Saturday, December 27, 2008

Mendelian Rules of Inheritance

By RJ Sullivan

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.

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 nothing more than the manner in which a gene is expressed. For example, a dimpled chin is a widely recognized example of a genetic trait. Whether or not you have a dimpled chin determines your phenotype. In other words, if you have a dimple in your chin, the trait is expressed. This also means the trait is dominant. A trait that is hidden, no dimpled chin for example, is what is known as a recessive trait.

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 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 are examples of autosomal disorders, which are diseases that are linked to the 22 pairs of chromosomes that do not determine our gender. Interestingly enough, there are also dominant inherited disorders which are quite lethal and sex-linked disorders that have to do with mutations in the genes located on our X or Y 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.

In addition, A and B are codominant; this means if inherited together, both alleles will be expressed. Like any other inherited trait, only one allele will be donated from each parent. Possible combinations are AA, AO, BB, BO, AB, and OO. Resulting phenotypes are type A blood for AA and AO genotypes, type B blood for BB and BO genotypes, type AB blood for the AB genotype, and type O blood for the OO genotype.

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