Testcross definition?

[Quinque]

Hi I have a quick question on test cross. So if I am trying to decide if something is AA or Aa, I know I can testcross with it an homozygous recessive "aa".

But, can I also cross the unknown with "Aa" to check if the unknown is AA or Aa?

Because online it says a test cross is always just homozygous recessive is used, but why cant a heterozygous be used?

Thanks so much!!!!

---

Members don't see this ad.

 

[8_man]

Don't think too deeply about it. Just know that a test cross ALWAYS uses a homozygous recessive organism to mate with.

Sure, you could use a heterozygous organism to cross with to determine the genotype in question, but that's not what the parameters of a test-cross are.

 

[dentalmagic]

Statistically it is just easier for us to use a homozygous recessive test. Lets say the unknown were Aa we did the test using a heterozygous genotype Aa, the ratio of observed phenotypes would be 3:1. If the unknown were AA, and we did the test with the same heterozygous genotype Aa, we would see the phenotype of the dominant allele.

So for the first test cross, to confirm that it is Aa, we would be looking for the recessive phenotype. But there is only a 1 out of 4 chance that we will get that phenotype. We might get all dominant phenotypes due to chance. And if we saw this, we might mistakenly conclude that the phenotype is AA, when it might actually have been Aa.

Because of this, we do a test cross using the homozygous recessive genotype instead. Because If the unknown were Aa, there is a 50/50 chance that we will see both phenotypes. If we see the recessive phenotype, we can confirm with greater certainty that the unknown genotype was Aa.

Hope this makes sense!