The roots of genetics can be traced back to the 1800s when an Austrian monk named Gregor Mendel first began studying inheritance patterns using garden-variety pea plants. The results of his studies provided important groundwork for modern genetic science and are much of the reason why the science of genetics has prospered to date.

The work of Gregor Mendel established fundamental rules for patterns of inheritance. The principles of genotypes and phenotypes must first be understood before mastering Mendel's rules. Although unfamiliar, these terms have a simple explanation.

A phenotype is the physical manifestation of a gene. A dimpled or cleft chin is an example of an inherited trait. Your phenotype is established by whether you have a dimple in your chin. In other words, you express the trait if you have a dimpled chin. In addition, this means the trait is dominant. A hidden trait, for example no dimpled chin, is known as recessive.

On the other hand, a genotype is an individual's actual genetic information, whether hidden or expressed. In other words, a genotype indicates both allelic copies of the trait. Don't forget, everyone gets two copies of a gene -- one comes from each parent.

Dominance rules in patterns of inheritance are pretty straightforward. If an individual inherits two recessive alleles, the trait will always be hidden or masked. This individual would be considered a recessive homozygote.

Recessive alleles are normally written out in lowercase letters. For example, if we indicate the trait for dimpled chin with a lowercase letter d, then a person who inherits two copies of the recessive allele, one from each parent, will have the genotype of dd. Again, this individual would be considered homozygous recessive.

Consequently, if an individual inherits two dominant alleles, the trait will always be expressed. This is also known as homozygous dominant. Dominant alleles are written with, you guessed it, capital letters; therefore, someone with two dominant alleles, DD, for the dimpled chin trait will surely have a dimpled chin.

So, what happens if someone inherits one dominant and one recessive allele, effectively Dd? This means that they are heterozygous for the trait; however, they will always express it. The reason for this is because dominant alleles are exactly that -- dominant. Even in the presence of a recessive allele, a dominant trait will always be expressed.

Another interesting point on recessive alleles and traits is that they are more significant than merely being hidden. In some cases, two copies of a recessive allele can mean that a person will inherit a genetic disease. Some commonly known recessive inherited diseases are Cystic Fibrosis, Sickle Cell Anemia, and Tay Sachs.

These recessive diseases are examples of autosomal disorders, which are disorders linked to the 22 chromosome pairs which do not determine gender. Also curious is that there are dominant inherited disorders which are very lethal and sex-linked disorders that are related to genetic mutations on the X and Y chromosomes.

Blood type is a special type of inherited trait that is governed by multiple alleles, or gene variants. Most often referred to as the ABO blood group, these three alleles, A, B, and O, can produce four phenotypes from six genotypes. Of the three possible alleles, A and B are always dominant to O.

Likewise, A and B are codominant which means that if inherited together, both A and B will be expressed. As with any other inherited trait, only one gene copy will be donated from a parent. Possible mixtures are AO, AA, AB, OO, BO, and BB. The resultant phenotypes are type A blood for the AO and AA genotypes, type AB blood for the AB genotype, type O blood for the OO genotype, and type B blood for the BO and BB genotypes.