Calculating and analysing outcomes

Genetics · Inheritance · note 5 of 5

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Calculating and analysing outcomesSpec 3.16

In short

The outcomes of a monohybrid cross can be given as a ratio, a probability or a percentage. From a Punnett square, count the boxes with that outcome out of four. For two carrier parents (Ff × Ff), the probability of an affected child is 1/4, 0.25 or 25%. Each child is an independent event, so the probability stays the same.

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.

Ways of giving the same outcome (Tt × Tt)
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.

Maths skill:

Show the Punnett square, count the favourable boxes out of the total, then convert to a ratio, fraction or percentage as the question asks.

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.

Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026

Frequently asked questions

What is a genotype and phenotype?

A genotype is the alleles an organism has for a gene, such as Tt. A phenotype is the characteristics that are expressed and can be observed, such as being tall. A heterozygous plant, Tt, has the phenotype of the dominant allele, so it is tall even though it also carries the allele for short.

What is a Punnett square used for in genetics?

A Punnett square is used to show the possible offspring of a genetic cross. The gametes from each parent are written along two sides, then combined in the boxes to give the possible genotypes. Counting the boxes gives the ratio, probability or percentage of each genotype and phenotype in the offspring.

What is the difference between homozygous and heterozygous?

Homozygous means having two identical alleles of a gene, such as TT or tt. Heterozygous means having two different alleles of a gene, such as Tt. A heterozygous organism shows the dominant characteristic, because a dominant allele is expressed in the phenotype even when only one copy is present.

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