Dominant/recessive traits and genes

M

malkiri

Registered Senior Member
What relationship do dominant and recessive traits have to genetics? For example, does a creature's genome contain two codings for a trait, and only the dominant trait is expressed? Or is there no direct mapping to the genotype? Does a trait's dominance or recessiveness change over time with evolution, or is it some relative property intrinsic to a trait? Maybe a clearer way of asking that is: what determines a trait's dominance or recessiveness?
Unfortunately all I know about this is what I learned in freshman high school biology. I'm hoping to apply this concept to a type of programming problem and am curious what the biological mechanisms are.
Thanks for your help!
 
complicated...any gene could have a dominant and recessive allele. Or be a hypomorph, be redundant in some processes, not in others. But i am sure the population biologists must have some tidy answers. I don't.
 
malkiri,

I sorry about these people here I would have expect more from spuriousmonkey or at least giving you the basics, shame on you spuriousmonkey :mad:

Ok first

yes most eukaryotic (basically non-bacteria) organisms have a diploid stage or a time when the organism has two sets of chromosomes and thus two sets of every gene (some organisms have even have 4-8 copies) now in humans as well as in most animals we are diploid throughout the majority of are lives (only sex cells are haploid). The two sets of genes are called alleles. Dominants and Recessives are stated depended on if an allele produces a functional product, A Dominant allele produces a functional product and Recessive allele does not, if the dominate allele is present functional product is being produces no matter if a recessive is present or not, this is why the recessive allele is called “recessive” because it is covered up by the dominate allele. Lets do a example: Brown eyes are caused by a dominated allele we will call “B”, blue eyes are caused by a recessive allele called “b”; Now remember there are two alleles per gene, so lets say we have a person that is heterozygous dominate (has “B” and “b” allele, Bb) this person would have Brown eyes, lets say this person as a child with another person that Homozygous recessive (has only two “b” alleles, bb) this other person as blue eyes. The child will have what color eyes? Well we can figure that out with a little multiplication chart like this (Bb allele pair) *(bb allele pair) = 2(Bb allele pair) and 2(bb allele pair) so out of four possible allele permutations 2 will Bb and 2 will be bb thus the child has a 50%-50% chance of having blue eyes. Ok lets say we have two heterozygous dominates (Bb) people breed (thus they both have brown eyes) what color eyes do their children have? Again (Bb alleles)* (Bb alleles) = 1(BB allele pair)+2(Bb allele pair) +(1bb allele pair) thus out of 4 permutations or lets say four children, 3 will have brown eyes and 1 will have blue eyes. Arrggg I need the rest my fingers! If you need more help please ask. This is the basics it gets more complicated in that there are many other things in genetics. some there are Multi-dominate alleles, recessive alleles that produce a effect, operons, gene expression control and feed back… god I can't write!
 
Thanks monkey & Fetus. I think I'm mostly wondering what determines dominance and recessiveness. I remember the general concept of inheritance (your description helped with that, thanks), I'm just a bit unclear on what makes brown eyes dominant & blue eyes recessive, for example.
Feel free to assume as many simplifications as you need...I don't need to get into the gory complications if it makes it easier to explain the basic idea.
Thanks again :)
 
Originally posted by malkiri
Thanks monkey & Fetus. I think I'm mostly wondering what determines dominance and recessiveness. I remember the general concept of inheritance (your description helped with that, thanks), I'm just a bit unclear on what makes brown eyes dominant & blue eyes recessive, for example.
Feel free to assume as many simplifications as you need...I don't need to get into the gory complications if it makes it easier to explain the basic idea.
Thanks again :)
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a constitutive active receptor could be dominant for instance. But not necessarily. You would have to assess things on a case to case basis.
 
Well, let me put up a disclaimer first before I add something here. I'm not a geneticist so this is not my long suit.

But I'll try to tackle at least part of this. Concerning structural genes, a dominant allele usually is the one that codes for a piece of mRNA that will eventually be translated into an enzyme that will catalyze a step in a metabolic pathway. The recessive allele generally does not produce, directly or indirectly, a functioning enzyme.
The allele for brown eyes apparently codes for an enzyme that results in the presence of melanin. Thus, the eyes are pigmented (brown, hazel, green, etc.). The allele for blue eyes does not accomplish the production of melanin, and the eyes are blue (not pigmented).

But it ain't always that straightforward about what is dominant and what is recessive. Consider the sickle-cell trait. It is generally considered that sickle-cell anemia is due to a recessive allele. An individual homozygous for that allele exhibits in their phenotype the condition. But, heterozygous individuals have some resistance to malaria. So, the same allele can be said to be recessive for the sickle-cell condition, but perhaps dominant for resistance to malaria.

It all gets kind of confounding. There are genes that code for enzymes involved in mRNA processing. There are apparently lots of regulatory genes that are involved with switching on and off the structural genes. There are inhibitor genes that, when dominant, cause the recessive phenotype to be expressed when the dominant allele is in the genotype. There are transposons that get involved. There is a phenomenon called incomplete penetrance, which means that only ceratin individuals with the dominant allele will actually express it in their phenotypes. Hopefully a bona fide geneticist will add their two cents here.
 
Wow, okay. I'm going to absorb but blissfully ignore the complications in that last paragraph, just because it's a level of detail I don't really need to handle.
So, if I'm understanding correctly, dominance isn't strictly an inherent characteristic of an allele, but rather an characteristic attributed to it based on the observation of the expression of a gene? Hopefully that makes sense.
 
Hi Malkiri: The idea that a dominant allele is the blueprint for the construction of an enzyme that does something is a pretty good way to look at it in a general sense, I believe.
 
Originally posted by chuck u farley
Hi Malkiri: The idea that a dominant allele is the blueprint for the construction of an enzyme that does something is a pretty good way to look at it in a general sense, I believe.
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except if you look at ontogeny.
 
Makiri and chuck u farley are right on track, but there are exceptions and a slight misdefinition. Most genes, but not all, have two alleles. These alleles can be both dominant, both recessive, or one dominant and one recessive (heterozygous). If both are the same it is called (homozygous). A dominant allele always expresses itself physically (the phenotype) over a recessive allele if it is present.

However, there are cases were a gene has more than just two alleles, but that is uncommon. But regarding the "coding for a trait" - the original question - most traits involve the expression of more than one gene or loci. Then it gets very complicated to predict the outcome. Also there is the condition of "codominance" where one dominant allele from one gene has an effect on the expression of another gene that produces the same trait. Many physical features involve more than one gene: height, skin color, hair color, just to name a few. And still, it gets much more complicated than that if you want to know more. But first - if you choose to do so - you'd best get a basic study of genetics down pat, because it really does get very very complicated.
 
ElectricFetus said:
If you need more help please ask....recessive alleles that produce a effect, operons, gene expression control and feed back… god I can't write
Hi! If you find the the time, could you explain more about the above? When do, if ever, recessive aleles produce an effect except when recessive-recessive? What is gene expression "control and feed back"?
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