What makes some alleles dominant?

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Offline Matthew

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What makes some alleles dominant?
« on: 18/10/2009 23:30:04 »
Matthew asked the Naked Scientists:

My question is about inheritance. Can you please tell me why one dominant allele, found in the pair of alleles in each chromosome, has the overriding mechanism for determining a particular characteristic?

Also, why do genetic diseases such as cystic fibrosis only occur when there are two recessive (or faulty genes)? Is this because if there is one dominant allele in the pair, this means the person remains healthy?

Thank you.

What do you think?


Offline Nizzle

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What makes some alleles dominant?
« Reply #1 on: 27/10/2009 10:42:02 »
There's a small misconception in your post about dominant alleles.
There is no overriding mechanism.

Dominance in genetics refers to which gene determines the effect on the organism.

We humans (and all other animals + a whole bunch of plants) are diploid organisms, which means that our entire genome is present in two copies in each cell.

So each gene is present twice, and this is what we call alleles. You get one allele from your mother and one from your father.

A dominant allele means that when it is present in your genome, it determines the effect of that gene on the body, no matter what the other allele is (recessive or dominant). [This is oversimplified for explanatory purposes. In the case of double dominant, there are sometimes quantity of transcription effects noticeable].
Example: Gene AB2C is coding for a protein that performs the chemical reaction A + B → C.
Let's say gene AB2C is the dominant allele of this gene and ab2c is the recessive, which can't perform this reaction.
People that have AB2C AB2C will be able to perform the reaction
People that have AB2C ab2c will also be able to
People that have ab2c ab2c will not be able to perform the reaction.
Since 1 copy of AB2C is already enough to perform the reaction, this gene is said to be dominant over ab2c, and only double recessive alleles will not be able to convert A and B to C.

The alleles however, do not have an effect on each other though, and a dominant allele does not have a mechanism specifically designed to suppress transcription of the recessive allele. [It might have a suppressive mechanism, like negative feedback, but that will affect both alleles in the same way]

Your thoughts about cystic fibrosis are correct, although i must say that double recessive does not always have to lead to an illness.
Some dominant/recessive alleles determine eye color or hair color, or how tall you'll become, or any numerous other condition that contributes to the fact that all 6 billion people on earth are unique.
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