Summary of genetics:

There are variations in the features of individuals in any population.  These variations are inherited from the parent organisms. If a variation is beneficial or advantageous to survival of a population, then the members of the population with that variation are more likely to survive and pass that variation on to the next generation.  In this way, adaptations result in the survival of the individuals best suited to any particular environment.  The chemical codes which cause these variations are carried on structures called chromosomes, made of a chemical called DNA, in the nucleus of every cell. 

Chromosomes exist in all body cells as similar, matched, or homologous pairs. One half of each chromosome pair came originally from the female parent organism (in the egg) and the other from the male parent organism (in the sperm).  Small sections of each chromosome control different features of the organism (eye color, hair color, fur pattern, etc.).   A small section of each chromosome in a chromosome pair holds the chemical code for the same feature as a matching small section on the other half of the pair (such as for eye color). The part of one chromosome that controls a particular feature is called an allele.  Taken together, the similar alleles controlling one feature on a pair of chromosomes make a gene.

  An allele can be dominant or recessive.  If a dominant allele is present in either half of the chromosome pair, or in both, then the trait (a variation of a feature) carried by that allele will be expressed (show up in the organism).  The traits carried by recessive alleles can only show up if the recessive allele is present on both halves of the chromosome pair. 

An allele is represented by a single letter related to the feature that it controls.  For example, a letter "e" could represent the allele for eye color.  Capital "E" would represent the dominant allele (in humans this would represent brown eye color), and lower case "e" would stand for the recessive trait (blue eye color in humans).  Someone with the genotype EE (referred to as "homozygous dominant") would have the phenotype of brown eyes in this simplified example.  The heterozygous genotype Ee would also result in brown eyes, because one dominant allele is present.  Only the homozygous recessive genotype (ee) would result in blue eyes.