This week we expanded our knowledge of different variations on dominance concepts (for example, incomplete dominance and codominance) and entered the world of pedigree analysis. Pedigrees are particularly useful in species (such as humans) where it is impossible or highly impractical to get the very large numbers of offspring required for the analytical approaches of Mendel. The patterns by which affected individuals appear on the pedigree provide ‘tell tale’ signs indicative of the dominance relationships of the alleles. We also entered the fun (IMO) world of epistasis where the interactions between gene products, for example in biochemical pathway contexts, can perturb the F2 dihybrid ratios away from the usual expected 9:3:3:1 for genes whose products do not interact. There are lots of different possible modifications of the 9:3:3:1 that result from epistasis, such as 12:3:1, 9:7, and 15:1. These different ratios result from different particular underlying biochemical explanations. You don’t need to memorize these, but do need to be able to reason your way to these modified ratios (and their corresponding phenotypes, genotypes) when given cross data. After our epistasis adventure, we moved into a review of mitosis and meiosis, emphasizing the dynamics of chromosomes during these cell division processes. We concluded Thursday’s lecture with an overview of sex chromosome transmission processes, and learned about nematode sex 🙂
Week 4 Sneak Peek: Of course, the main event will be the Tuesday Midterm during Week 4. Study hard! Bring your calculators! Know your probabilities! After the midterm, we will delve further into chromosome biology in Chapter 5.