Erika has taught college Biology, Microbiology, and Environmental Science. She has a PhD in Science Education.
Are you either tall or short, fat or thin, very dark or very light? It seems silly to think about what we look like as simply being either or, yet this is how most people think of genes and traits. But many phenotypes, or observable traits, are not determined by simple dominant or recessive alleles. People show a huge variation in skin tone, hair color, height and weight, and it's certain aspects of our genes that allow us to do this. Have you ever wondered how twins can be born with two entirely different skin tones? Maybe you have wondered why some people have albinism when no one else in their family does? After this lesson, you will be able to answer these questions for yourself with an understanding of how additive alleles and the additive gene effect work.
What Is an Additive Allele?
Genes are DNA sequences stored on chromosomes, and each gene codes for features like eye color or diabetes. Alleles are variations of a gene and the key to the variation seen in humans and other organisms. For example, alleles determine whether you have one of many possible eye colors, or no eye color at all.
When most people think of alleles, they talk about whether the allele is dominant or recessive. However, the alleles for many genes do not work that way. Alleles for a gene can also have an additive effect - but what exactly does that mean? Exactly what it says, in fact. The phenotype resulting from additive alleles depends on the number of alleles that are present. For example, the continuous variation in skin color is determined by additive alleles. The more alleles for high melanin that you have, the darker your skin, and the fewer alleles for high melanin you have, the lighter your skin will appear. Additive alleles differ from dominant alleles in that it only takes one dominant allele to inherit the phenotype, so if skin color was determined by a dominant allele it wouldn't matter how many alleles for low melanin you had if you had a single allele for high melanin. Additive alleles allow there to be levels of intensity instead of just 'yes the phenotype is present', or 'no it isn't present.'
Let's delve more into skin tone. There are three genes that determine how much melanin is present in the skin, so each person will inherit six alleles. This leads to an enormous variety in skin tones. As you can see in Figure 1, the more of the allele for high melanin that you inherit, the darker your skin. The more of the allele for low melanin that you inherit, the lighter your skin. And if you have an equal number of each, your skin will be in between. Figure 1 also shows us that parents who have three alleles for high melanin and three alleles for low melanin could possibly have children with skin tones ranging from very fair to very dark. This is the reason it is possible for twins to be born with entirely different skin tones. Although it may look odd, it isn't any different from fraternal twins inheriting different colored eyes or hair.
What Is the Additive Gene Effect?
When people think about the genetics of a trait, they usually think about the relationships between alleles. However, the relationship between separate genes is also important in determining physical features. Multiple genes interact to determine these traits, which is why there is such diversity. When more than one gene determines a trait, the outcome is determined by the interactions of all the genes; this is called a polygenic trait.
We just examined how the interactions of additive alleles determine skin color. However, genes can have an additive effect as well. When you have genes that code for the same trait and have an equal outcome on the phenotype, it is called the _additive gene effect. The genes that determine how much melanin a person has in their skin are good examples of the additive gene effect. Each of the three genes has an equal impact on the amount of melanin present. Of the three genes, none is dominant or recessive; their contribution to the amount of melanin present in the skin is equal.
To better understand this concept, let's contrast the additive gene affect to another type of gene interactions: epistasis. With epistasis, one gene masks the effect of the others. Genes that show epistasis are like bullies; these genes will completely overpower other genes even if the trait showing epistasis is recessive. Albinism is an example of a trait that shows epistasis. The reason why it can seem that a person has inherited albinism from nowhere is that it is a recessive trait; you have to inherit two recessive alleles. A person who with two recessive alleles for albinism will lack melanin no matter how many copies of the allele for high melanin they inherit, as shown in Figure 2. This is because the gene for albinism, the bully gene, dominates the expression of the genes for melanin production. The gene for albinism and the genes for melanin concentration are not additive because they do not contribute equally to the color of a person's skin. The gene for albinism has more influence on the physical appearance of a person's skin than other genes.
This lesson covered additive alleles and the additive gene effect. Genes determine how a person looks, and alleles are variations of genes. Alleles are the key to the variety seen in people. The phenotype of a trait determined by additive alleles depends on how many of the allele the person inherits. The more of the allele a person inherits, the more intense the phenotype will be.
Genes can also have a similar additive effect. The additive gene effect tells us that additive genes contribute equally to the phenotype - none of the genes will dominate the other. The more of the gene present, the more intense the phenotype will be. This is in contrasts to gene interactions that show epistasis, in which one gene dominates the other genes.
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