What Are The Four Exceptions To Mendelian Rules?

By Antonia Čirjak on May 29 2020 in Answer

Gregor Mendel
Gregor Mendel
  • Mendel’s Laws of Heredity contain the Law of Segregation, Law of Independent Assortment, and Law of Dominance. 
  • The three-allele gene is conneceted to the parents of different blood types; both genes are dominant and are later expressed in the offspring’s phenotype.
  • The reason why albinism happens is because of a defect in the tyrosinase gene which is the reason our bodies produce melanin.

Johann Gregor Mendel formed the Laws of Heredity and is often credited for being the father of genetics as we know it today. His experiments on pea plants allowed us all to understand better how genes work. Mendel’s Laws of Heredity are made up of three main laws. These are the Law of Segregation, Law of Independent Assortment, and Law of Dominance. 

The first one states that an offspring inherits traits based off of a specific pair of genes, getting one allele from each parent. The second law states that all genes that represent specific traits are separated from each other. Finally, the third law deals with different forms of genes and their dominance, stating that the offspring always inherits the traits of the dominant form. These rules are extremely important and are responsible for the majority of knowledge of genetics today. However, with time we managed to discover new things, which include some exceptions to these laws.

Incomplete Dominance And Pleiotropy

The fact that there are exceptions to Mendel’s laws does not mean he was wrong. It simply means we know a lot more about heredity and various diseases today, which helps us explore his theses further. The first exception to Mendelian Rules is the incomplete dominance, also known as codominance. The best way to explain this would be through the example of blood types. Blood types are a good example of a type of gene called the three-allele gene. This means that with parents of different blood types, both genes are dominant and are later expressed in the offspring’s phenotype. The only time we can see a regressive gene is in type O.

Blood types are a good example of a type of gene called the three-allele gene.
Blood types are a good example of a type of gene called the three-allele gene.

The second exception is pleiotropy. This happens when one defect in a gene causes other effects on the body. The reason this happens is that the gene with the defect produces proteins that are used throughout the entire body. One example of this is sickle cell disease. It occurs when there is just one defect in the hemoglobin. This small amino acid change that occurs in those proteins can have extreme effects on the body. Some of those include strokes, episodes of unbearable pain, bone damage, lung blockage, and many more. Many people all over the world are carriers for this disease, and a smaller, but still significant number of them have the disease.

Epistatic Genes And Continuous Variation

The third exception is epistatic genes. These genes are also called the “covering up genes”. Their function is to interfere with how other genes express themselves in the offspring. One common example is albinism or the different colors of Labrador Retrievers. People that have albinism lack the melanin pigment in their bodies. They actually did inherit all of the alleles for the different colors, however, with the lack of melanin, these colors can’t manifest themselves.  The reason why albinism happens is because of a defect in the tyrosinase gene. This gene is the reason our bodies produce melanin.

The last exception is continuous variation. Throughout his research, Mendel only focused on traits that were opposed to each other, such as comparing two colors on opposite sides of the spectrum. However, things like height and weight are more complex and they often vary and follow a curve. There are many things that can vary and are not simply controlled by alleles.

Eye color, which is controlled by more than two genes is one of those things. It can also change after birth, as the person gets older. Height is also controlled by numerous genes. These examples only enriched the theories laid out by Mendel, and in no way do they discredit the work he has done in the field of genetics. We still have a lot to learn, so we are sure that several more modifications will be made to his laws through time.

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