Chemically, what determines that an allele is dominant or recessive?

Answer 1

Great question. It's not the chemistry of the gene itself that determines whether or not inheritance is recessive or dominant. Actually, its the manner in which the gene product (protein) functions!

Ok, so every cell has 2 copies of each gene, 1 from your mother, 1 from your father. These genes code for proteins that determine every single chemical, structural, and functional aspect of the cell. Some chemical, structural, or functional endpoints require that BOTH copies of a gene be expressed to function properly. Some endpoints only require ONE copy to be properly expressed. This is what makes dominance and recessivity (respectively). Let me use examples.

Take the gene that causes sickle cell disease. If you inherit a mutation in this gene for sickle cell disease from your mother, but inherit a normal copy of the gene from your dad, you are not going to have sickle cell disease, because it is inherited recessively. Why? Because half of the hemoglobin produced is still normal, and this is enough for red blood cells to function perfectly normally!

Contrarily, there are phenotypes that are inherited dominantly. Dominance means that BOTH COPIES of a gene that you inherit ( 1 from each parent) must be normal for proper function. There are three reasons why a phenotype would be inherited dominantly:

1. haploinsufficiency - this means that in the case of a heterozygote (one normal gene, one mutant gene), that half the amount of normal protein will be produced. But contrary to sickle cell disease, where half the normal amount of hemoglobin did NOT cause disease, half the amount of protein here is WILL cause disease, and the mutant phenotype will be present.

2. gain of function - this means that in the case of a heterozygote, the normal copy of the protein can function properly, but the mutant protein is doing something HARMFUL. So one copy of a gene causes a mutant phenotype.

3. dominant negative - this means that in the case of a heterozygote, the normal protein and mutant protein are being expressed, but the mutant protein is affecting the normal protein, and thus it can't function. For example, collagen is formed by helical peptide subunits that form a TRIPLE HELIX. In mutations that affect collagen synthesis (like achondroplasia), the mutant proteins try to join with the normal proteins to form the triple helix, but since it is mutant, the triple helix doesn't work 100% right. So the mutant protein has a dominant negative effect on the normal protein.

Now, of course if you are homozygous for a dominant phenotype, the effect is usually significantly worse than being heterozgous, because NO NORMAL PROTEIN IS PRODUCED AT ALL. Hope this clears it up for you.

Answer 2

It is actually not as complex as you would think; usually it is just that a dominant allele produces a product that overwhelms a recessive allele. For example, blonde/brown hair, blonde pigment will easily be overwhelmed by brown pigment, same with eye colour.

In seemingly more complex cases like straight vs curly hair, it would be due to the dominant protein/s ability to do something the other allele's protein can't, or by blocking the protein produced by the other allele.

For example, in recessive diseases, if you have one good copy of the gene this "dominates" because you still have a working protein being made, but dominant diseases occur when the mutant gene produces a product that stops the normal protein from working too.