Calculating and analysing outcomesSpec 3.16
You need to calculate and analyse the outcomes of monohybrid crosses and pedigrees for dominant and recessive traits. Outcomes can be given as a ratio, a probability or a percentage.
| Outcome | Boxes out of 4 | Probability | Percentage |
|---|---|---|---|
| Short plant (tt) | 1 | 1/4 or 0.25 | 25% |
| Tall plant (TT or Tt) | 3 | 3/4 or 0.75 | 75% |
As a ratio, this is 3 tall : 1 short. A 3 : 1 ratio means 1 in 4 offspring are short, not 1 in 3.
Probability for a recessive disorder
A disorder is caused by a recessive allele (f). Two parents do not have the disorder but both are carriers (Ff). What is the probability that their child has the disorder?
- Gametes from each parent: F or f.
- Possible offspring: FF, Ff, Ff and ff.
- Only ff has the disorder. That is 1 out of 4 equally likely outcomes.
Answer: Probability = 1/4 = 0.25 = 25%.
Each child is an independent event. If the first child has the disorder, the probability for the next child is still 1 in 4. Probabilities describe chance. They do not guarantee the actual numbers in a family.