A phenotype ratio is the percentage of organisms within any given population that possess a particular physical characteristic, such as being left-handed or having blue eyes. A genotype is the specific combination of genes in an organism that correspond to a certain trait, whereas a phenotype is the physical expression of those traits.
Humans, for example, inherit a total of 46 chromosomes from their parents, 23 each from the mother and father. Each of the chromosomes have small sections, called genes, which determine a particular trait. Chromosome strands of the same length that have the same gene location on the strand are called homologous chromosomes. For example, if a person has two chromosome strands that line up, one from the mother and one from the father, and the gene for eye color is located in the same place on each strand, that chromosome pair is considered homologous. If the genetic trait for blue eyes is called B and the genetic trait for brown eyes is called T, and both of the person's homologous chromosomes are labeled B (BB), then that person would be certain to have blue eyes. If one of the chromosomes were labeled B and the homologous chromosome was labeled T, then there's a 50-50 chance that the person would have blue eyes. If both chromosome strands were labeled T (TT), then the person would certainly have brown eyes. In this scenario, a phenotype ratio would focus on the percentage of people in the studied population who have either blue eyes or brown eyes. Such studies are often used in the field of medicine to determine the cause of certain diseases or other hereditary conditions.