Download what causes dominance

MadSci Network: General Biology
Re: why are some gnenes dominant? What makes
them dominant?
Date: Thu May 6 22:17:36 1999
Posted By: Nicole Davis, Grad student, Department of
Genetics, Harvard Medical School
Area of science: General Biology
ID: 925061598.Gb
Message:
Dear CJ,
Greetings! Thanks for your great question!
In your question, you ask why some genes are dominant and what makes them
so. To begin, I think we need to define and clarify some terms.
A gene is a segment of a very long piece of DNA called a chromosome.
Humans have 46 chromosomes (2 of each of the 22 autosomes, and 2 sex
chromosomes), and on these chromosomes there are tens of thousands of
genes! A gene is a functional unit of DNA--it is passed on from one
generation to the next, along with the other genes on a chromosome. The
collection of genes that an individual has is responsible, in part, for
determining certain qualities, such as height, eye color and hair color.
Each gene, through the ordered sequence of different molecules within the
DNA segment, constitutes a "code" that is used to make a protein. It is
these proteins which ultimately carry out the function of the genes.
Alleles are alternative forms of a particular gene. Let's use the flowers
on pea plants as an imaginary example. These plants have a gene that
controls the color of the flower petals. The petals can be different
colors, either white or purple, because there are different forms, or
alleles, of the petal color gene.
Dominance describes the relationship between two alleles of a gene. So,
why are some genes dominant? Well, to be precise, the question should be
reworded: why are some alleles dominant? In general, it is because a
Query:
dominant allele masks or interferes with the other allele. Because genes
(and their alleles) function through the proteins they encode, it is really
the protein made by the dominant allele that is responsible for these
effects. I bet you're wondering how a protein can mask or interfere with
another protein. Here is one example:
1. Lets go back to the flowers on the pea plant, where the C allele is
dominant to the c allele. These alleles for the petal color gene code for
a protein that makes a purple pigment. Imagine that the C allele has a
mutation in its DNA that makes the protein it encodes unable to produce the
purple pigment. The c allele, however, makes a normal protein that is
fully functional. So, referring to the table below, why isn't the
heterozygous plant (which has the two different alleles C and c) light
purple instead of white? It has half the amount of normal purple
pigment-making protein, right? In genetic terms, this is given the fancy
name of "haploinsufficiency", which basically means that having half the
amount of normal protein is not enough, and is the same as not having any
at all. Let's imagine that you go to the store and want to buy a loaf of
bread that costs $2, but you only have $1. In terms of buying the bread,
the fact that you have $1 really doesn't matter-you still don't have enough
to buy it. In this sense, having $1 isn't really any different from having
$1.99 or no money at all. Unless you can meet the cost of $2, you cannot
buy any bread. In the same way, unless the pea plant has two c alleles, it
cannot make a purple flower.
Genotype
(the alleles present)
Phenotype
(what you see in the plant)
cc
CC
Cc
purple
white
white
Keep in mind that this is only one example of how an allele can be dominant
to another. There are several other ways that this can happen in biology.
I hope this information clarifies your question. The concept of dominance
is often not easy to understand so please feel free to email me if you have
further questions!
Things to remember:
1. Traits, not genes, are dominant or recessive.
2. It is traditional to refer as genes as dominant or recessive.
3. Since then, lots of advances have occurred in regard to genetics.
4. Because of this new knowledge, we now know that protein gene
products influence phenotype.
5. Dominant and recessive refer to the nature of inheritance of
phenotypes, not to genes, alleles, or mutations (can't restate this
enough).
The problem with discussing genetics in terms of dominance and
ressiveness is that the traits are framed as exclusive alternatives.
Only one will dominant, it's either one or the other that is
expressed, not both.
This makes it hard to get people to understand that traits may be
expressed together (a compound phenotype) or that the traits may
actually interact with each other. I made a bit or an error before, in
saying that the dominant trait overpowers the recessive trait, (it's
just the way I was taught how to explain it, until I actually got
interested in genetics and looked stuff up for myself. still that
description seems to stay put. my apologies).
If you look back at what Mendel did with the peas, a go beyond
high school textbooks, you can see that he knew that there were
"non-Mendelian" cases (where no dominance occurs). Specifically at
the peas, he noted that the size and form of the leaves, stem
length, seed coat color, flowering time, and peduncle length, did not
follow his dominant /recessive model. Mendel new that dominance
was not the exclusive norm. He deliberately focused his work on
traits that he could work a model on.
Today, when docs decide to label human disease as dominant or
recessive, this too has a slightly different meaning. Dominance
generally describes and condition found - fully or partially - in
heterozygotes. The heterozygote need not resemble the
homozygote at all. Dominant in this setting means, a condition
where just one relevant alleles present (don't believe me, check out
the US national human genome research institute. They define
dominant as: "a gene that almost always results in a specific
physical characteristic, for example, a disease, even though the
patient's genome possesses only one copy. With a dominant gene,
the changes of passing on the gene (and therefore the disease) to
children is 50-50 in each pregnancy"
www.genome.gov/glossary.cfm?key=dominant).
Recessive alleles are found only where two copies are present. So
you could imply here that dominant refers to the causal effect of a
single allele. What was I going on about again? Oh yeah....
Mendel used d/r to refer as traits. This is on appearances only, and
describes no properties to hidden alleles, which cannot be directly
inspected. Traits are not always reliable benchmarks. But many
conditions can be d or r, depending on different mutations (even at
the same locus). MIM said that they would stop classifying
autosomal traits as dom. or rec. was that so often the precise same
phenotype was dominant when caused by one mutation but
recessive when caused by another mutation in the same gene.
You cannot always define traits as dominant or recessive
consistently.
Alleles, d/r, is what biologists (outside the field of genetics)
generally use. However, this isn't reliable either. An allele can
appear to be dominant or recessive, depending on how it is paired
with other alleles. i.e. blood type (in case anyone is wondering, I’m
AB+, thus sparked curiosity in genetics and future career path).
Given the blood enzymes are A, O, and B. A is dominant to O, in
the sense that AO corresponds to AA. When AB is about, both
alleles are expressed, here A and B are codominant. But there is
also cis-AB. cis-AB can generate antigens to both A and B. when A
and cis-AB are coupled, the phenotype is AB. in this case, A is
recessive to cis-AB. the very same allele - seeding the type A blood
antigen - can be dominant, codominant, and recessive, depending
on whether it is couple with O, B, or cis-AB allele. But the behavior
of A remains fundamentally the same, regardless (ironic?). A will
always yield A antigens. Labeling it as dominant obscures the
understanding of this.
Then there is another problem in characterizing phenotype. You
cannot always describe all aspects of a genetic condition as
uniformly d or r. think of sickle cell anemia. If you think of this trait
as circulation and survival, the functional hemoglobin is dominant.
However, if the trait is malarial resistance, that non-sickly
hemoglobin is recessive. If the trait is, oxygen physiology, than the
functional hemoglobin is incompletely dominant. So basically, a trait
is d or r depending on how you interpret the trait.
But many syndromes have many distinct traits. Like with the sickle
cell anemia; circulation, oxygen physiology and malaria resistance
are addressed independently, although they are related genetically.
Dominance cannot accommodate this unity of features.
Tay sachs disease is another example. Is recessive, incompletely
dominant, or codominant, depending on the level of the phenotype.
Dominance is descriptive, not explanatory. Dominance embodies a
linear model of causality, which emphasizes singular causes, which
obscures the context and overstates the effects. It also raises the
problem of articulating the role of two parallel genomes (this issue
at hand here). Dominance does not cause the full phenotype, but
contributes causally. That would change the way were taught (from
Aa and AA to "allele D with haplophenotype potential of D". I don't
think my biology teacher would have liked that). You would have to
describe that the potential effect of a single allele is not overstated.
It does not yield a full phenotype on its own. It contributes to only a
haplophenotype (half-phenotype).
The d/r model, Punnet squares and all that, shows that a phenotype
is currently conceived as singular. Yet a diploid phenotype is double.
Alleles are active in pairs. Diploid organisms are diphenic, each
allele functions partly independently, in parallel with its homolog.
And then the phenotype is compound, two traits expressed in
tandem and variously overlapping or intersecting.
So, in all, I’ll stand by my previous post that "dominance" has to do
with the biochemical behavior of the proteins that are the products
of those particular genes.
Vetticus
(sorry it's so long. seemed like such an easy question...)
Was going to edit out some spelling mistakes, but too many.... can't
be bothered.
"At more advanced levels, the concepts of expressivity and
penetrance help further modify the basic all-or-none model of
dominance. Most students never encounter these qualifications." so
sorry to hear that.