Here's how the Punnett Square gets more complicated ...
|Here are the parental genotypes:||Assuming no crossover:||Assuming 10% crossover:|
Phenotypic ratio 3:1
Phenotypic ratio ??
Up until now, when we've dealt with Punnett Squares, we've always assumed that all the gametes appear with equal frequency. That is, if there are 4 kinds of gametes (4 rows and 4 columns in the table) then each type appears 25% of the time. Therefore we could count the squares in the phenotype part of the table (the purple area) to get the phenotypic ratios. For example, if 9 out of 16 phenotype squares were dominant-dominant, then 9 out of 16 offspring would (on average) have this genotype.
THIS "phenotype counting" DOES NOT WORK ANYMORE! The reason is that some of the gametes are less common than others. So, we have to go back to the drawing board on figuring out the proportions of these different phenotypes.
Foreshadowing question: In a regular deck of cards, your chances are choosing a face card are 3/13 (or 0.23). Say you choose 1 card each from 2 decks. How likely is it that BOTH cards will be face cards? What rule of probability did you use?
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