How does test cross work? If you have W_ x ww, how does this tell you the unknown?

Question

W=violet w=white

W_ x ww

If you use WW x ww you get all white. If you use Ww x ww you get half white & half violet. So how do you know which is the correct one?

When you cross WW x ww you get all violet. When you cross Ww x ww you get 1/2 violet & 1/2 white. How does this tell you what the parent is???

Answer 1

You can figure out the parental alleles once you know what the children turn out to be like.

So, in the scenario you gave, if you get ANY violet, then you know the first parent HAS to be Ww. BUT if the children are all white, then the first parent is probably WW (but not for sure, because what if a violet just randomly doesn't occur for many, many generations).

Answer 2

Looking at the offspring tells you what the unknown parent's genotype must have been. In your example, you don't know if the parent is WW or Ww.

So you find out by doing the test cross.
Cross the unknown with ww.
-- If you get half white and half violet, then the unknown parent had to be Ww.
-- If you get all violet, then the unknown parent had to be WW.

Answer 3

You just answered your question.

A test cross is basically a practicall way of knowing if a parent if dominant homozygous, or heterozygous. By looking at the results, you will know the unknown.