Data from the Fall Semester Class
The class recognized that the three dark green plants Dr. Specht had provided them produced similar progeny and all had the same genotype. They combined the data from the progeny of these three plants and got the following results:
The ratio observed among these progeny for the leaflet number trait was about 3:1. Do you remember seeing this ratio in other experiments?
The class also combined the data from the four selfed pale green, three leaflet plants.
What data set can we use to study the inheritance of two traits at a time? The dark green parent was passing on the dark green alleles to all of its selfed progeny. Therefore we do not learn anything about the inheritance of that trait. The data set we need to focus on is the one from progeny of the dihybrid pale green, three leaflet parents because all phenotypes for both traits were observed.
The dihybrids Dr. Specht obtained would have the genotype Y1Y2 Ll. The question we have is how are the genes from these two gene pairs getting passed along to the progeny that were grown and characterized by the genetics class? It is easy to get confused by a data set with four phenotype categories. Again, simplifying the data helps us think about trait inheritance.
Looking at the data one trait at a time reveal the same 3:1 ratio we observed when segregation at a single gene pair could explain the phenotype variation. It appears that segregation is again happening in this dihybrid cross. If the segregation of the color gene pair was independent of the leaflet number gene pair segregation, than we would make the following phenotypic predictions:
The numbers observed obviously coincide with the expected numbers if we propose that the genes controlling each trait are inherited independently from each other. The 9:3:3:1 ratio is indicative of the segregation of one gene pair proceeding independently of the segregation at another gene pair. This is the principle of independent assortment.