Here's why:
* Homozygous recessive means the organism has two copies of the recessive allele for the trait in question. This allows the recessive allele to be expressed.
* Unknown genotype: The organism with the unknown genotype could be homozygous dominant, heterozygous, or homozygous recessive.
* By crossing with a homozygous recessive organism: The offspring's phenotype will directly reveal the genotype of the unknown parent.
Here's a simple example:
Let's say we're looking at flower color, with purple (P) being dominant to white (p).
* Unknown genotype: We have a purple flower, but we don't know if it's PP (homozygous dominant) or Pp (heterozygous).
* Testcross: We cross the purple flower with a white flower (pp).
* Results:
* If all offspring are purple, the unknown parent was PP (homozygous dominant).
* If half the offspring are purple and half are white, the unknown parent was Pp (heterozygous).
This is because the homozygous recessive parent (pp) can only contribute the recessive allele (p), so the offspring's phenotype directly reflects the allele contributed by the unknown parent.
