Mendel’s Law of Dominance can also be simply stated as:
“In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.”
Mendel’s Pea Plant Experiment
We all know that Mendel was breeding peas to observe the hereditary effects of different types of breeding.
In a cross of parents that are pure for contrasting traits:
Here pure means monohybrid whereas contrasting traits mean any two form a particular trait. For example, say height- Pea plant could be of two possible heights : either tall or short.
So, Mendel takes two pea plants: One short pea plant (thus having genes “tt” for expressing height) and one tall pea plant ( Having a genes “TT” which express or determine its height.)
Then he bred them by pollinating the flower of one monohybrid short pea plant with pollen of monohybrid tall pea plant. However, when he planted these seeds after harvesting, he found that all of the offspring plants were tall. Why was this so?
We all know that in sex gametes, genes break up and one of them forms a pair with an allele from the other cell. For pea plants, the two sets of gametes were TT and tt. Hence, during meiosis, the only possible combinations is Tt.
So why does only T expresses itself in a Tt pair?
Why was a plant whose one parent was tall and another short expresses only the tall trait?
This anomalous behaviour was exhibited in various other crossbreeding experiments in which two contrasting trait exhibiting plants were used.
So Mendel thought, maybe one gene suppressed the other or prevented the other gene from expressing it. And thus he devised the Law of Dominance which states that:
In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.
Such a trait is known as a Dominating trait. The suppressed trait is known as Recessive trait. Also, the recessive trait freely expresses itself in the absence of the dominant state. And this is what Mendel’s Law of Dominance is all about.
So in the pea plant, the seed color yellow always dominates seed color green. seed shape round always dominates seed shape wrinkled. So here, round (RR) and Green (GG) are the dominant traits whereas wrinkled (rr) and yellow (gg) are recessive traits.
We know from Mendel’s Law of Dominance that if there exists two contrasting traits, one of the traits will always suppress the other, thereby expressing itself (Dominating trait) . In the pea plant experiment example above, T suppresses t, thus making the plant offspring tall.