Mendelian Genetics Study Tips Here are some study tips that I put together for you. In addition to reviewing these and your notes (very important!), it would also be wise to take a fresh look at the problems on Genetics #3 to make sure you remember how to work those problems. Similar problems will make up about 75% of the quiz. When doing problems with punnet squares, here are some hints to keep in mind: If a 1:2 ratio of offspring is present, there must be a lethal allele. If offspring ratio is 3:1, parents were both heterozygous If offspring ratio is 2:2, one parent was heterozygous and the other was homozygous recessive If something bred with itself ALWAYS gives birth to similar offspring, it must be homozygous for the recessive trait If something bred with itself can sometimes have offspring with a different trait, the parent's trait must be dominant If two colors (or versions of the particular trait) are showing up at the same time, it must be codominance (black and white cows, for example) If there is a third trait (often color) that is in between the other two, it is incomplete dominance (yellow + blue = green, or curly + straight = wavy) Blood types show simple dominance, codominance, and are an example of a gene with multiple alleles. AA and AO make type A blood, BB and BO make type B blood, AB makes type AB blood, and OO makes type O blood Hair color exhibits incomplete dominance and epistasis. The black/brown gene is epistatic over the red/blonde gene, which only shows up if the individual is homozygous recessive for the black/brown gene. If there are more than two traits being crossed at a time, use the product rule (cross each trait with itself in a four-box square and multiply the results) Additionally, you should be able to define genetics, allele, gene, phenotype, genotype, test cross, and epistasis.