With a dominant allele, the homozygous and heterozygous genotypes look the same. A tall plant can be TT or Tt, and only more evidence can tell them apart.
This lesson follows probability for one offspring versus a family count. A phenotype is evidence, but it is not always enough evidence.
Why do two genotypes look the same?
The dominant allele T shows whenever it is present, so TT and Tt both give a tall plant. Only tt gives a short plant.
| Genotype | Alleles | Phenotype |
|---|---|---|
| TT | Homozygous dominant | Tall |
| Tt | Heterozygous | Tall |
| tt | Homozygous recessive | Short |
Worked example with an invented plant
A tall plant of unknown genotype is crossed with a short plant (tt).
Question: How do the offspring show whether the tall plant is TT or Tt?
If the tall plant is TT, its gametes are all T and the short plant’s are all t. Every offspring is Tt, so all are tall.
If the tall plant is Tt, its gametes are T or t, and the short plant gives t. Offspring are Tt and tt in equal numbers, so about half are short.
Written answer: “The phenotype alone cannot tell TT from Tt. If a test cross with tt gives only tall offspring, the plant is likely TT. If any offspring are short, the plant must be Tt.”
The wording “likely TT” is deliberate. A few tall offspring could appear by chance even from Tt, so more offspring give stronger evidence.
The mistake that loses marks
The common slip is to write “TT” for every tall plant. That claims more than the evidence shows.
| Claim | Supported by “the plant is tall”? |
|---|---|
| “The plant is TT” | No |
| “The plant is Tt” | No |
| “The plant is TT or Tt” | Yes |
| “A short plant is tt” | Yes, since short is recessive |
The fix is to give the possibilities, then state what would settle them.
A routine for any phenotype
- Ask whether the phenotype is dominant or recessive.
- If recessive, write the homozygous recessive genotype.
- If dominant, write both possibilities.
- Add what test or family information would decide.
The simple inheritance cross explorer lets you run both test crosses side by side.
Check yourself
Invented plant: a red-flowered plant (R dominant) is crossed with a white-flowered plant, and all twelve offspring are red. State what you can and cannot conclude.
Answer
The white parent is rr. The red parent is likely RR, since all twelve offspring are red. It could still be Rr by chance, because the chance of twelve red from Rr is (1/2)¹², which is small but not zero.
What to study next
Practise naming what you still need in identifying information that is missing before assigning a genotype. To revisit the test cross ratios, see completing monohybrid crosses.
If you want a teacher to set you test-cross puzzles, see online one-to-one Biology tuition.