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.
In our Genetics article, we looked at co-dominance. Co-dominance is a type of dominance where the offspring show similarity to both parents. This is due to the blending of alleles rather than one allele masking the other. The co-dominant phenotype might look like a blend of two phenotypes. For instance, a mix between red (genotype RR) and white (genotype rr) flowers might result in pink flowers (genotype Rr.) This is sometimes called incomplete dominance. The co-dominant phenotype could also result in both features appearing. For instance, a flower with red and white blotches.
Define Multiple Alleles
Let's start by looking at the definition of an allele.
An allele is a different version of the same gene.
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
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.
Note that sometimes the allele IO is written as a small i to denote that it is recessive. Either is fine!
| Phenotype |
IA IA or IA I | A |
IB IB or IB I | B |
IA IB | AB |
I I | O |
Figure 2. Genotypes and phenotypes of different blood groups
In the example below, a blood type A person (IAI) will be crossed with a blood type B person (IBI.)
| IA | I |
IB | IAIB (AB) | IBI (B) |
I | IAI (A) | I I (O) |
Figure 3. Genetic cross of blood type A and blood type B
Thus, the offspring phenotypes are:
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?
Epistasis: when the expression of one gene is affected by the expression of one or more independently inherited genes.
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.
In many animals, several genes interact with one another to determine coat colour. In mice, gene A controls the distribution of melanin, which creates a black colour in the coat. The dominant allele A leads to hairs with black bands, whereas the recessive allele a leads to uniformly black hairs in homozygotus recessive individuals. The gene B modifies gene A: the dominant allele B allows melanin to be produced, whereas the recessive b leads to no pigment.
Depending on the exact combination of alleles in an individual, different phenotypes can appear.
An individual with the phenotype AaBb, for instance, would have banded hairs, which is known as the agouti phenotype. Agouti is grey-brown in colour and is the most common phenotype in the wild. The genotype aaBb leads to a uniformly black mouse, while the genotype aabb leads to an albino phenotype since the coat is uniformly coloured but melanin is not expressed.
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.
| Ab | AB | ab | aB |
Ab | AAbb (Agouti) | AABb (Agouti) | Aabb (Agouti) | AaBb (Agouti) |
AB | AABb (Agouti) | AABB (Black) | AaBb (Agouti) | AaBB (Black) |
ab | Aabb (Agouti) | AaBb (Agouti) | aabb (Albino) | aaBb (Albino) |
aB | AaBb (Agouti) | AaBB (Black) | aaBb (Albino) | aaBB (Albino) |
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 refers to the exchange of alleles between homologous chromosomes during crossing over.
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.
Independent assortment: genes for different traits can segregate independently during the formation of gametes.
Crossing over: the sharing of genetic material between two non-sister chromatids in a homologous pair.
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.
Multiple Alleles - Key takeaways
Co-dominance occurs when both alleles are expressed in a phenotype, rather than having one allele mask the other.
Co-dominant alleles might result in both phenotypes appearing in one organism or in an individual expressing a blend of the two phenotypes.
There can be multiple alleles of one gene present in a population, rather than just one dominant and one recessive allele. In a population with many variations of the same gene, multiple alleles arise.
Diploid organisms, which contain two copies of each gene, are able to express two alleles at the same time.
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.
Recombinant offspring have different combinations of characteristics from their parents.
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