Re: why are some gnenes dominant? What makes them dominant?

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 
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		      Phenotype
(the alleles present)      (what you see in the plant)

	  cc	                       purple
	  CC		               white
	  Cc	                       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!

-Nikki