You have learned that genes are segments of chromosomes. So far, we have been focusing on pairs of genes, which are found on pairs of chromosomes, or homologous chromosomes. Each person has a pair for each trait. When we talk about multiple alleles, that means that there are more than two kinds of gene for that trait in the population as a whole, not in any individual. An individual has only a pair of alleles on a pair of chromosomes. For example, there are A, B, and O blood groups. Any one person has a combination of two of these alleles, such as AA or AO or AB. Within the human population, there are three possibilities of allele, making this type of inheritance multiple alleles.
Polygenic inheritance pattern refers to many--poly--genes. It describes a trait that is the result of a combination of proteins from genes on different locations within a chromosome or on chromosomes that are not homologous, meaning not paired. The gene on one chromosome pair contributes part of the trait, and the genes on other pairs contribute other components. A popular question about genetics refers to eye color, which is a good example of the polygenic inheritance pattern. The appearance of the eye results from the general color of the iris, the rim of the iris, flecks of other colors, each resulting from the action of a different gene pair. The polygenic inheritance pattern is also responsible for height, skin color, and hair color, among others. Keep in mind that each of these genes still has the possibility of having two recessive alleles, two dominant, or one of each, or the alleles can be incompletely dominant. Because of the numerous combinations, there is a range of phenotypes, not just two or three, as in a single gene. With a single gene pair, there is an either/or situation; the characteristics are discontinuous, or distinct from each other. One has widow's peak, or does not; one has polydactyly, or does not; one can taste phenylthiocarbamide, or not taste it, or have a delayed reaction (This phenotype is due to incomplete dominance). There are many heights to which humans grow; one can be four-foot-ten; six-foot seven, or anything in between. There is a range of skin colors, not just chocolate or vanilla; the same applies to weight, hair color and texture, foot size, and some genetic disorders. When two individuals at opposite ends of the continuum mate, the probability is greatest that the offspring's characteristic will be intermediate. If a tall person marries a short person, their offspring will probably be average in height. When the different phenotype frequencies are graphed, their distribution resembles a bell curve, with the greatest number of individuals placed in the middle, where the "in-between", or intermediate, phenotype is shown, and the smallest numbers on the two ends, where the most distinctive phenotypes are shown.
To summarize, the term "multiple alleles" refers to one pair of alleles per person, but several possibilities within the population of that species. The term "polygenic inheritance pattern" refers to several genes on different loci, or chromosome locations, or genes on more than one chromosome pair, each contributing to the trait.