In The Gno

Electron microscopy of chromosomes

These terms might sound scary, 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.  Some people call this a “cleft chin.”  If you have a dimpled chin, you express the trait.  Essentially, your phenotype is a “YES – I have a dimpled chin.”  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.  A genotype, on the other hand, is the actual hereditary information of an individual.  This means that a genotype represents the copies of the alleles one inherits for a particular trait.

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 usually written with lowercase letters.  So, if we say that the letter “d” represents the trait for “dimpled chin,” then someone that inherits two recessive alleles (one from each parent) has the genotype of “dd,” and 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, so 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.

One last note on recessive alleles and traits…  They are more significant than merely being “hidden.”  In some cases where, 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.  We will explore genetic diseases and autosomal vs. sex-linked disorders at a later date.

I hope you have enjoyed reading about the expression of traits and dominance rules.  Remember, what you “know” is not as important as what you “gno.”