Complicating the picture of gene expression is the many interacting genes/gene products involved in skin pigmentation. A simple model explains the influence that multiple genes can have on a characteristic. Complexity is compounded by other gene properties, such as multiple alleles and epistasis.
An Analogy of Doses
Skin pigmentation is the result of the interaction of many expressed genes: skin color is a polygenic characteristic. What is the nature of polygenic inheritance? This module uses a simple three-gene model, each with two alleles. The dominant form codes for a “dose” of dark skin and the recessive form codes for a “dose” of light skin. The darkest skin is due to six dominant “doses” and the lightest skin is due to six recessive “doses”. Varying combinations of the alleles result in seven discrete colors. Mendel’s laws (reviewed in slides) show that there are different combinations of the three expressed genes for each of the seven colors. The model shows the result of expressed alleles adding to a phenotype of one of seven discrete colors. However, skin color varies continuously in modern populations, a reflection of the many genes involved. Sorting out the role that each interacting gene product plays in the expression of skin color is a task not easily accomplished. Research has determined the role of a handful of genes, but not necessarily how these interact and certainly not how the constellation of genes involved in determining skin color interact. The problem is to determine the genetics of these tangled relationships.
Figure:The pipe cleaners represent a duplicated pair of matched (homologous) chromosomes of a parent with what we’ll call the Ee genotype, ready to separate during meiosis I. The beads represent the alleles E (pink) and e (white). Upon separation the two kinds of alleles will segregate. During meiosis II, the duplicated alleles split apart.
The fact that one gene can have more than two alternative versions, or multiple alleles, also adds complexity. This is certainly the case for some of the known genes involved in skin color, such as MC1R and TRYP2. Additionally, the relationship between alleles is not always “dominant” and “recessive”. Rather, there is dominance variation, the most common of which is co-dominance. Epistasis, where modifier genes influence the expression of other genes, also contributes to the task of sorting out polygenic inheritance. For example, the genes responsible for determining the internal environment of melanosomes, where melanin pigment are made, regulate, or modify, the synthesis of melanin.