Multiple Alleles

Why are there some people with blood type AB? Why can a red and a white flower mix result in pink offspring? To answer these two questions, we have to discuss co-dominance, multiple alleles, and epistasis, which are conditions that result in special phenotypic ratios.

Define Multiple Alleles

Let's start by looking at the definition of an allele.

Mendel initially showed that there could be two alleles, a dominant and a recessive, for a single trait. However, we have since learned that there could actually be several alleles. In a population with many variations of the same gene, multiple alleles arise.

In both haploid and diploid organisms, new alleles come about by spontaneous mutations. A mutation causes the sequence of amino acids to change, producing multiple alleles in a population. Some of these mutations have minor effects, some major. Scientists mostly focus on the phenotypes that certain alleles create and then classify alleles by the phenotypes they create. Multiple alleles combine in many different ways and produce many phenotypes. These phenotypes are caused by the proteins that the alleles encode. Even though each gene encodes for the same type of protein, alternative alleles can cause high variability in the functioning of proteins.

Multiple alleles exist when the population presents multiple variations of a gene. Organisms with two copies of each gene are called diploid organisms and can express two alleles simultaneously. Haploid organisms and cells only have one gene copy, but the population can still have many alleles.

Codominance and Multiple Alleles

Blood type is an example of multiple alleles because the gene controlling human A, B, and O blood groups has three alleles. Figure 1 below shows the possible genotypes (alleles present) and phenotypes (blood group). In this case, alleles Iᴬ and Iᴮ are both dominant, whereas I is recessive.

How to draw a genetic cross diagram for co-dominance and multiple alleles

We construct genetic diagrams in a similar way when showing co-dominance or multiple alleles. However, instead of big and small letters, genotypes are represented with a big letter for the gene and a superscript letter for the alleles.

Iᴬ Co-dominant

Iᴮ Co-dominant

I Recessive

Fig. 1 - Genetic cross diagram for co-dominance

How to predict blood type inheritance

Blood types are represented by the letter I (for immunoglobulin). There are three alleles: A, B, and O. A and B are dominant, whereas O (I I) is recessive.

Because A and B alleles are co-dominant, when an individual inherits both the A and B alleles, they will both be expressed in the blood type AB. However, when they inherit a dominant allele and the O (I) allele, only the dominant allele is represented in the blood type. This is because allele O represents an absence of antigens. When a person inherits two O alleles, no antigens are expressed, resulting in the blood type O.

Figure 2. Genotypes and phenotypes of different blood groups

In the example below, a blood type A person (I^AI) will be crossed with a blood type B person (I^BI.)

Figure 3. Genetic cross of blood type A and blood type B

Thus, the offspring phenotypes are:

• 25% AB

• 25% A

• 25% B

• 25% O

Polygenic traits vs. multiple alleles

Multiple alleles is the term used to describe cases where more than two alleles of the same gene are present in the population, such as blood type. Polygenic traits refer to traits that are determined by multiple genes. Many traits are polygenic, particularly complex traits like behaviour.

What is epistasis?

There are cases where the presence of one gene can affect the expression of another gene. This leads to changes in the traditional Mendelian ratios as explained in our Genetics text. In epistasis, when two genes on different chromosomes affect the same feature, an allele of one gene can affect the expression of another in the phenotype.

Depending on the exact combination of alleles in an individual, different phenotypes can appear.

Fig. 2 - A uniformly black mouse

An example of epistasis and mice

When a mouse with the genotype AaBb is crossed with the individual AaBb, all offspring in the F1 generation will have banded hairs, resulting in the agouti phenotype. However, in the F2 generation, there will be nine agouti mice, four albino mice, and three black mice. This is different from the 9:3:3:1 ratio found in Mendelian crosses, which is a strong indicator that something is modifying the phenotype.

Figure 5. Summary of epistasis in mice

What leads to this phenotypic 9:3:3:1 ratio? Let’s break the result down colour by colour.

• To be albino, the mouse must produce no melanin. To do this, the individual has to have two recessive a alleles. The effects of its alleles for the B gene are cancelled out. Only four offsprings meet these conditions as we can see in the Punnett square above.

• To be black, the individual must have two recessive alleles and one or two dominant B alleles. This means they can have the genotypes aaBb or aaBB. Only three offsprings meet these conditions in the Punnet square above, making black mice the rarest phenotype.

• Finally, to have the agouti phenotype, individuals must have one or two copies of both the dominant alleles, A and B. This means their genotypes have the format A_B_. Most of the offspring have that genotype, making agouti the most common phenotype.

What are recombinant offspring?

This is when offspring have different combinations of traits than the parents, caused by crossing over or mutations.

Recombination results in new combinations of parental characteristics in the offspring. These are called recombinants. We can see the result of this in the appearance of the offspring. However, the proportions expected from independent assortment are not shown.

Thus far, we have only looked at examples that assume crossing-over does not occur. However, in reality, nature produces recombinant offspring that have different combinations of characteristics from their parents. This is because crossing over can occur during meiosis. Genetic material is exchanged between chromosomes, breaking linkages between genes and literally recombining them. You can find more information on crossing over in our article on Meiosis.