Genes come in different varieties, called alleles. Somatic cells
contain two alleles for every gene, with one allele provided by each
parent of an organism. Often, it is impossible to determine which two
alleles of a gene are present within an organism's chromosomes based
solely on the outward appearance of that organism. However, an allele
that is hidden, or not expressed by an organism, can still be passed on
to that organism's offspring and expressed in a later generation
