Epistasis, where modifier genes influence the expression of other genes, also contributes to the task of sorting out polygenic inheritance. Rather, there is dominance variation, the most common of which is co-dominance. Additionally, the relationship between alleles is not always “dominant” and “recessive”. This is certainly the case for some of the known genes involved in skin color, such as MC1R and TRYP2. The fact that one gene can have more than two alternative versions, or multiple alleles, also adds complexity. During meiosis II, the duplicated alleles split apart. Upon separation the two kinds of alleles will segregate.
The beads represent the alleles E (pink) and e (white). The problem is to determine the genetics of these tangled relationships.įigure: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. 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. Sorting out the role that each interacting gene product plays in the expression of skin color is a task not easily accomplished. However, skin color varies continuously in modern populations, a reflection of the many genes involved. The model shows the result of expressed alleles adding to a phenotype of one of 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. Varying combinations of the alleles result in seven discrete colors. The darkest skin is due to six dominant “doses” and the lightest skin is due to six recessive “doses”. The dominant form codes for a “dose” of dark skin and the recessive form codes for a “dose” of light skin. What is the nature of polygenic inheritance? This module uses a simple three-gene model, each with two alleles.
Skin pigmentation is the result of the interaction of many expressed genes: skin color is a polygenic characteristic.
0 kommentar(er)
