Incomplete and Co-Dominance – Types & Examples

Incomplete and Co-Dominance 

Introduction

Gregor Johan Mendel was an Austrian monk who discovered important facts about heredity. Mendel was the first person to predict which traits would be passed from parents to offspring.  

He discovered the basic principles of genetics and hence is popularly known as “The Father of Genetics.” 

Between 1856-1863, Mendel conducted hybridization experiments on the garden peas (Pisum sativum). During that period he chose some distinct characteristics of the peas.  

Mendel carried out his experiments on inheritance with the pea plant taking seven contrasting characters of the plant, which included:  

1. Stem height 
2. Pod shape 
3. Pod color 
4. Flower position 
5. Flower colour 
6. Seed color 
7. Seed shape 

Mendel selected pea plants for his experiments because pea plants can be grown quickly and maintained easily. They are naturally self-pollinating but can also be cross-pollinated manually. It is an annual plant; therefore, it is possible to study several generations in a short period. It has several contrasting characters. 

Explanation

Mendel’s laws of inheritance 

Mendel discovered the fundamental laws of genetics and inheritance. He discovered that genes come in pairs and are inherited as separate units, one from each parent. Mendel analyzed the segregation of parental genes and their expression in the offspring as dominant or recessive traits. 

Mendel’s laws of heredity are usually stated as: 

1. The Law of Segregation: When gametes form, alleles are separated so that each gamete carries only one allele for each gene. 

2. The Law of Independent Assortment: The segregation of alleles for one gene occurs independently to that of any other gene. 

3. The Law of Dominance: An organism with alternate forms of a gene will express the form that is dominant. 

What are the extensions to Mendel’s principles? 

As the study of inheritance expanded beyond Mendel’s seven traits, biologists noticed a diversity of relationships between alleles that code for the same character. These allelic interactions were not exclusively dominant or recessive. These types of interactions led to the extension of Mendelian principles.  

However, the fundamental principles of Mendelian genetics still hold true. In the sections to follow, we consider some of the extensions of Mendelism. 

Mendel had studied traits with only one type of inheritance in pea plants. The inheritance of the traits studied by Mendel followed the relatively simple pattern of dominant and recessive alleles for a single characteristic. 

Later, it was discovered that there are various important modes of inheritance, brought to light after Mendel’s work, that do not obey the dominant and recessive, single-gene model. 

Incomplete Dominance: 

Incomplete dominance is a phenomenon in which a dominant allele or a form of a gene does not entirely hide or mask the effects of a recessive allele, and the organism’s resulting phenotype shows a blending of both alleles. 

An example of incomplete dominance is seen in snapdragon (Antirrhinum majus). A cross between a homozygous parent with white flowers and a homozygous parent with red flowers produces offspring with pink flowers. 

The allele for red color is dominant over the allele for white color, but heterozygous flowers having both alleles are pink in color. 

This inheritance pattern is called incomplete dominance, meaning that  the traits coded by both the alleles of the heterozygote appears in the phenotype. In such a case, we can say that the allele for red flowers is incompletely dominant over the allele for white flowers. 

In this case, the F2 genotype and phenotype ratio will be the same, i.e., 1 : 2 : 1. Thus, the mendelian monohybrid ratio of 3 : 1 gets modified. 

Examples of Incomplete Dominance: 

An example of incomplete dominance that is seen in humans is wavy hair. There are two alleles for the texture of hair, curly or straight.  

If an individual is homozygous for either type of these alleles, they can either have curly hair or they have straight hair. 

However, if a person is heterozygous for hair texture, meaning they inherit one allele for curly hair and one allele for straight hair, the person will have wavy hair.  

This is because the two alleles blend to create a unique phenotype, the hallmark of incomplete dominance. 

Co-Dominance: 

As opposed to incomplete dominance, co-dominance occurs when phenotypes of both parents are simultaneously expressed in the same offspring. In co-dominance, the expression of alleles is uniformly conspicuous, meaning that both the alleles have an equal chance for expressing their effects. The offspring express both the parental phenotypes. 

No allele is dominant or recessive, and the dominating relationship is absent. An example of co-dominance is observed in the ABO blood groups of humans.  

The alleles A and B are expressed as A or B molecules present on the surface of red blood cells.  

The homozygotic forms (𝐼^𝐴 𝐼^𝐴 and 𝐼^𝐵 𝐼^𝐵) express either the A or the B phenotype, and heterozygotes (𝐼^𝐴 𝐼^𝐵) express both phenotypes equally. 

 The 𝐼^𝐴 𝐼^𝐵 individual has blood type AB. 

In a self-cross between heterozygotes expressing a co-dominant trait, the three possible offspring genotypes are phenotypically distinct.  

In this case, the F2 genotype and phenotype ratio are the same, i.e., 1: 2: 1. Thus, the mendelian monohybrid ratio of  3: 1 gets modified. 

Examples of co-dominance: 

In some animals, co-dominance is seen by a mix of coat colors in a progeny of parents with different coat colors. Consider a cross between a black-furred male dog and a white-furred female dog. It leads to the production of an offspring with a black-and-white coat.  

This suggests that the color coat traits of both the parents are co-dominant as both alleles were expressed together in the progeny.  

Sickle cell anemia is a disease wherein the red blood cells become thin and stretched out. If a person has a single copy of the sickle cell allele, then half their red blood cells become abnormally shaped. If this happens, co-dominance occurs because both normal and sickled shapes are mixed and seen in the blood. 

Summary

• Gregor Mendel is known as the father of genetics.
• He performed his experiments on garden pea (Pisum sativum). He studied seven different contrasting traits of the pea plant.
• He gave three laws for genetic inheritance which are known as Mendel’s principles.
• The law of segregation states that alleles are separated so that each gamete carries only
  one allele for each gene.
• He performed his experiments on garden pea (Pisum sativum). He studied seven different contrasting traits of the pea plant. He used the pea plant because it was an annual plant and easy to maintain.
• Law of independent assortment states that segregation of alleles for one gene occurs independently of the other gene.
• Law of dominance states that an organism with alternate forms of a gene will express the form that is dominant.
• However, Mendel’s law could not explain all the genetic interactions that were discovered
  in the phenotypes of various organisms.
• As a result extensions to this principles were created.
• Incomplete dominance is a phenomenon in which a dominant allele or a form of a gene
  does not entirely hide or mask the effects of a recessive allele, and the organism’s
  resulting phenotype shows a blending of both alleles.
• In this case, the F2 genotype and phenotype ratio will be the same, i.e., 1: 2: 1. Thus the
  mendelian monohybrid ratio of 3:1 gets modified.
• Co-dominance is a mode of inheritance in which both the alleles of a gene pair in a
  heterozygote are fully expressed. As a result, the phenotype of the offspring is a combination of the phenotype of the parents.
• In this case, the F2 genotype and phenotype ratio are the same, i.e., 1: 2: 1. Thus the mendelian monohybrid ratio of 3: 1 gets modified.