Before naming a genotype, list what the question gives, what it leaves out and what would settle the matter. A careful answer lists the possibilities and stops where the evidence stops.
This lesson closes the set on inheritance evidence rather than family assumptions. It uses the ideas from two genotypes, one phenotype.
What is the three-column method?
- Known: facts stated in the question.
- Unknown: what the genotype could be.
- Needed: what extra fact would decide it.
The third column is the real skill. It shows that you know where the evidence ends.
Worked example with a fictional family
This family is fictional. The trait is recessive, with A the dominant allele and a the recessive allele.
- I-1 and I-2 are unaffected and have an affected child, II-1.
- They also have an unaffected child, II-2.
Question: What can you say about II-2’s genotype, and what is the chance that II-2 carries the recessive allele?
| Known | Unknown | Needed |
|---|---|---|
| II-1 is aa, so both parents are Aa | Whether II-2 is AA or Aa | II-2’s own offspring, or a test cross |
| II-2 is unaffected |
Now calculate. A cross Aa × Aa gives AA, Aa, Aa, aa. II-2 is unaffected, so the aa outcome is ruled out and three outcomes remain: AA, Aa, Aa.
Two of the three are Aa, so the chance that II-2 is a carrier is 2 in 3.
Written answer: “II-2 is AA or Aa. The chance that II-2 is a carrier is 2 in 3, because two of the three unaffected outcomes from Aa × Aa are Aa. The genotype cannot be fixed without further evidence, such as II-2’s offspring.”
The mistake that loses marks
The common slip is to use 1 in 2 as the chance of being a carrier, or to say “II-2 must be Aa”. Neither follows from the evidence.
| Claim | Supported? |
|---|---|
| “II-2 is AA” | No |
| “II-2 is Aa” | No |
| “II-2 is AA or Aa, with a 2 in 3 chance of Aa” | Yes |
The fix is to remove the aa outcome first, then count what remains.
A routine for any genotype question
- Write what is known, including the phenotype and relatives.
- Write the possible genotypes.
- Remove any that the evidence rules out.
- State what extra fact would decide the rest.
The claim, evidence and explanation revision desk gives a template for this write-up. Use the mistake log to record assumptions you caught.
Check yourself
Fictional tall plants: two tall plants (T dominant) produce one short offspring. State their genotypes.
Answer
The short offspring is tt, so each tall parent passed a t. Both parents are Tt. The evidence fixes both genotypes, because each parent must carry t.
What to study next
Test the whole set with the practice on inheritance evidence. To strengthen the basics, return to completing monohybrid crosses.
If you want a teacher to practise the pause with you, see online one-to-one Biology tuition.