Alleles give organisms variety, and for each gene, there are a variety of alleles. For example, alleles for sickle cell anemia determine if you have sickle cell disease, if you're a carrier, or if you have no hint of this condition at all. Alleles on the genes controlling eye color determine the color of your eyes. There are even alleles that help determine the serotonin you have access to! There are countless ways alleles affect you, and we shall explore them below.
Definition of an Allele
An allele is defined as a variant of a gene that gives a unique characteristic. In Mendelian inheritance, monk Gregor Mendel studied pea plants with only two alleles possible for a gene. But, as we know from analyzing many genes in humans, animals, and plants, most genes are actually polyallelic - there is more than one allele for that gene.
Polyallelic gene: This gene has multiple (more than two) alleles, which decide its phenotype. Genes examined in Mendelian inheritance have just two alleles, but many other genes observed in nature have three or more possible alleles.
Polygenic trait: This trait has multiple (more than one) genes dictating its nature. Traits examined in Mendelian inheritance have just one gene determining their characteristics (for example, only one gene determines pea flower color). Still, many other traits observed in nature have two or more genes dictating them.
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.
AA, AB, AO, BO, BB, OO.
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
These alleles are usually denoted by a capital letter (for example, A), juxtaposed to a recessive allele, written in the lower case version of that same letter (a).
Dominant alleles are assumed to have complete dominance, meaning they determine the phenotype of a heterozygote, an organism with both dominant and recessive alleles. Heterozygotes (Aa) have the same phenotype as homozygous dominant organisms (AA).
Let's observe this principle with cherries. The dominant trait for cherry color is red; let's call this allele A. We see that homozygous dominant, and heterozygous cherries have the same phenotype (Fig. 2). And what about homozygous recessive cherries?
Recessive Allele Definition
Recessive alleles are exactly as they sound. They "recede" into the background whenever a dominant allele is present. They can only be expressed in homozygous recessive organisms, which leads to certain important realities.
Dominant alleles are often written in capitals (A), while recessive alleles are written in lower case letters (a), but this is not always the case! Sometimes both alleles are written in capitals, but they have different letters (like in this made-up genotype - VD). Sometimes, the dominant allele is written in capitals, and the recessive allele is too. In this case, the recessive allele has an asterisk or apostrophe next to it (like in this made-up genotype - JJ'). Be aware that these stylistic variations can exist in different texts and exams, so don't get tripped up by them!
For instance, we know that most deleterious mutations (deleterious means harmful) in humans are recessive. There are "autosomal dominant" genetic diseases, but these are much fewer than autosomal recessive diseases. This is due to many factors, such as natural selection, which essentially works by eliminating these genes from the population.
Autosomal dominant disorder: Any disorder in which the gene encoding it is located on an autosome, and that gene is dominant. An autosome is every chromosome that is not an X or Y chromosome in humans.
Autosomal recessive disorder: Any disorder in which the gene encoding it is located on an autosome, and that gene is recessive.
Most deleterious mutations are recessive, so we'd need two copies of those recessive alleles to have the deleterious trait. Scientists have discovered that within every human being, there are one or two recessive mutations that we carry, that if they were dominant, or if we happened to have two pairs of that allele, it would cause either our death within the first year of life or a severe genetic disease!
Sometimes, these genetic diseases are more common in certain populations (like sickle cell anemia in people with West African ancestry, cystic fibrosis in people with North European ancestry, or Tay Sachs disease in people with Ashkenazi Jewish ancestry). Outside of those with a known ancestral link, most mutations happen completely at random. Thus, the odds that two parents would both have an allele with the same mutation and pass that single allele onto the same offspring are very slim. We can see that the recessive nature of most deleterious alleles means the odds remain in favor of producing a standard healthy offspring.
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
If you suspect that you have already seen a codominant allele in this lesson, you're correct! ABO, the human blood type, is an example of codominance. Specifically, the A allele and the B allele are codominant. Neither is "stronger" than the other, and both are expressed in the phenotype. But both A and B are completely dominant over O, and so if one allele of a gene is O, and another allele is anything other than O, the phenotype will be that of the non-O allele. Remember how the BO genotype gave a B blood group phenotype? And the AO genotype gave an A blood group phenotype? Yet the AB genotype gives an AB blood group phenotype. This is due to the dominance of A and B over O, and the codominance shared between alleles A and B.
So ABO blood types are an example of both a polyallelic gene and codominant alleles!
Incompletely Dominant Alleles
Incomplete dominance is a phenomenon that occurs when neither allele at the locus of a gene dominates the other. Both genes are expressed in the final phenotype, but they don't express completely. Instead, the phenotype is a mix of both incompletely dominant alleles.
For example, if a kitten's fur color exhibited codominance and had the Bb genotype, where B = dominant black fur and b = recessive white fur, the kitten would be part black and part white. If the gene for kitten fur color exhibits incomplete dominance and has the Bb genotype, then the kitten would appear grey! The phenotype in a heterozygote is neither the phenotype of the dominant nor the recessive allele nor both (Fig. 3). It is a phenotype that is in between the two alleles.
Figure 3 Codominant vs. incomplete dominant kitten coats. Chisom, StudySmarter Original.
Sex-linked Alleles
The vast majority of sex-linked disorders are on the X chromosome. Generally, the X chromosome has more alleles than the Y chromosome because it is literally bigger with more space for gene loci.
Sex-linked alleles don't follow the principles of Mendelian inheritance because sex chromosomes behave differently than autosomes do. For example, males have one X and one Y chromosome. So, if males have a mutated allele on their single X chromosome, there's a high likelihood that this mutation may display in the phenotype, even if it is a recessive mutation. In females, this recessive phenotype would not be expressed, because of a dominant normal allele on the other X chromosome, as females have two Xs. Males only have one X chromosome, so if they have a mutation at a gene locus, that mutation can be expressed if there's no dominant normal copy of that gene on the Y chromosome.
Alleles Exhibiting Epistasis
A gene is considered epistatic to another if its phenotype modifies the expression of that other gene. An example of epistasis in humans is baldness and hair color.
Suppose you inherit the gene for auburn hair from your mother, and you inherit the gene for blonde hair from your father. You also inherit a dominant gene for baldness from your mother, so no hair grows on your head from the day you are born.
Thus, the baldness gene is epistatic to the hair color gene because you are required not to express baldness for the gene at the hair color locus to determine your hair color (Fig. 4).
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.
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