Beyond Mendel

Overview

This module looks at examples of inheritance to which Mendel's principles appear not to apply. Upon closer examination, however, we will see how Mendel's principles can be extended to explain these examples.

Objectives

1. Understand the concepts of incomplete dominance and codominance, and the difference between the two. Learn to identify examples of these modes of inheritance.
2. Understand how the existence of multiple alleles of a gene can affect the inheritance pattern.
3. Realize that genes produce polypeptides, and understand how this relates to allelic variation.
4. Understand how genes can interact to produce particular phenotype. Understand the concept of epistasis.
5. Understand the concept of pleiotropy.
6. Understand the concept of continuous variation, and how quantitative traits are inherited.

Extensions of Mendelism

Although Mendel formuated the postulates that provide the basis of our understanding of genetic principles, there were many types or modes of inheritance that Mendel simply didn't encounter. These modes of inheritance were encountered when invstigators began using Mendel's postulates to study inheritance in other organisms. Some of these modes of inheritance appear, at first glance, to use different rules than those Mendel proposed. So was Mendel wrong? Not really - his postulates fit the data that he collected, but as stated above, there were situations he didn't observe in his study of the pea. This is a common occurrence in science: a theory is proposed, which is used as long as it is useful in explaining some aspect of nature. When data are encountered that don't fit the theory, the theory may have to be modified(if possible) or discarded. Mendel's ideas as originally presented may not fit every possible mode of inheritance, but they still provide the basis for explaining those other types of inheritance.

Incomplete Dominance

One early example of an inheritance pattern that seemed to contradict predictions based on Mendel's principles was flower color in snapdragons. It was observed that when a red-flowered plant was crossed with a white-colored plant, the offspring were neither red nor white, but pink-flowered! Moreover, when the pink plants were self-fertilized (or crossed among themselves), the F2 generation was 1/4 red, 1/4 white, and 1/2 pink:

In this case, there is no dominance. The phenotype of the F1's is midway between the two parents. There is one allele for white flowers (we'll call it w) and one allele for red flowers (we'll call it W). If a red-flowered plant is WW and a white-flowered plant is ww, then a cross of these two plants should produce all heterozygotes (Ww). These are the pink plants. Our study of Mendelian principles tells us that when these are self pollinated, they should produce 1/4 WW, 1/4 ww, and 1/2 Ww. This corresponds exactly to what is observed above in the F2 generation.

Why is there no dominance in this case? The best explanation is that this gene produces a product that gives the flowers color. Plants with two copies of the red allele (WW) produce twice as much of this product as heterozygotes (Ww), and therefore have a much deeper red color. Plants with no copies of the red allele produce no flower color product, abd therefore have white flowers.

Clear-cut examples of incomplete dominance such as this are fairly rare. More often, one allele will have partial dominance over another.