[Daniel Jenkins]

Daniel Jenkins  asked the Naked Scientists:
   Would it be possible for a human to possess all of the dominant genes, no recessive genes? And if so, what might that person look like?
What do you think?

[CliffordK]

The human body has between 20,000 and 100,000 genes. 
You will have two copies of each gene, except for those on the sex chromosomes (which men have an X&Y, and women have an active and inactive chromosome).  These code for much more than what is superficially visible.

It would be rare to be homozygous dominant for 20,000+ genes, and would certainly indicate a significant amount of inbreeding, or perhaps a member of a tribe that has had little interaction with the rest of the world.

There are several genes that have 3 distinct phenotypes, and thus can't fall completely in a dominant/recessive category.  Sickle Cell and Cystic Fibrosis genes are two of these.  The heterozygous forms are believed to offer disease protection from Malaria or the Plague.  The homozygous forms lead to devastating diseases. 

There are some systems such as skin color that likely involve several genes giving a continuum of colors from very dark to very light colors.

Certainly some things like hair color have multiple possible colors, black, blond, brown, red, etc.

I wonder if there are any "loops".  A is dominant over B, B is dominant over C, but C is dominant over A?

If you consider racial mixes of skin and hair..., White + Black ==> Brown.  Brown + Brown ==> Brown.  The tight curls of "African descent hair seems to endure for several generations.  So which genes are truly dominant?  Probably at least dark hair and the tight African curly hair, but otherwise it seems to depend on multiple genes.

Sorry, I don't have a lot of specifics about things like cleft chin & etc.

[The Penguin]

Brilliant reply Cliff. Yes, there certainly are "loops" in which there are many variable genes. Remember that a gene codes for a protein. Slight differences in the gene alone can yield the same phenotype while the genotype is considered the same yet varies slightly in the nucleotide sequence. To answer the question, to be "dominant" and "good", a gene must produce a desirable protein. In many cases the protein a gene codes for is bad, while in other cases the lack of a protein that could have been coded for is bad, and sometimes the protein can differ in many different ways. To be absolutely dominant might imply that you create all the good proteins and none of the bad. If this is the case it would be statistically impossible for this to occur.