Dominant and Recessive Genes

If you attended biology classes in vain, you may be surprised to learn that not all recessive genes are rare, and some genetic disorders have a dominant inheritance pattern. Sounds like rocket science? Then read on to find out what these terms denote and whether it is possible to say an allele is definitely dominant or definitely recessive.

Image Credit: wikipedia.org

Confusing terms

When most of us hear the terms in question, the first thing that comes to our mind is red hair versus dark hair, blue eyes versus brown eyes, etc., and that the dominant allele “dominates”, and that is why red-haired people are rare. In fact, it’s not that simple.

Every species that reproduces sexually has each gene present in two copies. These gene copies are called alleles. There can be certain differences in alleles, which affect the way the protein expressed by the gene works. It results in different phenotypes, i.e. different sets of traits, behavior and physiological properties (for example, whether a person is allergic to something, with a rare condition, or suffers from an inherited disease), products of behavior, etc. So, to make it simple:

There are two copies (alleles) of each gene -> alleles can differ -> difference leads to changes in protein expression -> different phenotypes are produced.

Contrary to what we are used to hearing, inheritance patterns, or how traits are inherited, are complex, and you cannot label a particular allele as a dominant or a recessive one. And here is why.

Since the sexual reproduction implies involvement of two individuals, they may carry different alleles, which can be recessive or dominant depending on what function of the protein expressed by the gene is meant.

Dominant or recessive?

The classic example is hair color. Red hair color is determined by a recessive allele, whereas dark hair is a dominant phenotype.

If two dark-haired individuals with two dominant alleles have a child, their child will be dark-haired too.

If there is one recessive and one dominant allele, you get a dominant phenotype (and become a carrier of the recessive phenotype though you do not look like one).

If you combine two recessive alleles, you get a recessive phenotype.

It looks quite simple, but it is not. The thing is, alleles code for proteins with different functions, and they can be dominant when one function is concerned, and recessive when another function is considered.

A good example is the sickle-cell allele, which, if present in two copies, causes sick-cell disease that affects hemoglobin. This function (causing the disease) has a recessive inheritance pattern, i.e. in order to suffer from it, you must have two alleles of this kind.

However, when another function of it is concerned, it becomes dominant. Besides causing the disease, the sickle-cell allele makes its carriers malaria-resistant, and this phenotype is a dominant one.

So, an allele can be dominant and recessive at the same time.

Common myths

  • Many people believe that if a phenotype is a recessive one, it is twice as uncommon as the phenotype which is dominant. However, it is not true. Look at Scandinavians – the majority of them have light eyes, but the phenotype is recessive. The same is true of blonde hair.
  • Some people think that “dominant” means better. It’s another myth, because whether a trait is beneficial or not depends on the environment in which an organism lives. Imagine a mouse that has dark fur – it may be easier for such a rodent to live in places with dark ground, because the dark fur color (a dominant phenotype) will make it less visible, and it will be more difficult for predators to find the mouse. But if this mouse moves to a place filled with sand, it will not be an advantage any more.
  • Contrary to the widespread assumption, not all genetic disorders are coded by recessive genes. Some genes, like, for example, keratin genes, can have defects with dominant inheritance pattern.

References:

Dominant and Recessive Characteristics – Blinn.edu

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