HSC Exam Practice

Sample response. An allele is a particular version (form) of a gene at a specific locus. A homozygous genotype contains two identical alleles for a gene (e.g. AA or aa); a heterozygous genotype contains two different alleles for a gene (e.g. Aa).

Marking notes. 1 mark — defines allele as a form/version of a gene. 1 mark — correctly defines homozygous (two identical alleles, with example or notation). 1 mark — correctly defines heterozygous (two different alleles, with example or notation).

Sample response. A Punnett square models the possible gamete combinations from each parent and the genotypes that can form, giving the probability of each offspring genotype. Because each fertilisation event is independent, the predicted ratio (e.g. 3:1) is an expected probability per child — it does not force a specific family of four children to contain exactly three of one phenotype and one of the other.

Marking notes. 1 mark for identifying the Punnett square as a probability/gamete-combination model. 1 mark for explaining that fertilisation events are independent so outcomes per family are not guaranteed.

Sample response. Parents are both Tt. Gametes from each parent are T and t. The Punnett square gives offspring TT, Tt, Tt, tt. Genotype ratio = 1 TT : 2 Tt : 1 tt. Phenotype ratio = 3 tall : 1 short.

Marking notes. 1 mark — correctly identifies parental gametes (T and t from each). 1 mark — completes Punnett square correctly with all four offspring genotypes. 1 mark — states both genotype ratio (1:2:1) and phenotype ratio (3:1) correctly.

Sample response. Clue 1: an X-linked recessive trait affects more males than females, because males express the trait with only one recessive allele on their single X chromosome, while females need two copies. An autosomal recessive trait affects males and females in roughly equal numbers. Clue 2: an X-linked recessive allele cannot pass from father to son, because the father gives his Y chromosome to sons, not his X. Father-to-son transmission is permitted for autosomal recessive traits.

Marking notes. 1 mark — sex bias (more affected males) explained via X/Y chromosome logic. 1 mark — no father-to-son transmission of the X-linked allele. 1 mark — explicit contrast that both clues are absent or different in autosomal recessive inheritance.

Sample response. A father with an X-linked recessive condition has genotype XhY. He passes his Y chromosome to all of his sons; he does not pass any X chromosome to his sons. Sons therefore receive his Y plus their mother's X, so the affected Xh allele cannot reach his sons directly. (Daughters, by contrast, receive his Xh and so are obligate carriers.)

Marking notes. 1 mark — identifies that fathers pass Y (not X) to sons. 1 mark — concludes that the affected X-linked allele therefore cannot reach sons via the father; sons get their X from the mother.

Sample response (a). The condition is most likely X-linked recessive.

Sample response (b). The pedigree shows only affected males (II-2 and III-2) — no affected females are present, consistent with X-linked recessive inheritance where females require two copies of the recessive allele to express the trait while males express it with only one (single X chromosome). Unaffected parents (II-3 × II-5) produced an affected son (III-2), indicating II-3 must be a carrier mother — also consistent with X-linked recessive. There is no father-to-son transmission in the pedigree (I-1 is unaffected; II-2 has no recorded sons), and the structure does not contradict the rule that fathers pass Y, not X, to sons.

Sample response (c). I-2 is XAXa (carrier mother — must carry the recessive allele because her son II-2 is affected). II-3 is XAXa (carrier mother — she is unaffected but produced an affected son III-2 with an unaffected father).

Marking notes. Part (a) — 1 mark for correctly identifying X-linked recessive. Part (b) — 1 mark for sex bias (only affected males); 1 mark for using "unaffected mother × unaffected father → affected son" as evidence of a carrier mother; 1 mark for explicitly invoking chromosome transmission (X-from-mother, Y-from-father) and consistency with no father-to-son X transmission. Part (c) — 1 mark for I-2 = XAXa; 1 mark for II-3 = XAXa.

Sample response. Parents are XHXh (carrier mother) and XHY (unaffected father). The mother produces gametes XH and Xh; the father produces gametes XH and Y. The Punnett square gives four offspring genotypes, each with 25% per-fertilisation probability: XHXH (unaffected daughter), XHXh (carrier daughter), XHY (unaffected son), XhY (affected son). Therefore: (a) affected son = 25% of all children (50% of sons); (b) unaffected son = 25% of all children (50% of sons); (c) carrier daughter = 25% of all children (50% of daughters); (d) affected daughter = 0%, because the father can only contribute XH (not Xh), so no daughter can be homozygous XhXh. Each child's chromosomal sex depends on whether the father donates X or Y; the affected allele in this cross can only reach a child as Xh from the mother.

Marking notes. 1 mark — correct parent genotypes and gametes identified. 1 mark — Punnett square produces the correct four offspring genotypes. 1 mark — correct probabilities stated (25% each, or 50% of sons / 50% of daughters as appropriate). 1 mark — correctly explains that the affected-daughter probability is 0% due to the father's contribution. 1 mark — links the result to chromosome transmission (father provides X or Y; the Xh allele in this cross can only come from the mother).

Sample response. The claim conflates three independent ideas — dominance, "power", and frequency — and is wrong on each. Dominance describes a phenotypic relationship between alleles in a heterozygous genotype: a dominant allele is one whose effect is expressed when paired with a recessive allele. It says nothing about biochemical "power" or fitness, and it says nothing about how common the allele is in a population. Allele frequency depends on factors such as mutation rate, selection, drift and gene flow, not on whether the allele is dominant or recessive. As a named example, Huntington's disease is caused by a dominant allele yet is rare in the population — the dominant allele is far less common than the recessive (unaffected) allele. By contrast, the recessive allele for cystic fibrosis is relatively common in some populations despite being recessive. A defensible reformulation is: "A dominant allele is one that is expressed in a heterozygous individual, while a recessive allele is masked in the heterozygote. Dominance is independent of allele frequency, fitness and 'strength' — a dominant allele can be rare and a recessive allele common." Therefore the claim should be rejected on all three of its assertions; the only correct usage of "dominant" is the strict heterozygote-expression definition.

Marking notes. 1 mark — states an overall evaluative judgement (claim is incorrect). 1 mark — gives the correct definition of dominance as expression in a heterozygote. 1 mark — refutes "more powerful" using lesson framing (dominance is not about strength/health/fitness). 1 mark — refutes "more common" by separating dominance from frequency. 1 mark — uses a valid named example (e.g. Huntington's disease) showing a rare dominant allele, or another biologically defensible example. 1 mark — produces a defensible reformulation that uses precise terminology from the lesson.