I'm confused on how/why would you determine if the tested is AA or Aa if its always heterozygote, shouldn't it always be Aa (F1)?

Hi,

A back cross is kind of a double check to fully confirm that your F1 generation is heterozygous. It's largely done as a control experiment.

For example, if you cross a purple AA plant with a white aa plant, you get 4 purple plants. Put yourself in the perspective of a skeptic in that day that believes the genomes of these 4 purple plants don't contain the white a gene anymore.

"How do you know they are Aa offspring? What if you just get one gene "swallowing" the other?"

If you do the back cross, you can prove from the ratios that you still have the a gene in these flowers, proving them by ratios to be Aa. If you don't do this, then the skeptic just has to take your word for it.

From a problem solving perspective today, we may not need this as much as we know all of the variables in the problem (but sometimes they might be tricky and give you information from the back cross to determine the phenotypes of the parents).

Please let me know if anything is still unclear.