Edexcel IGCSE Biology · Spec 3.23-3.25
Monohybrid Crosses
How to complete monohybrid genetic crosses using Punnett squares and predict the offspring ratios.
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Monohybrid Crosses
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Explained
Setting out a cross so it earns every mark
A monohybrid cross follows a single gene. You are given the parents, and you work out what proportions of offspring are possible.
The vocabulary matters because the marks are attached to it. Genotype is which alleles are present, such as Bb. Phenotype is the characteristic that shows, such as brown eyes. Homozygous means two identical alleles; heterozygous means two different ones. A gamete carries one allele of the gene.
The four steps
- Write the parent genotypes, using a capital for the dominant allele and the same letter in lower case for the recessive one.
- Write the gametes each parent can produce. Each gamete gets one allele.
- Combine them in a Punnett square to give the offspring genotypes.
- Convert those genotypes to phenotypes and state the ratio.
Crossing two heterozygotes, Bb with Bb, gives BB, Bb, Bb and bb. Three of the four show the dominant characteristic, so the phenotype ratio is 3 to 1.
Choosing letters
Use a letter whose capital and lower case forms look clearly different. B and b are fine; S and s, C and c, and O and o are not, because a hurried capital and a hurried lower case look identical and the examiner cannot tell which you meant.
State what your letters stand for before you use them. It costs one line and removes any ambiguity.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme awards four separate marks for a cross: the parent genotypes, the gametes, the offspring genotypes, and the phenotype ratio. Setting the working out in full is therefore worth as much as arriving at the answer.
The guidance is generous in several ways. It allows any symbols and different letters from the ones it used. It accepts the genotypes and ratio read from a Punnett square rather than listed. And it allows an error carried forward, so correct offspring worked out from incorrect parents still score.
It is strict about one thing. The gamete line must not contain two alleles, and it prints examples of the incorrect combinations it will not accept. A gamete is haploid, so writing two alleles in one shows a misunderstanding of meiosis rather than a slip.
On a probability question that followed, the mark scheme multiplies two probabilities together, 0.75 by 0.5, to give 0.375, and accepts that as a decimal, a fraction of three eighths, or 37.5 per cent. If the final answer is wrong, one mark is still available for having found either individual probability.
That last provision is the reason to write the intermediate values down. Two separate probabilities on the page are worth a mark even when the multiplication goes astray.
Ratios and probabilities
A 3 to 1 ratio does not mean that four offspring will be three and one. It means each offspring has a three in four chance of the dominant phenotype, independently of its siblings.
Real families rarely match the ratio exactly, and small numbers depart from it most. Saying that fertilisation is random, and that ratios are predictions rather than guarantees, is a mark on its own when a question gives real data that does not fit.
To combine two independent events, multiply the probabilities. The chance of a first child being male and brown eyed is the chance of male times the chance of brown eyed.
The test cross
An organism showing the dominant phenotype could be homozygous or heterozygous, and you cannot tell by looking.
Cross it with a homozygous recessive individual. If any offspring show the recessive characteristic, the unknown parent must have carried a recessive allele, so it was heterozygous. If all the offspring show the dominant characteristic, it was probably homozygous.
Spec 3.23-3.25
What you need to know
- Set out the alleles for a monohybrid cross
- Use a Punnett square to find possible offspring genotypes
- Calculate phenotype ratios and probabilities
Active recall
Quick check
Answer each question before opening the answer.
What is a monohybrid cross?
A genetic cross that follows the inheritance of a single gene.
What offspring ratio do you expect from crossing two heterozygotes (Tt × Tt)?
3 : 1 (dominant phenotype : recessive phenotype).
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