Why Is The Allele For Round Seed Shape In Garden Peas Considered Dominant?

Main Difference – Dominant vs Recessive Alleles

Dominant and recessive alleles are two terms that are described in the Mendelian genetics. Alleles are variants of a gene that determines a character of an individual. For example, in sickle cell disease, there are two genes that determine whether red blood cells should be normal or sickle-shaped. These two genes are variants of the same gene. Mendel describes these two terms using the experiments done with garden pea. He observed there were two traits in garden peas, round shaped peas, and wrinkled peas. After crossing the plant with rounded peas and the plant with wrinkled peas, the resulting first generation only had rounded peas. Then he allowed self-pollination of first generation plants, and found out the round and wrinkle characters are in three to one ratio. Since the roundness of peas is the dominant character, which masks the wrinkle, he named the allele that determines roundness as dominant allele and the allele that is masked by the dominant allele as the recessive allele. Therefore, the main difference between dominant and recessive alleles is that dominant allele determines the dominant character whereas recessive allele determines the recessive character.

What is Dominant Allele

The dominant allele is the variant of a gene that determines the dominant character. Most of the time, the dominant allele is the allele that is found in the majority of a population. However, some dominant characters do not appear frequently in the population. Dominant allele, most of the time, carries the better character. However, this is not always true.

In order to understand the idea of dominance, please look at the following example.

Example: Consider the above rounded and wrinkled peas experiment. If a plant with all rounded peas (homozygous) are crossed with a plant with wrinkled peas (These are parent plants and denoted as "P". Round character is denoted as "R" and wrinkle character is denoted as "r". The first generation is denoted as "F₁ " and the second generation is "F₂ ". Note the how dominant and recessive alleles are expressed.

Difference Between Dominant and Recessive Alleles - Expression of Dominant and Recessive Alleles

Garden pea experiment by Gregor Mendel.

What is Recessive Allele

Recessive allele is the allele that is masked by the dominant allele. Usually, the recessive allele is not expressed, so the recessive character only expressed when there is no dominant allele. If we consider that previous example, the wrinkle character is not expressed in the first generation due to masking effect by the dominant allele. Only one-third of the population in the second generation shows the recessive character. However, this is not the case always. Some characters like the human eye color are controlled by two dominant alleles and one recessive allele. The majority of the people have brown color eyes and that is the recessive character. Fewer people have either blue color or green color eyes, but that is the dominant character. Difference Between Dominant Recessive Alleles

Difference Between Dominant and Recessive Alleles

Character

Dominant allele determines the dominant character.

Recessive allele determines the recessive character. (only with no dominant allele present).

F₁ generation

In the F₁ generation, the dominant character is expressed although both the dominant and recessive alleles are present in the individuals.

Population

Most of the time the dominant character is frequently found in the population than the recessive character (3:1 ratio).

Sometimes the recessive character is frequent than the dominant character due to the high frequency of recessive allele in the population.

Superiority

Most of the time the dominant allele determines the superior character

Recessive allele determines the inferior character.

Both dominant allele and the recessive allele are essential for the diversity and the stability of the population.

Difference Between Dominant and Recessive Alleles - infographic

Reference and Image Courtesy:

Pierce. A.B. (2012).Genetics: A Conceptual Approach: New York, NY: W. H. Freeman publications

"Autosomal recessive – mini" by Thomas Shafee – Own work. (CC BY-SA 4.0) via Wikimedia Commons