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.
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 merely how a gene is expressed. For instance, a dimpled chin is an example of a trait that is governed by genetics. Your phenotype is determined by whether or not you have a dimple in your chin. If you have a dimpled chin, you express the trait. By the way, this means the trait is dominant. A recessive trait, no dimpled chin for instance, is one in which the trait is hidden; hence, the term 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 of inheritance patterns are mostly unambiguous. If a person receives two recessive alleles, they will always hide or mask the trait. This person would also be known as a recessive homozygote.
Recessive alleles are typically indicated with lowercase letters. So, if we say the trait for dimpled chin is represented by a lowercase letter d, then an individual who inherits two recessive alleles, one that came from each parent, will have the dd genotype. Again, someone with this genotype is said to be a recessive homozygote.
Accordingly, if a person receives two copies of a dominant allele, the trait will always be expressed. The expression of two dominant alleles is termed homozygous dominant. Dominant alleles are typically indicated by, you guessed it, capital letters; thus, an individual who has two dominant alleles, DD, for the dimpled chin trait will absolutely 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 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 unique type of genetic trait which is governed by multiple alleles. Often spoken of as the ABO blood group, these three alleles, A, B, and O, can yield a possible four phenotypes and six different genotypes. In the ABO blood group, A and B are permanently 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.
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