Genetics: Non-Mendelian Traits

Biology Tutorial

Genetics: Non-Mendelian Traits


Genetics is the study of genes from one generation to the next. Gregor Mendel is the Father of Genetics, and his experiments with pea plants provided insight into how traits pass from parents to offspring. We now know that our chromosomes, which are made of DNA, work in pairs. That means that for any gene, which is a section of DNA coding for a protein related to a specific trait, is found on a specific chromosome- and we have two copies of it. Genes may have different forms, called alleles. With Mendelian traits, we had two alleles. One of them was dominant, which means it was expressed in the phenotype, the physical appearance of the trait, if the individual had a copy of it. The other is recessive, and it is only expressed if the individual only has recessive alleles for that gene. We use Punnett squares to make predictions about the probability of the offspring having a certain genotype, the combination of alleles.
With non-Mendelian traits, we do not have only a dominant and a recessive allele to consider. This includes co-dominance, incomplete dominance, sex-linked traits, multiple alleles, and polygenic traits.

Sample Problem

Let’s say that we have Jasmine growing flowers. She has true-breeding red flowers and true-breeding white flowers.

Remember, true-breeding means that, when crossed with other true-breeding flowers, they produce offspring with the same phenotype. They are homozygous for the allele.

Jasmine decides to recreate Mendel’s experiment with flower color, so she crosses a true-breeding red flower with a true-breeding white flower. All of the offspring are pink flowers.

Jasmine is surprised, and continues the experiment. She crosses two of the F1 generation (the pink flowers). The F2 generation have about 25 red flowers, 25 white flowers, and 50 pink flowers.

What has happened? How does Jasmine know that flower color for this species is not Mendelian?


The flowers are a real example from a species called snapdragons. Their flower color is an example of incomplete dominance.

With incomplete dominance, we have the homozygous individuals expressing the phenotype like we would expect from a Mendelian trait. The difference is most noticeable in the heterozygous individual; we have a third phenotype. This third phenotype is a blend of the other two phenotypes.

In this example, we have red flowers (RR), white flowers (WW), and pink flowers (RW). We still only have two alleles in this example.

To decide if you are looking at an example of incomplete dominance, assign a letter to the true-breeding traits and make a Punnett square. Normally, we pick a capital letter from each description, R for red and W for white. Be careful picking your letters! Some letters can be mistaken for one another, depending on your handwriting.

To produce the F1 generation, Jasmine crossed a red flower (RR) with a white flower (WW).

….R R – it doesn’t matter which side you put which parent on



All four possibilities are the same genotype: RW. With what we know about incomplete dominance, it is logical to think that red and white combine for the pink phenotype Jasmine saw.

Don’t stop here. Be sure to check all of the information that was given to be confident that your conclusion is accurate.

To produce the F2 generation, Jasmine crossed two pink flowers (RW).

….R W



So, we have one option for RR, two for RW, and one for WW. This is a 1:2:1 ratio for RR:RW:WW and to the corresponding phenotypes red:pink:white.

Look back at the data Jasmine gathered. She had about 25 red flowers, about 50 pink flowers, and about 25 white flowers. That is 25:50:25 or 1:2:1 for red:pink:white. This matches the theoretical ratio given by the Punnett square.

We can confidently say that this flower color is a case of incomplete dominance. Jasmine knows it cannot be Mendelian, because there is a third phenotype.

Note: Check with your teacher for what style of symbols you should use to express the different non-Mendelian alleles. There are multiple styles, and some teachers have preferences they expect you to use on quizzes and tests.

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