What Are Mendelian Traits? Everything You Need to Know

The word “Mendelian” is one of those last-name adjectives that are used to describe something that someone once invented and, as such, their last name has been used to describe that very thing they have discovered or invented.

Mendelian actually takes after an Augustinian monk who has changed the field of genetics completely. He has helped us understand everything we need to know about trait inheritance and not just why we are the way we are, but also helped us be able to relate our traits to our parents. Even more so, he has helped us be able to predict what our offspring would look like given our appearances. In other words, he has helped us really understand inheritance and inheritance patterns. Meet Gregor Johann Mendel who was once a scientist and a monk who was very fond of gardening. To be more specific, the gardening of the pea plant.

He conducted experiments with pea plants during the period between 1856 and 1863. Focusing on seven characteristics including plant height, pod shape and color, seed shape and color, and flower position, his findings helped establish many of the rules of heredity, which are now referred to as the laws of Mendelian inheritance.

The reason that Mendel decided to settle on the aforementioned traits is that he saw that they were inherited independently of other traits. Seed shape, for example, came in two forms, namely round and angular. He studied some 28,000 plants after which he came up with conclusions that would affect the way we look at genetics and traits forever. Overall, he concluded that interbreeding various varieties of pea plants yielded results that always followed certain patterns of trait inheritance. If, for example, he crossed angular peas and round peas, the offspring traits were the following: one in four peas had pure recessive traits, two out of four or one-half of the plants were hybrids, and one out of the four was a purebred dominant.

So, hang on: what is recessive here and what is dominant? Let’s start by saying that offspring receive two copies of one gene, one from the mother and one from the father. These gene copies or varieties are referred to as alleles. Alleles code for proteins that manifest themselves as traits, or phenotypes. Those varieties of genes that code for phenotypes is also called genotypes. In other words, the genotypes determine the phenotypes. Going back to dominant versus recessive, dominant traits are those traits that are expressed (or visible, or apparent to the eye) even if just one copy of the allele is present. Recessive traits, or phenotypes, on the other hand, are only visible if both of the alleles are present. In the case that one dominant and one recessive allele are inherited together, the dominant allele will be expressed. So, in the case of eye color,for example, the trait of brown eyes is always dominating over the trait of blue eyes. To juxtapose that onto the explanation above, the allele that is coding for brown eyes is dominant and a person will almost always have brown eyes under two circumstances, namely with one or two alleles for brown eyes. In other words, a person can have one brown eye and a blue eye allele, and that person still has brown eyes. A person will only have blue eyes if they inherit two blue eye alleles. In other words, recessive alleles can only be seen if both of the copies that are inherited are recessive. This means that a person with a recessive trait can only have a genotype of two recessive alleles, otherwise, the recessive trait would be masked by the trait dominant allele. (Of course, there are exceptions to this rule, but those are beyond the scope of our original story.)

Going back to Mendel and his peas and just how do they relate to the term ‘Mendelian Traits’? Well, it is very similar to the eye color example. The gene for pea shape has two alleles, namely round and wrinkled. The round allele is dominant and the wrinkled allele is recessive. The cross between a purely round and a purely wrinkled pea shape plant will yield offspring that is all dominant in phenotype but a hybrid (one dominant allele and one recessive allele) in genotype. If that offspring is crossed to one another, the following offspring are obtained: one-quarter of them will be purely dominant round, one half of the offspring will be hybrid in genotype by having a round phenotype and one-quarter of them will be wrinkled with two recessive alleles for wrinkled peas. Out of the offspring, the allele genotypes will be purely dominant, hybrid dominant (whereby these offspring have one copy of each allele) and purely recessive. This ¼, ½, ¼ distribution is what is termed a Mendelian pattern of inheritance. The first round of offspring is typically referred to as F1 and the second round as F2 (see figure).

parental generation

The image denotes a way of drawing and delineating parental phenotypes as well as the phenotypes of the offspring. SS and Ss are genotyped, whereby capital letters refer to dominant traits and lower caps refer to recessive traits.

There is much more to be said about Mendelian Inheritance of traits as well as the topics that were discovered following Mendel as for many things in genetics as well as the patterns of inheritance, his discoveries paved the way.

Now, when someone mentions Mendelian Inheritance, you will know exactly not only who Mendel is and also why he is often regarded as the Father of Genetics.

Laura Day