How Are Traits Inherited?
We all know that DNA is the chemical that carries the instructions that are used by our cells to produce each one of us. During most of the life of a cell, its DNA looks like a bunch of uncoiled ropes stuffed into a plastic bag. If a cell is destined to divide into two daughter cells, it duplicates its DNA first so that each daughter cell receives the same sets of instructions as were present in the parent cell. Just before a cell is ready to divide, the DNA (with its associated proteins) becomes coiled into very discrete packages called chromosomes. This process of coiling facilitates the easy transfer of identical sets of chromosomes to each daughter cell during the process of mitosis. We have 46 chromosomes in each of our cells (except sperm and eggs which have 23 chromosomes). These 46 chromosomes are actually 23 pairs (one member of each pair came from our mothers and the other member of each pair came from our fathers). Each pair of chromosomes (numbered 1-22 and X and Y in humans) therefore has a duplicate set of instructions.
Most cells in our bodies divide to produce two daughter cells with the exact same number of chromosomes (46) as the parent cell through the process of mitosis. We describe these cells as having a diploid number of chromosomes. However, cells that are destined to become egg and sperm produce daughter cells with only half the number of chromosomes (23) through the process of meiosis. Sperm and egg are identified as having a haploid number of chromosomes. Understanding the difference between mitosis and meiosis is essential to understanding heredity. To help visualize this process, J. Stein Carter has developed a simple animation comparing the two processes.
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).
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).