Commonly confused
Genotype vs phenotype
One is the gene code an organism carries. The other is what that code looks like. They are linked, but not the same.
Genotype
The alleles an organism carries, written as letters: TT, Tt, or tt. It is the instruction, even if you cannot see it. TT and Tt are different genotypes.
Phenotype
The trait you can see: tall or short. TT and Tt pea plants have the same phenotype (tall), even though their genotypes are different.
The trap
The key point: the same phenotype can come from more than one genotype. A tall plant can be TT or Tt, and you cannot tell by looking. That is why the test cross exists. If a question asks how many genotypes give the tall phenotype, the answer is two, not one.
Quick check
A tall pea plant is seen. Its phenotype is ‘tall’. Its genotype could be:
- AOnly TT
- BOnly tt
- CTT or Tt
- DOnly Tt