Genetic Crosses With Punnett Squares
Laura Armstrong
Teacher
Recall Questions
This topic requires prior knowledge of key terms in inheritance.
What is a genotype?
The set of alleles an organism has for a gene (e.g., Bb, BB)
What is the phenotype?
The physical characteristics shown, determined by genotype and environment
What does homozygous mean?
Having two identical alleles for a gene (e.g., BB or bb)
Topic Explainer Video
Check out this @lauradoesbiology video that explains genetic crosses with punnett squares, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Genetic Crosses and Punnett Squares
Gregor Mendel – The Father of Genetics
- Gregor Mendel was an Austrian monk.
- In the mid-1800s, he carried out experiments on pea plants.
- He discovered that characteristics are inherited in clear and predictable patterns.
- Mendel proposed that "units" (now called genes) are passed from parents to offspring.
- His work was not recognised until after his death, but it laid the foundation for modern genetics.
- In the late 19th Century behaviour of chromosomes during cell division was observed.
- In the mid-20th Century the structure of DNA was determined and the mechanism of gene function worked out.
Key Point: Mendel was the first to show how dominant and recessive alleles work using genetic crosses, long before the discovery of DNA.
What is a Genetic Cross?
- A genetic cross shows how alleles from parents can combine in offspring.
- It uses a Punnett square to predict probabilities of inherited traits.
- Traits are controlled by alleles, which can be dominant or recessive.
How to Draw and Use a Punnett Square:
- Identify the parents’ genotypes.
- Split them into gametes (one allele per gamete, as gametes are haploid so will only have one allele for each gene).
- Complete the Punnett square by combining gametes to show what will happen at fertilisation.
- Count the outcomes and determine the phenotypic ratio or probability.
Example 1: Brown Eyes (B) Dominant over Blue Eyes (b)
Cross: Heterozygous parent (Bb) × Heterozygous parent (Bb)
|
B |
b |
|
|
B |
BB |
Bb |
|
b |
Bb |
bb |
- Offspring Genotypes: 1 BB, 2 Bb, 1 bb
- Offspring Phenotypes: 3 brown eyes, 1 blue eyes
- Probability of brown eyes = 3/4 = 75%
Example 2: Tall (T) is Dominant over Short (t)
Cross: Homozygous tall parent (TT) × Short parent (tt)
|
T |
T |
|
|
t |
Tt |
Tt |
|
t |
Tt |
Tt |
- Offspring Genotypes: All Tt
- Offspring Phenotypes: All tall
- Probability of tall offspring = 4/4 = 100%
Example 3: Cystic Fibrosis – a genetic disorder caused by a recessive allele (n)
Cross: Carrier (Nn) × Carrier (Nn)
|
N |
n |
|
|
N |
NN |
Nn |
|
n |
Nn |
nn |
- Offspring Genotypes: 1 NN, 2 Nn, 1 nn
- Offspring Phenotypes: 3 no disease (1 unaffected, 2 carriers), 1 has cystic fibrosis
- Probability of child with cystic fibrosis (nn) = 1/4 = 25%
Key Terms
- Punnett square – Grid used to predict outcomes of genetic crosses.
- Dominant allele – Always expressed, even if only 1 copy is present (e.g., B).
- Recessive allele – Only expressed if two copies are present (e.g., b).
- Homozygous – Same alleles for a gene (BB or bb).
- Heterozygous – Different alleles for a gene (Bb).
- Phenotypic ratio – Ratio of physical traits in offspring.
- Carrier – Has one allele for a genetic disease, they are heterozygous. They will not have the disease themselves but can pass the recessive allele on to their offspring.
No answer provided.
Exam Tip:
If you take foundation tier - you will be provided with a punnett square to simply fill in. If you take higher tier - you will have to construct your own punnett squares. In which case, always label your Punnett square clearly with the parental genotypes and phenotypes, and the offspring genotypes and phenotypes.
You will likely be asked for a probability of a certain outcome. This may be presented as a fraction, a decimal or a percentage - but always check in case they have specifically asked for your answer in a certain way.
No answer provided.
Practice Question
In guinea pigs, the allele for rough fur (R) is dominant to the allele for smooth fur (r). Two heterozygous rough-furred guinea pigs are crossed. Use a Punnett square to predict the probability of a smooth-furred offspring. (4 marks)
Model Answer:
Rr x Rr
|
R |
r |
|
|
R |
RR |
Rr |
|
r |
Rr |
rr |
- Offspring Genotypes: 1 RR, 2 Rr, 1 rr
- Offspring Phenotypes: 3 rough fur, 1 smooth fur
- Probability of smooth fur (rr) = 1/4 = 25% or 0.25
More Practice
Try to answer these practice questions from the TikTok videos on your own, then watch the video to see how well you did!