sex chromosomes. A male will be hemizygous for X-linked genes.
(sorry for late reply, I just found this board)
Good Question and a very simple answer:
Recessive genes are simply “lack of information”.
Example - blue eyes are lack of pigmentation. If either parent gives the information to add pigment, the eyes are brown. Indeed, all babies are born with blue eyes.
Think of other examples - detached ear lobe, skin color, blood types, … In every case, the “recessive trait” is the “do nothing”.
The child is a mixture of each parents genes (some, not all come across, random). Thus, skin color is a mixture between parents. If both parents say “pigment the eyes”, the child probably has darker eyes than if they get it from just one. There is no “brown or blue”, but a gradual continuum. Couldn’t someone easily test this?
Some Implications -
Information gets lost over time. That is why isolated groups, such as Pacific Islanders, show more recessive traits, like rare blood types.
It takes more information to make an African than a northern European. My grand-dad was upset when I destroyed his claim that his blue eyes and blond hair (me too) were superior traights. I told him those were “nothing”, just lack of pigmentation. The default is straight hair. It takes more info to curl hair. In a related vein, the most complicated language sounds come from the Bushmen of Botswana, who are thought to be the source of mankind. They look a little like every “race” on earth. Many of their sounds (clicks, …) have been lost as people spread out and became more isolated.
History -
I figured this out in the late 1970’s and thought it was so obvious I wondered why it wasn’t explained in my junior high biology class. In undergrad, I mentioned it to a biology major and was told I was crazy. I later thumbed thru all the biology books in the library and found no mention, though I found academic papers with bizarre theories like genes competing similar to natural selection in organisms.
I finally found this answered (same as me) by the brilliant Cecil Adams in his “The Straight Dope” column, which was carried in alternative (i.e. hippie) magazines in the 1980’s. That is why I came here, but don’t find it here. Much in print that pre-dates the internet seems lost to history. I hope someone can find it.
I am a simple engineer. To me, evolution and natural selection is a trivial explanation: “If you live to make babies, there will be more babies like you”, QED. Who could argue otherwise? I said “sure, obvious” in 9th grade. The rest of class, “don’t understand, my parents said God …, we’re not monkeys, …”. The amazing thing is how long it took for people to realize this and why many still can’t understand it today. Recessive genes seem to be on the same path. It seems Biologists are still stuck in learning by rote memory and have the same attitude as those who opposed Darwin long ago. Sorry to rant, back to making things.
In many cases, it is the case that the recessive gene is “do nothing”. It may even be most cases. But I would be extremely reluctant to say “all” in science, especially in a science as messy and complicated as biology.
That’s not accurate. Almost all recessive trait genes still encode for a protein, they aren’t just a series of stop codons mutation. The only difference is that the protein they encode for is overwhelmed by the dominant version.
The example of blood type that you used is perfect illustration of this. People with type O blood still have a perfectly functional gene that encodes for for a surface antigen. That gene encodes for exactly the same amount of information as the A, B or AB types. The only difference is that the dominant blood types have mutated to paste one extra sugar molecule onto that antigen.
The recessive O gene doesn’t encode for nothing, it just encodes for a perfectly functional protein. There is a mutation where the blood type gene does produce no surface antigens, known as Bombay syndrome. That is the only case where you might be able to argue that the recessive gene is encoding for nothing. But it is exceedingly rare because it is literally a defective gene. Under the standard blood type system, the recessive gene encodes the same amount of information as the dominant.
That is due to a small founder pool and lack of genetic influx, not due to the loss of information over time. All humans have existed for exactly the same amount of time. If information was lost over time, then all humans would have lost that information.
Sorry, but that’s not in any sense true. Even if we accept that recessive genes code for no information, which we’ve just seen isn’t correct, the frequency of recessive traits is just the same in Africa as anywhere else. You are concentrating on two very superficial appearance traits, while ignoring the plethora of other traits.
Pure baby-blue eyes, yes. Green, hazel, yellow or other eye colours, no. If blue eyes were just “nothing” then all blue eyes would be the same colour, which of course they are not. Those sorts of baby-blue eyes are incredibly rare.
And for skin colour, definitely not. If white skin were the result of no encoding, then Cuascasians would all look like albinos, which of course they do not. People wit pale skin have genes that encode for less active melanocytes, but that isn’t an no information, that is information in itself. The only way what you say could be construed as true is if Caucasians had melanocytes that were genetically silenced.
Not really true. Keratin is a protein. Proteins have a conformation by default: they always want to fold. If there was no selective pressure to hair conformation, and hence no encoded information for shape, we would expect a randomly wavy shape. Both curly hair and straight hair are the result of very specific encoding for those shapes.
That is also incorrect. “Complicated sounds” in language doesn’t really mean anything. Any sound that you are not raised hearing will be complicated to an adult. Conversely children don’t find it any harder to make the sounds in San languages than in English.
If by complicated, you mean high diversity then, no. While one branch of the San languages has a large number of sounds, other branches with more Bantu influence have fewer then English. And languages with as many or more sounds are found in New Guinea, Polynesia, South East Asia , South and Central America and even Europe.
They are thought to be a basal group on the human evolutionary tree. That isn’t anything at all like being the source of mankind.
To give an analogy, turtles are a basal group of the reptilian evolutionary true. That doesn’t mean that turtles are the source of snakes, birds and crocodiles. It just means that they split off from the branch that led to snakes, birds and crocodiles before those other groups differentiated. Doesn’t mean they haven’t evolved beyond all recognition since that split occurred. Nobody believes that the first reptiles looked anything like a turtle. The first reptiles, including the ancestors of turtles, looked and acted identical to modern lizards to the extent that a layman wouldn’t be able to tell them apart.
In the same way, the ancestors of the San and pygmies split off from the main stem of humanity before the rest of us differentiated. That doesn’t mean that they are the source of humanity or that they look, act or speak any more like the original humans any more than turtles look or act like the first reptiles.
These sorts of misunderstanding occur because of the way evolutionary trees are drawn. To save space, they are drawn with the group that diverged last being placed near the top, which suggests that the groups that diverged sooner were the source of those groups. Of course that’s not correct. It would be just as accurate to say that the non-San group split off before the San and pygmy groups differentiated and place the San and pygmy groups at the top.
IOW it’s just as correct to say that the non-San groups are the source of that part of humanity on the San branch, as the way that you put it.
I think a lot of the theories on the Mendelian traits have been disproven or updated right. I believe there are multiple genes involved in hair, skin, and eye color and the dominant/recessive is an oversimplification that doesn’t hold up to reality.
In those cases, that’s true. Even ABO blood type isn’t simple dominant recessive. however for many other traits the dominant recessive model is accurate. Even in more complex cases like eye colour, it’s sufficiently accurate to declare that blue eyes are “recessive” in the sense that the trait is inevitably swamped by darker eye colours and not vice versa.
Blake, According to Wikipedia it has been shown that blue eye color being a recessive trait has been proven to be incorrect, although maybe you meant something different, like that while technically it isn’t recessive from a pragmatic construct to understand inherited traits it works most of the time?
It is an interesting topic though I am white and have eyes that are kind of in between blue and green and my mom and dad have green and blue eyes, respectively. My wife is Hispanic and has olive skin and brown eyes and both her parents have brown eyes as well. Our son came out with kind of moderately tanned skin with really light blonde hair and green eyes, which was kind of surprising because I and my immediate family all have brown hair, Our daughter has pale skin, light brown hair and very blue eyes, both my kids look completely Caucasian and look like my clones and look nothing like my wife, strangers are often surprised when they see that my wife is the mother. I do believe however that my wife has some French in her ancestry so there are probably some blue eyes in her lineage if you go back far enough, I don’t think blue eyes always get swamped though.
It’s even more complicated than that. There is a place on chromosome 15 where a defective gene sequence is somehow capable of turning off the healthy contribution from the other parent. That’s not even the weird part, though. Depending on whether the defective sequence comes from the mother or the father, the child will have one of two different syndromes. When the defect is maternal, the child has Prader-Willi syndrome, and when it is paternal, Angelman’s syndrome.
Since these come from unaffected parents, they seem to be spontaneous mutations, although it’s possible that some people may be immune to the “turning off” effect, and so have the mutation, but not the disease, because their healthy pair functions for them.
The gene that gives dark brown eyes is dominant, it will always “swamp” blue eyes. You don’t have to search backwards for “non-hispanic” ancestry for those blue eye genes, since those genes occur in hispanic populations as well, even though they’re more rare than in Northern European populations.
I don’t know which part you read, but it’s incorrect. Dark brown eyes are dominant, and the wikipedia article on eye color states as much. Not that Wikipedia is some sort of perfect source, but since you referenced it in the first place.
naita, we must be reading two different articles because the wiki article on eye color states
This is the citation that it gives :
http://www.sciencedaily.com/releases/2007/02/070222180729.htm
You’re over-interpreting it and later in the same article it states that dark brown eyes are dominant. When you look beyond dark brown into amber, green, grey, blue you get the complexity involving more genes and an end to the simple “one gene” explanation, but as you just quoted yourself:
. That gene gives brown eyes if you have one or two copies.
Or, as I originally said, even in more complex cases like eye colour, it’s sufficiently accurate to declare that blue eyes are “recessive” in the sense that the trait is inevitably swamped by darker eye colours and not vice versa. Note once again that this is not perfect since eye colour is a complex trait. but it’s sufficiently accurate.
Ok I think I understand now after doing a little more reading there are basically the two genes for eye color brown/blue and green/hazel, the brown allele is dominant over all the others but the green/hazel alleles have incomplete dominance so the brown eye gene is dominant but eye color itself is not a simple Mendelian Dominant/Recessive trait because more than one gene is involved.