Law of Dominance — Definition & Role

What Is the Law Of Dominance?

One of the Principles of Mendelian Inheritance is the Law of Dominance (also sometimes called the Principle of Dominance).

Let's pretend that cherries carry gene X which codes for color. Each cherry has two copies of gene X, and those copies are called alleles. We can say that allele A codes for red color, and allele a codes for yellow color.

The Law of Dominance says that when an organism is heterozygous for a trait, only the dominant allele will produce a phenotype.

So how does this apply to the cherry gene?

Let's look at the allele pairs. The first cherry is homozygous for the red allele and the second cherry is homozygous for the yellow allele. The third cherry is heterozygous, meaning it has one red allele and one yellow allele. Since this cherry is red, the Law of Dominance would say that the red allele (A) is dominant because only this allele produced a phenotype in a heterozygous organism.

This law only applies to simple traits that follow Mendelian Genetics. However, most traits are more complex and have different inheritance patterns, like codominance. This law would not apply to those traits.

What Does It Mean If an Allele Is Dominant?

The Law of Dominance says that when an organism is heterozygous for a trait (meaning it has two different alleles for that gene), the allele that is expressed is the dominant one. It is one of the Principles of Mendelian Inheritance, alongside the Law of Segregation and the Law of Independent Assortment, which describe patterns of inheritance.

Unfortunately, there's no easy way to just know if an allele will be dominant or recessive. Scientists usually have to figure out the relationship between alleles by crossing two true-breeding, (homozygous) parents together and seeing which allele is expressed in the offspring. This is the technique that Mendel himself used when studying pea plants.

For Example:

Take these two, true-breeding, lumpy people here. If you just look at them, how can you know which traits are dominant? There's no way you can necessarily say that the purple phenotype would be dominant to pink (or vice versa).

But if we cross these two homozygous individuals together, we can see which phenotypes are expressed in the offspring.

Based on the phenotype of the lumpy hybrid baby, we now know that purple is dominant to pink, and more lumps is dominant to less lumps. Another way of saying the same thing is that pink and less lumps are recessive.

The dominant gene is whichever one shows up in the phenotype of a heterozygote. The allele that is masked by the dominant allele (the one that is not expressed) is recessive.

(Video) Mendel's Law of Dominance

by Biology Professor

In this video, Biology Professor discusses one of Mendel's laws of inheritance: the Law of Dominance.

This law basically says that we will see only the dominant trait in the phenotype of an organism. Recessive traits are ones that do not show up in the phenotype.