Mendelian and Non-Mendelian Genetics

Hey guys! In our last video we took a look at chromosomes, well in this video we are going to take a look Mendelian genetics.

Gregor Mendel, who was an Austrian monk, presented a theory of inheritance that he discovered when working with pea plants.

Before Mendel’s theory, everyone just thought that inheritance was like mixing two things together. For example, if you mix yellow and blue together, then you get green. Likewise, people believed that the parents’ looks were just kind of mixed together homogeneously in their offspring. I mean seems pretty legit, when all you have to go off of is someone’s looks.

Well, Gregor Mendel wasn’t convinced, so he did a little research, and what he discovered was that it doesn’t quite work that way.

Dominant and Recessive Traits

He discovered that there are specific units of inheritance that get passed onto the offspring.

Alleles get passed within the chromosome that come from each parent. However, even though they get a chromosome from each parent the children do not always possess traits that the parents pass down. This is because, some traits are dominant, while some are recessive. The alleles that come together are what determine one’s phenotype. Phenotype refers to the observation of a trait, like brown eyes, red hair, dimples, etc. This phenomenon is what Gregor Mendel discovered when studying pea plants.

He also noticed that there are three potential pairings of hereditary factors: dominant-dominant, dominant-recessive, and recessive-recessive. If a child possesses the pairing dominant-dominant or dominant recessive, then that child will show the dominant phenotype. Only if the child receives the pairing recessive-recessive, will it then have the recessive phenotype.

Punnett Square

This can be easily demonstrated with a Punnett square. A Punnett square is a square, with four squares within it.


In this Punnett square, we have our first parent on top, with the pairing recessive-recessive (that means two recessive alleles). A lowercase y is used to notate that the allele is recessive. On the left side, we have the second parent with the allele pairing dominant-recessive. The dominant allele is notated by a capital Y.

When we cross the alleles from each parent, we always cross by row by column, and you move from left to right, always putting the dominant allele to the front (or left side). Here is what that looks like.

For our first parent in our first column, we have a recessive y, and for our second parent in our first row, we have a dominant y. So, then we would write Yy, in our first box, and so on.

Non-Mendelian Traits

Non-Mendelian traits, on the other hand, are traits that don’t have complete dominance or recessive alleles from a gene. Mendelian genetics only account for a display of complete dominance, or very straightforward genetic patterns. However, there are patterns of genetic inheritance that cannot be tracked using Gregor Mendel’s work. Any patterns of genetic theory that do not fit within Gregor Mendel’s framework are referred to as Non-Mendelian genetics.

We will take a look at four different types of Non-Mendelian genetics.

  1. Incomplete dominance. So, instead of a child displaying either a trait from a dominant allele, or a recessive allele, the child would show a mixture of traits. So, more than one trait are displayed to some degree, and this is where the term incomplete dominance comes from.
  2. Co-dominance. In a co-dominant pattern of inheritance, both alleles are displayed equally, and both are displayed in the offspring’s phenotype.
  3. Multiple alleles. The non-Mendelian pattern of inheritance happens when there is the potential for more than two alleles to express a single characteristic. Like blood type, for example. There are three possible alleles for blood type.
  4. Sex-linked traits. Sex-linked traits are exactly what they sound like. They are genes that are located on the sex chromosomes and are inherited that way. So, if a gene is located on the Y chromosome, then it can only be inherited by males.


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by Mometrix Test Preparation | This Page Last Updated: June 24, 2022