Re: Do the zygotes of the same population vary only in their genes?

Hi Lydia, 

Thanks for writing.  Your question deals with the genetic similarities or 
differences between GAMETES (haploid sex cells) within a species.  You used 
the term "zygote" in your question, which actually refers to the diploid 
cell resulting from the union of a haploid egg and a haploid sperm.  In 
other words, a zygote is a fertilized egg.   

The gametes within any particular population contain very similar genetic 
information, with only very small differences in the genetic code.  These 
subtle differences have considerable impact, however, as they account for 
the wide variation in traits that one sees in a population (e.g., eye color, 
height, genetic predisposition to disease).  Furthermore, not only do the 
gametes in a population (or gene pool) differ from one another genetically,  
the gametes within an individual are not genetically identical either.  This 
is due to the way in which the chromosomes segregate and exchange miniscule 
portions of DNA during meiosis.  In this way, genes are combined in novel 
ways in every generation, which, among other things, facilitates evolution.  

You are absolutely right in stating that all heritable variation within 
populations is due to variation in genes.  Heritability is an expression of 
how much of the observed variation in a phenotype, or trait, is due to 
differences in genotype, or genetic makeup.  The 
"extra-genetic variation" you are wondering about does not come from the 
gametes, but from the environment.  The effects of the environmental factors 
we are exposed to throughout our lives (radiation, nutrients, chemicals, 
etc.) can influence phenotype tremendously, and can definitely account for 
differences within a population.  

I admire your conviction to teach your children the facts correctly.  I hope 
that this information is helpful to you, and I would be delighted to hear 
from you again if you have additional questions.

Warm regards, 

Jen