Mendelian Patterns — Autosomal Inheritance, Sex Linkage, Punnett Squares
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.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
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?
Know
- How to model monohybrid crosses using Punnett squares.
- Key features of autosomal dominant, autosomal recessive and X-linked inheritance.
Understand
- That dominant does not mean common, stronger or better.
- That probabilities describe chances for each offspring, not a fixed family outcome.
Can Do
- Interpret simple pedigree evidence to infer likely inheritance patterns.
- Separate autosomal inheritance from sex-linked inheritance using chromosome logic.
Core Content
Mechanism · a probability model
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.
Genotype probability
The chance of each allele combination, such as Aa or aa.
Phenotype probability
The chance of the observable trait, which depends on how the alleles are expressed.
Independent events
Each fertilisation event is separate. Previous births do not change the probability for the next child.
What to write in your book
- Punnett square = probability model combining parental gametes.
- Each parent contributes one allele per gene (meiosis).
- Heterozygous parent → two allele types in gametes; homozygous → one.
- Each fertilisation is independent; "dominant" ≠ common/better.
A Punnett square predicts the _____ of each genotype, not a guaranteed family outcome.
Autosomal patterns · both sexes affected equally
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.
Autosomal dominant
- Usually appears in every generation.
- An affected individual usually has at least one affected parent.
- Heterozygous individuals show the trait.
- Unaffected individuals are often homozygous recessive.
Autosomal recessive
- Can skip generations.
- Two unaffected carriers can have an affected child.
- Affected individuals are usually homozygous recessive.
- Carrier status is common in pedigree interpretation.
Punnett square outcomes show ratios of possible offspring genotypes, not a guaranteed family pattern.
What to write in your book
- Autosomal = gene on a non-sex chromosome; both sexes affected similarly.
- Autosomal dominant: appears every generation; affected child usually has an affected parent.
- Autosomal recessive: can skip generations; two carriers → affected child.
- Aa × Aa → genotype 1:2:1, phenotype 3:1 (each child 25% affected).
A dominant allele is always more common in a population than a recessive allele.
A heterozygous individual carries two different alleles for a particular gene.
Sex-linked traits can only be inherited from the mother.
X-linked traits · hemizygous males
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.
X-linked recessive
More common in males because one recessive allele on the X chromosome can be enough to show the trait.
Carrier female
A heterozygous female may not show the trait but can pass the recessive allele to offspring.
Affected father
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.
Sex-linked reasoning must track whether the allele is on X or Y, and which parent passes which chromosome.
What to write in your book
- Sex-linked usually = X-linked. Females XX, males XY.
- Males are hemizygous → one recessive X allele can be expressed.
- X-linked recessive is more common in males.
- Carrier mother × unaffected father → each son 50% affected, each daughter 50% carrier.
Why are X-linked recessive conditions expressed more often in males than females?
Pedigree reasoning · match the pattern to the model
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.
Clues for autosomal recessive
- Unaffected parents produce an affected child.
- Males and females can both be affected.
- The trait can disappear in one generation and reappear later.
Clues for X-linked recessive
- More affected males than females.
- An affected son often has a carrier mother.
- There is no father-to-son transmission of the X-linked allele.
What to write in your book
- Pedigree symbols: squares = males, circles = females, shading = affected.
- Autosomal recessive clues: unaffected parents → affected child; both sexes; skips generations.
- X-linked recessive clues: more affected males; affected son ← carrier mother; no father-to-son transmission.
- Father gives sons a Y, not an X.
Which pedigree clue most strongly supports X-linked recessive over autosomal recessive?
Worked example · the same controlled sequence each time
Use the same sequence each time so the reasoning stays controlled and you do not mix phenotype language with genotype language.
1. Define symbols
State what each allele symbol means and whether the gene is autosomal or sex-linked.
2. Infer parental genotypes
Use the information given in the stem or pedigree. Do not invent extra alleles.
3. List gametes
Write the possible gametes produced by meiosis from each parent.
4. Complete Punnett square
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."
Activities
Build the Cross
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.
Haemophilia Reasoning
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.
Punnett squares
- Model possible genotype combinations formed when parental gametes fuse. They show probabilities, not guaranteed family outcomes.
Autosomal inheritance
- Controlled by genes on non-sex chromosomes, so males and females are usually affected with similar frequency.
Dominant and recessive
- Dominant means expressed in a heterozygote. Recessive means not expressed when a dominant allele is present.
Sex-linked inheritance
- X-linked traits follow different inheritance patterns because males have one X chromosome and one Y chromosome.
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ApplyBand 4(3 marks) 1. A heterozygous parent Aa is crossed with a homozygous recessive parent aa for an autosomal trait. State the possible offspring genotypes and determine the probability of each genotype.
AnalyseBand 5(4 marks) 2. Explain two pedigree clues that would support an autosomal recessive inheritance pattern rather than an autosomal dominant pattern.
AnalyseBand 5–6(5 marks) 3. An unaffected father and a carrier mother are expecting a child. The trait is X-linked recessive. 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.
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Multiple choice
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Short Answer 1
Parent genotypes are Aa × 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.
Short Answer 2
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.
Short Answer 3
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.
Dominant vs common
Dominant describes expression in a heterozygote. It does not describe how frequent the allele is.
Autosomal vs sex-linked
Autosomal genes are on non-sex chromosomes; X-linked genes follow different transmission patterns.
Probability
Punnett squares predict the chance per child — they do not force a family to match the ratio.
Rapid-fire questions on Punnett squares, autosomal and X-linked inheritance and pedigree reasoning. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
Return to the statement from the start of the lesson. Rewrite it more accurately using the language of probability rather than certainty.