In the 1800s, an Austrian Monk named Gregor Mendel first began performing breeding experiments using garden-variety pea plants. His studies earned him the nickname the father of genetics and his findings laid important groundwork for modern rules of inheritance patterns.
Mendel’s work provided basic laws establishing the rules for inheritance patterns. Understanding these patterns requires grasping the concepts of genotypes and phenotypes. These terms might sound unfamiliar, but there is a simple explanation for each.
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.
Conversely, a genotype is the full genetic information of an individual. So this means a genotype represents both of copies of the gene for a particular trait. Remember, everyone inherits two copies of a gene — one is from each parent.
The rules of dominance in inheritance patterns are relatively clear-cut. An individual who inherits two recessive alleles will always mask or hide the trait. This individual would also be considered homozygous recessive.
Recessive alleles are usually written with lowercase letters. So, if we say that the lowercase letter d represents the trait for dimpled chin, then someone who inherits two recessive alleles, one from each parent, has the genotype of dd. Again, this person is said to be 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 fascinating feature of recessive traits is that they hold more significance than merely being hidden. In certain cases, inheriting two copies of a recessive allele may indicate that a person will have a particular genetic disease. A few common recessive inherited disorders are Sickle Cell Anemia, Tay Sachs, and Cystic Fibrosis.
These examples are known as autosomal disorders, or diseases linked to the 22 pairs of chromosomes that do not determine gender. Also interesting is that there are dominant inherited disorders which are highly lethal and X- or Y-linked disorders related to mutations in genes located on our sex-determining chromosomes.
Blood type is a distinctive inherited trait that is controlled by multiple alleles. Known as the ABO blood group, the three alleles of A, B, and O can present four different phenotypes from six genotypes. A and B are forever dominant to O in the ABO blood group.
Furthermore, A and B are codominant meaning if inherited together, both will be expressed. As in any inherited trait, only one allele will come from each parent. The possible combinations are AO, AA, BO, BB, OO, and AB. The phenotypes that result are type A blood for AO and AA genotypes, type B blood for BO and BB genotypes, type O blood for the OO genotype, and type AB blood for the AB genotype.