Have you ever wondered why some people have blood type AB or why mixing a red and white flower can create a pink offspring? The answer lies in co-dominance, multiple alleles, and epistasis - unique conditions that lead to special physical characteristics. In our previous article, we explored co-dominance, which is when offspring show traits from both parents due to blending alleles rather than one allele dominating over the other. This results in a phenotype that may look like a blend of two traits or both features appearing. For example, when a red flower (genotype RR) and white flower (genotype rr) mix, the resulting pink flower has a genotype of Rr. This is also called incomplete dominance. With co-dominance, the possibilities are endless, including flowers with red and white blotches. Understanding these genetic concepts like multiple alleles can help us comprehend why we certain traits and how they are inherited.

What are multiple alleles?

Alleles are different versions of the same gene. Mendel discovered that a single trait could have two alleles, one dominant and one recessive. However, we now know that multiple alleles can exist in a single population. These alleles arise from spontaneous mutations that cause changes in the sequence of amino acids. Scientists focus on the phenotypes that different alleles create and classify them based on these traits. The combination of multiple alleles leads to a diverse range of phenotypes, all determined by the proteins encoded by the alleles. Even though genes encode for the same type of protein, alternative alleles can cause significant variations in protein function. Diploid organisms have two copies of each gene, allowing for the simultaneous expression of two alleles. Haploid organisms and cells have only one gene copy, but there can still be many alleles in the population. Understanding multiple alleles can help us understand the variability of traits within a population.

Blood type

Blood type is an example of multiple alleles. 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.

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

‍

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!

‍

In the example below, a blood type A person (IAI) will be crossed with a blood type B person (IBI.)

‍

Thus, the offspring phenotypes are:

25% AB 25% A25% B 25% O

Polygenic traits vs. multiple alleles

Multiple alleles refer to cases where more than two different alleles of the same gene are present in a population, such as blood type. On the other hand, polygenic traits are traits that are influenced by multiple genes. Complex traits like behavior are often polygenic.

Epistasis is another genetic concept that describes how one gene's expression can be affected by one or more independently inherited genes. This can lead to changes in traditional Mendelian ratios, as explained in our previous Genetics article. When two genes on different chromosomes influence the same feature, an allele of one gene can affect the expression of another gene in the phenotype.

For example, in mice, the distribution of melanin that creates a black color in the coat is controlled by gene A, which has a dominant allele (A) and a recessive allele (a). Gene B modifies gene A, with the dominant allele (B) allowing melanin to be produced and the recessive allele (b) leading to no pigment. Depending on the combination of alleles in an individual, different phenotypes can appear.

An individual with the AaBb genotype, for instance, would have banded hairs, which is known as the agouti phenotype. Agouti is grey-brown in color and is the most common phenotype in the wild. The aaBb genotype leads to a uniformly black mouse, while the aabb genotype results in an albino phenotype since the coat is uniformly colored but melanin is not expressed. Understanding epistasis and the interaction between genes can help us understand the complex traits that make up an organism.

‍

An example of epistasis and mice

When a mouse with the genotype AaBb is crossed with another AaBb individual, 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.

To be albino, the mouse must have two recessive a alleles and no dominant B allele. This means that the effects of its alleles for the B gene are cancelled out. Only four offspring meet these conditions, as seen in the Punnett square above.

To be black, the individual must have two recessive a alleles and one or two dominant B alleles. This means they can have the genotypes aaBb or aaBB. Only three offspring meet these conditions in the Punnett 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.

Understanding the interaction between genes is crucial to understanding the diversity of traits within a population. Epistasis is just one example of how different genes can interact to produce unique phenotypes. By studying these genetic interactions, scientists can gain a better understanding of the underlying mechanisms that govern the traits we observe in organisms.

What are recombinant offspring?

Recombination refers to the exchange of alleles between homologous chromosomes during crossing over, which can produce offspring with different combinations of traits than their parents. This can lead to new combinations of parental characteristics in the offspring, known as recombinants. However, the proportions of these recombinants may not be what is expected from independent assortment.

Independent assortment is the principle that genes for different traits can segregate independently during the formation of gametes. Crossing over, on the other hand, is the sharing of genetic material between two non-sister chromatids in a homologous pair. This can result in the breaking of linkages between genes and the recombination of genetic material, producing new combinations of alleles in the offspring.

It is important to note that in reality, nature produces recombinant offspring with different combinations of characteristics from their parents, due to the occurrence of crossing over during meiosis. This process can break the linkages between genes and recombine them, leading to the production of new combinations of alleles in the offspring.

In summary, understanding the principles of recombination and independent assortment is crucial to understanding the diversity of traits within a population. By studying these genetic processes, scientists can gain a better understanding of the underlying mechanisms that govern the traits we observe in organisms.

Multiple Alleles

How do multiple alleles of a gene arise?

Through mutations that are different or at different positions in the gene.

What is the difference between multiple alleles and polygenic traits?

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

How do you write multiple alleles?

Multiple alleles can be written as superscripts. For instance, the gene for blood types is represented by the letter I (for immunoglobulin.) The alleles are written as superscripts.

What are the three alleles for blood type?

Blood types are represented by the letter I (for immunoglobulin.) There are three alleles for blood type: A, B, and O. A and B are dominant, whereas O is recessive. They are written as IA, IB, and IO respectively.

How many alleles do humans have?

Humans have two copies of each gene, inheriting one from each parent. For each gene, heterozygous individuals have two different alleles, while homozygous individuals have two copies of a single allele.

‍