What makes a gene recessive or dominant?

Question

What makes a gene recessive or dominant?

Answer 1

This is a great question.

What makes an allele recessive is nearly always that it has a "loss of function" mutation.  This means that that allele either produces no protein product, or one so altered that it cannot perform its job.  

If you are heterozygous and have one dominant (i.e., functional) allele and one recessive (i.e., non-functional) allele, then the functional allele will code for the protein product and you will exhibit the dominant trait.  It's only when you have two copies of the non-functional recessive that you exhibit that trait.

When alleles show incomplete dominance (this is there the heterozygote has an intermediate phenotype), it's usually related to something like color where quantity of the product matters.  For example, in flowers, you might have a red allele (which results in the red pigment product), a recessive white allele (which results in no product), but in the heterozygous condition you end up with pink because there is less red pigment made than in the homozygous RR condition.

In codominance, two different alleles are both expressed individually.  In ABO blood types, for example, Type AB is a codominant expression because your A allele is producing the A antigen and your B allele is producing the B antigen, so both are displayed.  In this example, O is recessive because the enzyme to add the final sugar doesn't work.  But, if you are Ao, you have Type A blood since the A allele does make a functional enzyme to add that antigen.

Answer 2

A gene is recessive if the associated trait requires two copies.

A gene is dominant, if a single copy is sufficient to express the trait.

In incomplete dominance, the trait is not fully expressed with a single copy (e.g. pink rather than red).

In codominance, there are two or more traits that can be expressed over a recessive (e.g. A and B blood types -- or both -- over O).

Answer 3

A dominant gene is so often expressed while as a recessive gene is not as explained by G.Mendel. It may be the nucleotide sequence in a gene which makes it dominant or recessive.

Answer 4

You have gotten some very good answers here. However, I'm not sure that anyone of them is complete in and of itself. Gene dominance or recessive is complex, at best.  You can bet that evolution (and selective pressures) has a hand in determining which gene gets expressed as a dominate trait.  It is my understanding that at each allele, there is a bio-chemical process for determining which gene will dominate. There must be selective forces acting at and on that particular allele.  There is some evidence to suggest that which ever gene gets selected as dominate, it will release a biochemical factor which will hold the other gene in a recessive condition.  Both dominate and recessive genes produce a protein product that differs from one another by the amount, the level, of protein product produced. The recessive gene may produce significantly less protein than the dominant gene.  Somehow, it is the dominant gene that holds, through some biochemical pathway, the recessive gene in check.

It makes one wonder what would happen if you could somehow inject a biochemical substance that would neutralize the dominate gene for say brown eyes in a heterozygous dominate person for that eye color trait?  If selective genes within the genome could be biochemically neutralized, and the recessive genes allowed to become dominate, would there be an immediate phenotype change in eye color?

See: GENEALOGY-DNA-L Archives

http://archiver.rootsweb.ancestry.com/th...

Answer 5

A dominant gene usually results from one of the parents being 'affected' therefore any offspring they have would have a 50/50 chance of having the condition that the affected parent has...

A recessive gene is where one of the genes in the pair has a fault and is passed on to the gene pool of the offspring etc. so when somebody has condition caused by a recessive genetic problem it is the result of both the parents passing on a faulty gene to the same component (ie a cleft palate or Cystic Fybrosis) or another way to look at it is a 1 in 4 chance of their offspring being affected.

Hope this helps?

Answer 6

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