Wednesday, 16 January 2013

Ratios, ratios, ratios

In classical genetics, phenotype and even genotype ratios are a mainstay for determining inheritance.  You can memorize the ratios, but it's important you understand what they're based on.

In this exercise, assume we're dealing with simple dominant/recessive relationships for two traits in a bean plant.  The bean can have yellow or orange pods and can be inflated or constricted.  Constricted or orange pods are novel (new) traits.  You cross a true-breeding orange, constricted bean line with one that is true-breeding for yellow, inflated pods.  The F1 seeds are planted and all grow up to bear fruit which are yellow and inflated.  The F1 are allowed to self-fertilize.
You have several tasks:
  1. Design appropriate gene symbols.
  2. Use a branch diagram to show the expected phenotypic distribution of the F2 progeny.
  3. Of the F2 progeny, those with orange pods are selected and allowed to self.  What is the expected phenotypic distribution among these select F3 progeny?
  4. Demonstrate the expected ratios of progeny from a test cross for the original F1 plants using a Punnett square.
Here's a full solution, but if you want to just see the solution for part 3 (as my students wanted) you can see a shorter version in the second YouTube video.

Here's just the answer to 3:

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