Incomplete Dominance

Gregor Mendel introduced the theory that in all living organisms, the hereditary traits are passed down from parents to offspring through a set of genes, which constitutes its genetic makeup.

He was also the first one to propose based on his experiments with pea plants that genes which are generally in pairs do not unite; instead, they segregate when sex cells fuse. During segregation, each member from a pair attaches itself to one sex cell each.

Mendel states in his law of segregation that for a particular trait, every organism has two alleles, one each from a male and female organism. If these two alleles are identical, then the organism is referred to as homozygous. If they are dissimilar, they are called heterozygous.

In the case of heterozygous organisms, the trait of dominant allele hides the trait carried by repressive allele. Thus, an organism’s genotype is the sum of all alleles that it receives from its parents. 

Do You Know? 

The word gene was coined only in the 29th century!

What Is the Law of Complete Dominance?

Mendel’s law of dominance based on his experiments with pea pod states that the dominant allele trait will completely mask the character of a recessive allele, which will also influence an organism’s phenotype. His pea plant experiment serves as a complete dominance example.

Let us look at it – 

Mendel took a purebred white, and purple flower and crossbred them. The resulting hybrid was a purple coloured flower and not a combination of the two colours. Thus, the purple flower was a dominant trait which completely hid the traits of the white flower. Nonetheless, when he allowed the hybrid plant to self-fertilize, the recessive traits appeared.

However, the study of genes in recent times has shown that gene alleles do not behave in such a straightforward way. It results in several other patterns of dominance, such as incomplete dominance. 

Let Us Examine What is Incomplete Dominance.

Incomplete Dominance

For instance, if a homozygous red(R1R1) and homozygous white-flowered plant (W1 W1) is crossbred, then the first generation will produce pink coloured flowers of the variety (R1W1). It shows that alleles of red and white coloured flowers were unable to dominate the other, thus resulting in incomplete dominance. Thus, the law of incomplete dominance says that when none of the two alleles exerts complete dominance over the other, the offspring will be a mixture of parents’ phenotypes. 

Aside from flowering plants, incomplete dominance takes place in human beings and animals as well. One such incomplete dominance example in human beings is that the growth of wavy hair. In this case, one parent has straight hair, and the other has curly hair, but none of the traits is entirely dominant, so it results in a new hair type – wavy hair.

Did You Know? 

The gene for six fingers is a dominant trait.

Complete Dominance vs Incomplete Dominance

Complete Dominance

Incomplete Dominance

One allele trait dominates or hides the recessive trait.

None of the allele traits dominates the other.

The dominant trait is expressed in the phenotype.

A new phenotype is created.

Codominance

Another pattern of dominance that does not follow Mendel’s law is codominance. In this type, the gene alleles of both parents are dominant, and together they form an entirely new phenotype for their offspring. Let us consider the incomplete dominance example of the red and white flower again.

In case of incomplete dominance, we saw that the colour of a hybrid flower turned pink as neither of the traits was dominant enough. However, in codominance, the bulb will show both red and white colours. Another instance of codominance is blood group in human beings. Individuals with AB blood group inherited one allele for A blood group from one parent, and another for B blood group from another parent, where both alleles are equally dominant.

Incomplete Dominance vs Codominance

The Main Difference Between Incomplete Dominance and Codominance Have Been Summarised in the Table Below - 

Incomplete dominance

Codominance

When none of the parent traits is completely dominant; therefore, they blend to form a new character. Nonetheless, the effect of one of them is slightly more than the other.

When both parent traits are dominant and are expressed equally in the offspring, and their effects are also similarly presented.

The hybrid offspring displays a phenotype different from its parents.

The hybrid doesn’t show any new phenotype.

Incomplete dominance example is snapdragon, Mirabilis.

Roan character in cattle is one example of this dominance trait.

Additionally, you can avail detailed study guides on genetics at Vedantu’s online portal along with charts and diagrams which will help to gain a better understanding of the concept. Alternatively, you can download a pdf version as well directly from your phone’s Vedantu app.

FAQ (Frequently Asked Questions)

1. What is Incomplete Dominance in Genetics?

Ans. When phenotypes of parents combine to develop a completely different phenotype for their offspring, it is known as incomplete dominance.

2. What is Codominance?

Ans. When both phenotypes of parent organisms are expressed in the child, it is known as codominance.

3. What is the Difference Between Incomplete Dominance and Codominance?

Ans.  In incomplete dominance, the alleles are not thoroughly dominant. On the other hand, in codominance, both alleles are entirely dominant.