What is a recessive gene?

A recessive gene is a section of genetic information with a low probability of being expressed in the individual.

In the reproductive process of living beings, the new organism receives two versions of each gene, one from the father and one from the mother. These versions, better known as alleles, contain either dominant or recessive genes in each section responsible for providing the infinite string of genetic information that will determine all the physical characteristics of the new individual. When the same recessive gene appears in both alleles, it will be expressed in the new organism. On the other hand, if one of the parents provides a recessive gene and the other provides a dominant gene, the dominant gene will be expressed, even if the individual has also inherited the recessive gene; that specimen will be able to transmit the recessive gene to its offspring.

Is it possible for a recessive gene to appear in an offspring of two parents with dominant genes?

Yes, it is possible, though it’s unlikely. Imagine that two individuals have brown eyes. The dark-colored eye gene is dominant – let’s call it (A). As we’ve already said, living beings that reproduce have two versions of each gene, so they also possess a second version, a second gene, even though it is not expressed. This second version could be the one for green eyes, which is recessive (a). If both parents have a combination (Aa), this means that they have brown eyes, but they have the gene for green eyes.

The combination of the Aa x Aa genes functions like distributive multiplication, and the possible combinations are:

• AA
• Aa
• Aa
• aa 

This result shows that the offspring will have only a 25% chance of being born with green eyes and a 50% chance of having brown eyes but passing the gene for green eyes on to a third generation. Similarly, they will have a 75% chance of having brown eyes and a 25% chance that their offspring will completely lose the green gene.

While each organism’s genetic code is produced by the crossing of genetic pairs contributed by its parents, this does not mean that there are only two unique versions of each gene. Some genes are recessive when compared to certain genes but dominant when compared to different versions. Therefore, dominance and recessivity depend on the relationship between the two alleles at hand.

What does knowing which genes are recessive and which are dominant tell us?

The notions of dominance and recessivity are fundamental for the entire development of genetics, and they are quite useful in different fields, such as:

• In medicine, this is used to diagnose and prevent hereditary diseases.
• In agriculture and livestock, it’s used to select desirable traits and breed out those that are less conducive for a certain purpose, such as better hardiness in certain climatic conditions, a larger or smaller size, the ripening time needed, etc.
• In forensic genetics, DNA tests are used to determine an individual’s line of parentage, criminal investigations, and victim identification.
• In biology, the rate of certain dominant or recessive genes is studied to investigate how environmental conditions impact the natural selection of species.

None of this would have been possible without the basic ideas of dominance and recessiveness.