At the beginning of the week, we went through mutation screening methods with Neurospora and associated complementation testing. These studies provide great examples of the type of logic that geneticists use – make sure you understand the logic that determines whether different particular genetic mutants go into the same versus different complementation groups. Next, we entered into classical Mendelian (aka transmission) genetics – how the results of thousands of crosses between pea plants of varying phenotypic traits and combinations of traits led to our current understanding of how different versions (alleles) of genes segregate, and how different alleles, and allele combinations, lead to different phenotypic outcomes. After revisiting the basic concepts and Punnett square approaches to understanding how alleles are transmitted, probability concept approaches were introduced that offer much faster and practical ways to solve these problems (especially when it comes to many genes). Mendel was EXTREMELY lucky to have worked with seven allele pairs (each pair for one of seven different genes) that each affect different “easy to score” phenotypic traits where there are very simple “full dominance” relationships between each of the allele pairs. We also entered into variations on dominance concepts such as incomplete dominance and codominance. Most phenotypic traits of interest to the health sciences (e.g. susceptibilities to genetic diseases and cancers, ability to defend against infectious microbes and parasites, aging) and agricultural sciences (e.g. plant drought tolerance, resistance to pests, livestock growth rates) are influenced by many genes. We will study these more complicated scenarios later in the term.
Week 3 Sneak Peek: Next week we will finish up Ch. 2, discussing more about variations on dominance concepts and epistasis. Ch. 3 will follow where discussion will turn toward the biology of chromosomes and the genetics of sex chromosomes. We will also cover the chi-square test.
