Welcome to Darwin’s home plate! The genetics and evolution is one of my favorite units simply because it involves so much problem solving and logic. As you saw in the last post, chapters 16-20 can kind of be considered in both the cell biology unit and the genetics and evolution unit. We want to follow the USABO/IBO syllabus, so we’re considering them part of cell bio.

Genetics and Evolution can be split into two parts (pretty self-explanatory). The genetics material is concentrated in chapters 13-15. Basically everything in these chapters is hugely important. However, as you read, you’ll notice they are a lot easier than some of the cell bio chapters. That’s because genetics isn’t just about knowing the definition of terms like epistasis; it’s about knowing how to apply or recognize them.

One of the toughest types of genetics questions you’ll come across is pedigree problems. Specifically, you should know how to do two things: identify the mode of inheritance and calculate the chances that [X] is a carrier/has the disease/is healthy. You can find some great resources for problems like this at this page. While pedigree problems may tie in extra concepts like STRs and penetrance, get a good understanding of the basics first so it’s easier to internalize and connect the more confusing topics.

Now for the evolution material! Evolution is one of the easier units of the book, so you won’t need to re-read it as many times as you’ll have to for units like Animal Anatomy & Physiology. However, you do need to make sure you really understand the concepts well, as while they are easy, they are extremely relevant. For example, make sure you know the Hardy Weinberg equations/conditions, the barriers to reproduction, the nuances of allele fixation, etc.

Part of the reason I love the genetics and evolution unit is that I’m a bit of a math geek, and the problem solving skills used in math contests are often applicable to complex genetics problems. However, whether you like math or not, I think this unit is one of the most fun ones, and I hope you enjoy studying it too. Just to show you how much I love these problems, I’ll leave you with a fun genetics problem to puzzle over!

The dominant/recessive mechanism seems a bit counterintuitive at first, as it seems like both alleles should be equally represented. However, this can be explained through molecular mechanisms like those responsible for Tay-Sachs disease. Tay-Sachs follows dominant inheritance because even having half the amount of working enzyme is sufficient to prevent the negative health effects. Now imagine a dihybrid cross (parents are both AaBb) where the offspring end up having one of only two different phenotypes, and these phenotypes are in the ratio of 9:7. What could be one molecular explanation for this outcome?