Practicing Non-Mendelian Inheritance

Gregor Mendel is known as the father of genetics. His experiments with pea plants created the foundation for our modern understanding of genetics. However, as with most scientific ideas, Mendel didn’t quite have the whole picture. Over the years, geneticists have found that not all traits have simple dominant/recessive traits. These cases of non-Mendelian inheritance are often sorted into three categories: incomplete dominance, codominance, and sex-linked traits.

Practice Question

The red parent has a genotype with two capital Rs. The white parent has a genotype with two lowercase Rs. All four offspring possibilities have genotypes with a capital R and a lowercase R. The offspring are all pink.

Figure 1. In this cross, the red and white parents have all pink offspring.

Which type of inheritance do you think is shown in Figure 1?

  1. incomplete dominance
  2. co-dominance
  3. sex-linked trait

Incomplete Dominance

In incomplete dominance, two phenotypes (like the red and white petals in Figure 1) are equally dominant, so the colors blend together in the offspring’s pink petals. Another example of incomplete dominance can be found in rabbits: when a long-furred Angora breeds with a short-furred Rex, the offspring have medium-length fur.


Now that we’ve talked about incomplete dominance, let’s look at co-dominance. If the parent flowers from Figure 1 had co-dominant inheritance, what do you think the offspring would look like the flower in Figure 2.

A flower with red and white petals. The petals are distinctly colored.

Figure 2. This flower exhibits co-dominance

Another example of co-dominance is blood typing in human beings. Ignoring positive and negative blood typing, there are four blood types:

  • Type A (IAIA, IAi)
  • Type B (IBIB, IBi)
  • Type AB (IAIB)
  • Type O (ii)


Given these alleles, is it possible for a child with AB blood to have a parent with O blood?

Sex-Linked Traits

Sex-linked traits occur when an allele is located on the X-chromosome. A lot of hereditary diseases, like hemophilia and cystic fibrosis, are sex-linked. These traits can either be dominant or recessive. If the trait is dominant, than every individual that receives an X-chromosome with the allele will display these traits. If the trait is recessive, then every male with the allele will display the traits. Female offspring will only display the traits if both of their their X-chromosomes have the recessive allele.

Let’s look at the difference between dominant and recessive sex-linked traits (Figure 3). In both examples, the father is unaffected and the mother only has one affected X-chromosome.

In X-linked Dominant Inheritance, there is an unaffected father, and an affected mother (who only has one affected X-gene). There is a 50 percent chance their offspring will be affected. In X-linked Recessive Inheritance, there is an unaffected father, and a carrier mother. There is a 50 percent chance their offspring will be unaffected, a 25 percent chance their offspring will be a carrier daughter, and a 25 percent chance their offspring will be an affected son.

Figure 3. Compare dominant and recessive sex-linked traits.

Practice Question

Colorblindness is a recessive sex-linked trait. Which of the following must be true for two parents to produce a colorblind daughter?

Remember that in incomplete dominance, the two traits blend together; in co-dominance, the two traits are equally expressed; and in sex-linked traits, the traits can be dominant or recessive, but they always appear on the X-chromosome.