Codominance and Multiple Alleles

What Are Codominance And Multiple Alleles?

Codominance and multiple alleles are the topics in genetics which play a great role in the study of inheritance. Codominance is expressed when two alleles in a heterozygous person are not masked thereby resulting in an organism exhibiting both alleles without reverting to the other, such as ABO blood grouping where both A and B are seen in an individual.

Multiple alleles mean that for a given locus there may be more than two alleles in the population which gives more variation. These concepts are important in genetics as they highlight more than one character involved and the pattern of inheritance different from simple dominance and recessiveness. Specifically, the following topics will be discussed in this article: codominance, multiple alleles, and examples of both in nature and uses in genetics and real life.

Basic Concepts

The basic definitions are given below-

Alleles And Genes

Concerning the two terminologies, genes are segments of the DNA responsible for the determination of certain traits since they instruct their formation, while alleles are modified forms of genes from the same family that are caused by mutation and occupy the same locus on a chromosome. For instance, for eye colour in human beings, some alleles code for blue, brown, green, etc.

Dominant And Recessive Alleles

Dominant Alleles overpower alleles that are recessive and do not show themselves in people who possess both the dominant and recessive alleles of that gene. For example in pea plants, the character of yellow seed colour dominated over green seed colour which is a recessive character.

Concept of Homozygous And Heterozygous

There are over 400 loci in Man, homozygous individuals possess two similar alleles at a particular locus that is BB for brown eyes and heterozygous individuals possess two dissimilar alleles at a peculiar locus that is Bb for brown eyes. This divergence determines the expression of traits by Mendel’s laws of inheritance.

Codominance

Codominance hence means a situation in a heterozygous individual where both genes are expressed fully and are not blended hence are easily seen. Codominance differs from incomplete dominance since the latter involves the blending of traits, while this is not the case with codominance.

Examples Of Codominance

Blood Groups In Humans (ABO System):

The ABO blood group system consists of the following three alleles IA which codes for antigen A, IB which codes for antigen B, and I which codes for the non-production of both the mentioned antigens. People with genotype IAIB on their red blood cells – A and B antigens are expressed, thus showing codominance.

Coat Color In Animals (e.g., Roan Cattle):

Roan cattle’s coat colour means that red and white hair are distributed in equal measures, which is because of the interaction of alleles for red and white colours.

Multiple Alleles

Multiple alleles mean the occurrence of more than two alleles in a particular gene. Alleles are specific variations of a gene that exists for a specific trait in a population of organisms. Nevertheless, within each single individual organism, the genes materialize in only two alleles each of which is contributed by one of the parents only.

Comparison With Simple Dominance And Codominance

In simple dominance, one gene form is dominant over another, thus, hiding the effect of the other gene when one is heterozygous. In codominance, however, the two alleles are both dominant and the phenotype of the offspring is not blended out. Multiple alleles are more than two alleles on a single gene but any organism produces only two multiple alleles.

Examples Of Multiple Alleles

ABO Blood Group System:

The ABO blood group system comprises three alleles; IA, IB, and i which outlines the presence of antigens in red blood cells. Proceeding from that, IA and IB are equal while Ia is a recessive factor.

Rabbit Coat Color:

Therefore, the coat colour of rabbits is controlled by multiple alleles. Examples are the alleles for brown, black and white coats -colours which working in combination, can give an actual coat pattern.

Table: Different Combinations of Multiple Alleles

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