A Fishy Way To Discuss Multiple Genes Affecting The Same Trait

Engagement: Introducing Complementation Using Human Deafness

I begin this unit by building on two concepts we have already covered in the course: 1) identifying autosomal inheritance patterns and 2) analyzing pedigrees. Specifically, students analyze a human pedigree on the inheritance of deafness (shaded individuals) to review these concepts (Figure 1A). Human deafness is a good example for introducing students to multi-gene traits because mutations in at least 57 genes cause deafness that is not associated with any other symptoms (http://deafnessvariationdatabase.org). I ask the class to tell me about the inheritance pattern for the pedigree in Figure 1A. Students are quick to volunteer that this pedigree is more consistent with an autosomal recessive inheritance pattern. I then ask students to tell me about the inheritance pattern in the Figure 1B pedigree. Students again say that the pedigree is more consistent with an autosomal recessive inheritance pattern.

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Figure 1. Human pedigrees used to introduce the concept of complementation.

One family where several members are deaf is shown in (A), a second family where several members are deaf is shown in (B).

https://doi.org/10.1371/journal.pbio.1001279.g001

Next, I tell students that a deaf person from the family in Figure 1A has a child with a deaf person from the family in Figure 1B, and that child can hear. This result is surprising because if we were considering a simple Mendelian inheritance pattern, two people who are deaf because of autosomal recessive mutations would have a 100% chance of having a deaf child. I then ask the class: “how can you explain the child from the mating between the two pedigrees?” Students often suggest that the sperm or egg that created the child had a spontaneous mutation that made a mutant allele normal (a possibility, but not very likely) or that the child has the genetic mutations but for some environmental reason, s/he is not deaf (another possibility, so I respond by saying let us assume all individuals with the mutation are deaf). Sometimes a student will suggest that the parents have mutations in two different genes.

I now introduce students to the terms “complementation” and “non-complementation” using the slides in Figure 2. I state that an example of complementation occurs when two deaf parents produce hearing children. These results suggest that the parents have mutations in DIFFERENT genes. I represent the two genes using the letters A and B (Figure 2A). Then I contrast complementation with non-complementation (Figure 2B). Here, two deaf parents produce all deaf children. This result suggests that parents have recessive mutations in the SAME gene, represented by the letter B.

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Figure 2. A slide introducing students to the terms complementation and non-complementation in the context of the human deafness example.

https://doi.org/10.1371/journal.pbio.1001279.g002