DNA is the chemical that carries the instructions that are used by our cells to produce each one of us. The instructions are arranged in a linear fashion along the length of each DNA molecule that makes up each chromosome. Each instruction is identified as an allele or a gene. Because we have two chromosomes, we have two instructions or two alleles. The location of each instruction is identified as a gene locus. Therefore, we have a gene locus with two alleles. Together, these two alleles are called a genotype because they carry the instructions for a specific trait (also known as a phenotype).
Traits that appear to be controlled by a single gene locus are classified as monogenic or single-gene traits. Each person has two alleles (genes) for a monogenic trait. These alleles may be identical or they may be different. If the alleles are different, they may interact with each other in different ways to produce the resulting trait (phenotype). If one allele masks the effect of the other allele, it is the dominant allele. The recessive allele is masked by the dominant allele. If both alleles are expressed, the alleles are codominant. Some phenotypes that exhibit simple dominance and recessive relationships include the attachment of your ear lobe (attached is recessive and free is dominant); hyperextensible thumb (a.k.a, double-jointed thumb which is recessive); short index finger (i.e., the index finger is shorter than the ring finger, and is said to be dominant in men and recessive in women); and the widow's peak (pointed front hairline is apparently inherited as a dominant). Type A and Type B blood are codominant.
Alleles located on the X chromosome are said to exhibit sex-linked inheritance. Remember that males have one X chromosome, and females have two X chromosomes. Therefore, a recessive sex-linked trait will not be expressed in females, but it might be expressed in males. Red-green colorblindness is an example of an X-linked trait. Why do sex-linked traits show up more frequently in males?
More frequently, multiple gene loci influence the expression of a single trait and one gene locus influences the expression of multiple traits. For example, eye color in humans is a polygenic trait (two blue-eyed parents can have brown-eyed offspring). The ability to roll your tongue is also a polygenic trait (see Tongue-Rolling is NOT a Simple Inherited Trait by Robert J. Huskey).
We will investigate the process of inheritance a through the development of some simple models and then using a computer simulation program entitled Genetics Construction Kit. A discussion of the value of constructing scientific models, especially in the field of genetics, is available from the National Center for Improving Student Learning and Achievement in Mathematics and Science.