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.
Quick check
What is an allele?
Show answer
A different version of the same gene.
What is the difference between genotype and phenotype?
Show answer
Genotype is the alleles an organism has. Phenotype is the observable characteristic that is expressed.
Which sex chromosomes do human females and males have?
Show answer
Females XX, males XY.
Two parents without a disorder have an affected child. Is the allele dominant or recessive?
Show answer
Recessive, and both parents are carriers.
What is codominance?
Show answer
Both alleles are expressed in the phenotype when both are present, as with IA and IB giving blood group AB.