Inheritance questions are about probability, not certainty. Punnett squares and pedigrees let us model likely genotype combinations, then interpret whether a trait is autosomal, recessive, dominant or X-linked.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
A couple are both heterozygous for an autosomal recessive condition. A student says, "That means one out of every four children in the family must have the condition, so if they have four children, exactly one will be affected."
Before reading on, explain why that statement is too strong. What does a Punnett square actually predict, and what does it not guarantee?
Wrong: Natural selection means organisms change because they want or need to.
Right: Natural selection acts on random genetic variations; organisms do not consciously adapt.
A Punnett square is a probability model. It combines the gametes that could be produced by each parent and shows the possible genotypes of offspring.
Because meiosis separates alleles into gametes, each parent contributes one allele for each gene. A monohybrid Punnett square tracks one gene at a time. If a parent is heterozygous, two different allele types can appear in its gametes. If a parent is homozygous, only one allele type appears in its gametes.
The chance of each allele combination, such as Aa or aa.
The chance of the observable trait, which depends on how the alleles are expressed.
Each fertilisation event is separate. Previous births do not change the probability for the next child.
Autosomal genes are located on chromosomes that are not X or Y. This means males and females are affected with similar overall probability, because both sexes carry two copies of each autosomal gene.
At HSC level, sex-linked inheritance usually means X-linked inheritance. Females have two X chromosomes, while males usually have one X and one Y. For many X-linked genes, males have only one copy of the allele, so a recessive allele on the X chromosome can be expressed in males even when only one copy is present.
More common in males because one recessive allele on the X chromosome can be enough to show the trait.
A heterozygous female may not show the trait but can pass the recessive allele to offspring.
Passes his X chromosome to daughters and his Y chromosome to sons, which helps explain pedigree patterns.
Haemophilia is a standard X-linked recessive example. If a carrier mother has children with an unaffected father, each son has a 50% chance of inheriting the affected X chromosome, while each daughter has a 50% chance of being a carrier.
Pedigrees use squares for males, circles for females, and shading for individuals showing the trait. The job is not to guess randomly. The job is to check whether the pattern matches the logic of an inheritance model.
Y chromosome, not an X chromosome.Use the same sequence each time so the reasoning stays controlled and you do not mix phenotype language with genotype language.
State what each allele symbol means and whether the gene is autosomal or sex-linked.
Use the information given in the stem or pedigree. Do not invent extra alleles.
Write the possible gametes produced by meiosis from each parent.
Combine gametes systematically and count genotype then phenotype outcomes.
Then finish by stating the result in full biological language, such as: "Each child has a 50% probability of being heterozygous for the trait and a 50% probability of being homozygous recessive."
Punnett squares model possible genotype combinations formed when parental gametes fuse. They show probabilities, not guaranteed family outcomes.
Autosomal traits are controlled by genes on non-sex chromosomes, so males and females are usually affected with similar frequency.
Dominant means expressed in a heterozygote. Recessive means not expressed when a dominant allele is present.
X-linked traits follow different inheritance patterns because males have one X chromosome and one Y chromosome.
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
A pea plant trait is controlled by an autosomal gene where T is dominant for tall and t is recessive for short. Cross two heterozygous plants.
Write the parent genotypes, list the gametes, draw the Punnett square, and state both the genotype ratio and phenotype ratio.
An unaffected father and a carrier mother are having children. The haemophilia allele is X-linked recessive.
Explain why a son can inherit haemophilia from this cross but the father does not pass the affected X-linked allele directly to his sons.
1. What does a Punnett square primarily show?
2. Which statement about a dominant allele is correct?
3. Two unaffected carriers for an autosomal recessive condition have a child. What is the probability that the child will be affected?
4. Which pedigree clue most strongly supports an X-linked recessive trait rather than an autosomal recessive trait?
5. A carrier female for an X-linked recessive trait has children with an unaffected male. What is the probability that a son will be affected?
6. A heterozygous parent Aa is crossed with a homozygous recessive parent aa for an autosomal trait.
3 marks
State the possible offspring genotypes and determine the probability of each genotype.
7. Explain two pedigree clues that would support an autosomal recessive inheritance pattern rather than an autosomal dominant pattern.
4 marks
8. An unaffected father and a carrier mother are expecting a child. The trait is X-linked recessive.
5 marks
Use a Punnett square or equivalent reasoning to determine the probability that their child will be:
a) an affected son
b) an unaffected son
c) a carrier daughter
Include the parent genotypes in your response.
Dominant describes expression in a heterozygote. It does not describe how frequent the allele is in a population.
Autosomal genes are on non-sex chromosomes. X-linked genes follow different transmission patterns because X and Y are inherited differently.
Punnett squares predict the chance of each genotype per child. They do not force a family to match the ratio exactly.
Return to the statement from the start of the lesson. Rewrite it more accurately using the language of probability rather than certainty.
1. B - Punnett squares show possible genotype combinations and their probabilities.
2. C - A dominant allele is expressed in a heterozygous individual.
3. B - A carrier cross Aa x Aa gives a 25% probability of aa.
4. C - No father-to-son transmission is a key clue for X-linked inheritance.
5. B - In the sons, half inherit the affected X chromosome from the carrier mother.
Parent genotypes are Aa x aa. The heterozygous parent produces gametes A and a. The homozygous recessive parent produces only a gametes. Possible offspring are Aa and aa. Probability: 50% Aa, 50% aa.
One clue is that two unaffected parents can produce an affected child, which fits autosomal recessive inheritance because both parents may be carriers. A second clue is that the trait can skip generations, which is common when heterozygous carriers do not show the phenotype. In autosomal dominant inheritance, affected individuals usually have an affected parent and the trait commonly appears in each generation.
The parent genotypes are XHXh for the carrier mother and XHY for the unaffected father. Possible offspring are XHXH, XHXh, XHY and XhY. Therefore:
a) affected son = XhY = 25% of all children, or 50% of sons
b) unaffected son = XHY = 25% of all children, or 50% of sons
c) carrier daughter = XHXh = 25% of all children, or 50% of daughters
Tick this once you can distinguish autosomal and X-linked reasoning and use Punnett squares without confusing probability with certainty.