There is usually not a one-to-one correspondence between a gene and a physical characteristic. Often a gene is responsible for several phenotypic traits and it is said to be pleiotropic. Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. Such a gene that exhibits multiple phenotypic expression is called a pleiotropic gene. For example, mutations in the vestigial gene (vg) in Drosophila results in an easily visible short wing phenotype. However, mutations in this gene also affect the number of egg strings, position of the bristles on scutellum, and lifespan in Drosophila. Therefore, vg gene is said to be pleiotropic in that it affects many different phenotypic characteristics. During his study of inheritance in pea plants, Mendel made several interesting observations regarding the colour of various plant components. Specifically, Mendel noticed that plants with coloured seed coats always had coloured flowers and coloured leaf axils — axils are the parts of the plant that attach leaves to stems. Mendel also observed that pea plants with colourless seed coats always had white flowers and no pigmentation on their axils. In other words, in Mendel’s pea plants, seed coat colour was always associated with specific flower and axil colours. Today, we know that Mendel’s observations were the result of pleiotropy, or the phenomenon in which a single gene contributes to multiple phenotypic traits. In this case, the seed coat colour gene, denoted a, was not only responsible for seed coat colour, but also for flower and axil pigmentation.
On the other hand, single characteristics can be affected by mutations in multiple, different genes. This implies that many genes are needed to make each characteristic. For example, if we return to the Drosophila wing, there are dozens of genes that when mutant alter the normal shape of the wing, not just the vg locus. Thus there are many genes that are needed to make a normal wing; the mutation of any one causes an abnormal, mutant, phenotype. This type of arrangement is called polygenic inheritance.
- Figure 6.4.1 Simple Genotype Phenotype Map by Alphillips6, CC BY-SA 4.0, via Wikimedia Commons
- Figure 6.4.2 Polygene00 by Maulits, CC BY-SA 4.0, via Wikimedia Commons
- Figure 6.4.1 Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. This relationship between genes and phenotypes is demonstrated by mapping one genotype; e.g., G1 to multiple phenotypes; e.g.; P1 and P2. [Back to Figure 6.4.1]
- Figure 6.4.2 A bell curve showing the typical distribution of phenotypes in Polygenic Inheritance. On the extreme left and the extreme right of the curve, a small frequency of outlier genotypes are represented. As the curve approaches the middle, the frequency of more common genotypes increases. At the very centre of the curve, a maxima is achieved, producing the typical bell shaped graph. [Back to Figure 6.4.2]