Biology • Year 12 • Module 5 • Lesson 14
Mendelian Patterns — Autosomal Inheritance, Sex Linkage, Punnett Squares
Lock in the core inheritance vocabulary, the language of probability, the structure of a Punnett square, and the difference between autosomal and X-linked transmission.
1. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: genotype, phenotype, allele, homozygous, heterozygous, autosomal, dominant, recessive, sex-linked, carrier. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | The observable trait produced when a genotype interacts with the environment. | |
| 1.2 | A gene located on a chromosome that is not a sex chromosome. | |
| 1.3 | An allele that is expressed in the phenotype when present in a heterozygous genotype. | |
| 1.4 | The full allele combination an organism has at a particular gene. | |
| 1.5 | An allele expressed phenotypically only when no dominant allele is present. | |
| 1.6 | A version (form) of a gene at a specific locus. | |
| 1.7 | A heterozygous individual who does not show a recessive trait but can pass the recessive allele to offspring. | |
| 1.8 | Having two identical alleles for a particular gene (e.g. AA or aa). | |
| 1.9 | A trait controlled by a gene located on a sex chromosome (usually the X at HSC level). | |
| 1.10 | Having two different alleles for a particular gene (e.g. Aa). |
2. Complete the Punnett square — Aa × Aa
Two heterozygous pea plants are crossed for plant height. T codes for tall and is dominant; t codes for short and is recessive. Fill the four offspring genotype boxes, then complete the genotype and phenotype ratios below. 8 marks
| T | t | |
|---|---|---|
| T | ||
| t |
2.1 Genotype ratio (___ TT : ___ Tt : ___ tt):
2.2 Phenotype ratio (___ tall : ___ short):
2.3 What is the probability that any single offspring is heterozygous?
2.4 The two parents have four offspring. Is it guaranteed that exactly one will be short? Briefly justify (1 sentence).
3. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version. 8 marks (1 T/F, 1 correction where needed)
3.1 A dominant allele is always more common in a population than a recessive allele. T / F
3.2 A Punnett square shows the probability of each offspring genotype, not the exact birth order. T / F
3.3 An autosomal recessive trait can skip a generation when carriers do not show the phenotype. T / F
3.4 A father with an X-linked recessive condition passes the affected X-linked allele directly to his sons. T / F
4. Function recall
Answer each in 1–2 sentences using precise lesson terminology. 10 marks (2 each)
4.1 What is the function of a Punnett square?
4.2 What does it mean for an allele to be dominant? (Define in terms of phenotype expression, not frequency.)
4.3 Why does a male only need one recessive allele on his X chromosome to express an X-linked recessive trait?
4.4 Why are males and females affected by autosomal traits with similar overall probability?
4.5 What is the function of including parent genotypes as the first step when answering an inheritance question?
5. Cloze paragraph — autosomal vs X-linked
Fill each blank using a term from the word bank. Each term is used once. 8 marks
Word bank: autosomes • carriers • dominant • heterozygous • probability • recessive • sex-linked • X chromosome
A Punnett square models the (5.1) __________________ of each possible offspring genotype, not a guaranteed family outcome. Autosomal genes are carried on the (5.2) __________________ rather than on the sex chromosomes, so males and females are affected with similar overall probability. An allele is (5.3) __________________ when it is expressed in a heterozygote, and (5.4) __________________ when it is masked in the presence of a dominant allele. Two unaffected (5.5) __________________, each (5.6) __________________ for an autosomal recessive condition, can have an affected child. Sex-linked traits are usually carried on the (5.7) __________________, and a (5.8) __________________ recessive trait such as haemophilia appears more often in males because they have only one copy of the gene.
Stuck? Revisit lesson § Key Terms, Card 2 (autosomal) and Card 3 (X-linked).Q1 — Term–definition matches (10 marks)
1.1 phenotype • 1.2 autosomal • 1.3 dominant • 1.4 genotype • 1.5 recessive • 1.6 allele • 1.7 carrier • 1.8 homozygous • 1.9 sex-linked • 1.10 heterozygous. 1 mark each.
Q2 — Aa × Aa Punnett square (8 marks)
Completed boxes (4 marks, 1 per cell): top-left TT; top-right Tt; bottom-left Tt; bottom-right tt.
2.1 Genotype ratio = 1 TT : 2 Tt : 1 tt. [1 mark]
2.2 Phenotype ratio = 3 tall : 1 short. [1 mark]
2.3 Probability heterozygous = 2/4 = 50%. [1 mark]
2.4 No — the 1/4 figure is a per-child probability, not a guaranteed family outcome; with only four children, fertilisation events are independent so any combination of tall and short can occur. [1 mark]
Q3 — True / false with correction (8 marks)
3.1 False. Correction: dominance describes whether an allele is expressed in a heterozygote — it does not describe how common the allele is. A dominant allele can be very rare (e.g. Huntington's) and a recessive allele can be very common. [1 + 1]
3.2 True. [1 + 0]
3.3 True. [1 + 0]
3.4 False. Correction: a father passes his Y chromosome to his sons, not his X — so an X-linked recessive allele cannot be transmitted father-to-son via the X. Sons receive their X from their mother. [1 + 1]
Q4.1 — Function of a Punnett square (2 marks)
A Punnett square is a probability model that combines the possible gametes from each parent to show the possible offspring genotypes and the probability of each genotype (and therefore each phenotype) per fertilisation event. [2]
Q4.2 — Dominant allele (2 marks)
A dominant allele is one whose effect on the phenotype is expressed when only one copy is present (i.e. in a heterozygous genotype). Dominance is about expression in a heterozygote — it says nothing about frequency, fitness or health. [2]
Q4.3 — Why one X-linked recessive allele can affect a male (2 marks)
Males have one X and one Y chromosome, so for genes carried on the X they have only one copy of the allele. If that single allele is the recessive one, there is no second X-linked dominant allele present to mask it, so the trait is expressed. [2]
Q4.4 — Autosomal traits and sex (2 marks)
Autosomal genes are located on chromosomes that are not sex chromosomes. Both males and females inherit two copies of each autosomal gene (one from each parent), so the probability of inheriting any particular genotype is the same regardless of sex. [2]
Q4.5 — Why state parent genotypes first (2 marks)
Stating parent genotypes commits you to a specific model before listing gametes — it forces you to translate the wording of the question (e.g. "carrier mother", "affected father") into precise allele notation. Without it, gametes and Punnett-square outcomes cannot be derived reliably, and phenotype/genotype language tend to get mixed up. [2]
Q5 — Cloze paragraph (8 marks)
5.1 probability • 5.2 autosomes • 5.3 dominant • 5.4 recessive • 5.5 carriers • 5.6 heterozygous • 5.7 X chromosome • 5.8 sex-linked. 1 mark each.