Alleles add variety to living things. Each gene has different versions, or alleles. For instance, alleles for sickle cell anemia decide if you have the disease or not. They can also tell if you carry the disease but don't show any symptoms. Your eye color is also determined by alleles. Even the amount of serotonin you produce can be influenced by alleles! There are many ways that alleles can affect you, and we'll look at some of them here. Let's explore the fascinating world of alleles!

Definition of an Allele

Alleles are unique characteristics that come from different versions of a gene. Gregor Mendel, a monk, studied pea plants and found only two alleles were possible for each gene. But in humans, animals, and plants, most genes are polyallelic, meaning they have more than two alleles. This means that a gene's phenotype is determined by multiple alleles. While Mendelian inheritance only examines genes with two alleles, there are many other genes in nature with three or more possible alleles. Polygenic traits, on the other hand, have multiple genes dictating their nature, unlike the traits studied in Mendelian inheritance, which are determined by just one gene. In nature, there are many traits that are influenced by two or more genes.

Example of a Polyallelic Gene

An example of a polyallelic gene is human blood type, which has three possible alleles - A, B, and O. These three alleles are present in two genes (a gene pair). This leads to five possible genotypes.


Now, some of these alleles exhibit dominance over the others, meaning that whenever they are present, they are the ones that are phenotypically expressed. This means that we have four possible phenotypes for blood type (Fig. 1):

A  (AA and AO genotypes), B (BB and BO genotypes), AB (AB genotype)O (OO genotype)

Types of Alleles

In Mendelian genetics, there are two kinds of alleles:

The dominant allele The recessive allele

Dominant allele definition

Alleles are often represented by a capital letter (such as A) paired with a lower-case version of the same letter to represent the recessive allele (a).

Dominant alleles are believed to have complete dominance, so they determine the phenotype of heterozygotes, which are organisms with both dominant and recessive alleles. Heterozygotes (Aa) have the same phenotype as homozygous dominant organisms (AA).

To see this principle in action, let's look at cherries. The dominant trait for cherry color is red, represented by the allele A. Both homozygous dominant and heterozygous cherries have the same red phenotype (shown in Figure 2). But what about homozygous recessive cherries?

Recessive Allele Definition

Recessive alleles are called that because they "recede" into the background when a dominant allele is present. They can only be expressed in homozygous recessive organisms, which has important implications.

Dominant alleles are often represented by capital letters (A), while recessive alleles are written in lowercase letters (a). However, there are other ways to represent alleles, such as using different capital letters for different dominant alleles or using an asterisk or apostrophe to denote a recessive allele.

Most harmful mutations in humans are recessive, which means that an individual needs two copies of the recessive allele to exhibit the deleterious trait. Some genetic diseases are more common in certain populations, such as sickle cell anemia in people with West African ancestry or Tay Sachs disease in people with Ashkenazi Jewish ancestry.

However, most mutations happen randomly, and the odds that two parents will both carry the same recessive allele for a genetic disease are low. This is why the recessive nature of most deleterious alleles means that the chances of producing a healthy offspring are still in favor.

Non-Mendelian Types of Alleles

The following are some categorizations of alleles that don't follow Mendelian inheritance.

Codominant alleles Incompletely dominant alleles Sex-linked alleles Alleles exhibiting epistasis

Codominant Alleles

ABO blood types in humans are an example of codominance, where both alleles (A and B) are expressed in the phenotype. However, both A and B alleles are completely dominant over the O allele, so if one allele is O and another is A or B, the phenotype will be that of the non-O allele. For example, the BO genotype gives a B blood group phenotype, while the AO genotype gives an A blood group phenotype. The AB genotype gives an AB blood group phenotype, which is due to the codominance shared between alleles A and B, as well as the dominance of A and B over O. ABO blood types are also an example of a polyallelic gene, meaning that there are multiple possible alleles at the same gene locus.

Incompletely Dominant Alleles

The example given for incomplete dominance is actually an example of codominance, where both alleles are expressed equally in the phenotype. Incomplete dominance occurs when neither allele at the locus of a gene dominates the other, resulting in a phenotype that is a blend of both alleles.

For example, if a flower has a gene for petal color that exhibits incomplete dominance and has a red (RR) and white (WW) allele, then a heterozygous genotype (RW) would result in a pink phenotype. The phenotype in a heterozygote is not completely dominant or recessive but rather a blend of the two alleles.

Codominant vs. incomplete dominant kitten coats
Codominant vs. incomplete dominant kitten coats

Sex-linked Alleles

Sex-linked disorders are commonly found on the X chromosome, as it is larger than the Y chromosome and has more space for gene loci. Sex-linked alleles do not follow the principles of Mendelian inheritance because sex chromosomes behave differently from autosomes. In males, who have one X and one Y chromosome, a mutated allele on their single X chromosome is likely to be expressed in the phenotype, even if it is recessive. This is because there is no dominant allele on the Y chromosome to mask the effects of the mutated X allele. In contrast, females have two X chromosomes, so a recessive phenotype is only expressed if both X chromosomes carry the mutated allele. This means that males are more likely to be affected by sex-linked disorders, as they only have one X chromosome. Females can be carriers of these disorders, as they have two X chromosomes and may carry a mutated allele on one of them without displaying symptoms.

Alleles Exhibiting Epistasis

The example given is actually an example of a different type of gene interaction called "dominance" and not epistasis. In this scenario, the dominant gene for baldness from the mother is preventing the expression of any hair color genes inherited from the father, resulting in no hair growth. Epistasis occurs when the expression of one gene is influenced by another gene, but the example given does not demonstrate this. An example of epistasis in humans is the interaction between the genes for albinism and hair color. The gene for albinism is epistatic to the gene for hair color, as it prevents the expression of any pigmentation genes, resulting in white hair regardless of the individual's hair color gene. It's important to note the distinction between different types of gene interactions to better understand the complexity of genetic inheritance.

How and When Does the Segregation of Alleles Occur?

We've mostly discussed alleles in gene pairs, but when do alleles segregate? Alleles segregate according to Mendel's Second Law, which states that when a diploid organism makes gametes (sex cells), it packages each allele separately. The gametes contain a single allele and can go on to fuse with gametes from the opposite sex to create progeny.

Alleles - Key Takeaways An allele is a gene variant present at the locus of a gene that codes for a specific trait. In Mendelian genetics, there are two kinds of alleles - dominant and recessive. In non-Mendelian inheritance, there are several more kinds of alleles; incompletely dominant, codominant, and more. Some alleles are located on autosomes and others on sex chromosomes, and the ones on sex chromosomes are termed sex-linked genes. Epistasis is when the allele at a particular locus affects or facilitates the phenotype of an allele at another locus. According to Mendel's Law of Segregation, alleles segregate independently and equally into gametes.


What is an allele?

An allele is a variant of a gene that codes for a specific trait. 

What is a dominant allele?

A dominant allele will show its phenotype in a heterozygote. Usually, dominant alleles are written in capital letters like this: A (vs a, the recessive allele).

what is the difference between a gene and an allele?

A gene is a piece of genetic material that codes for proteins that determine features. Alleles are variants of a gene. 

what is a recessive allele?

A recessive allele will only display its phenotype in a homozygous recessive organism.

How alleles are inherited? 

You typically inherit one allele from each parent, so you end up with a gene pair (two alleles).

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