Practice a College Board-style free response question on Heredity. Write your response, then reveal the model answer to see exactly what earns each point.
Free Response Question · Unit 5 · Sex-Linked Inheritance in Drosophila
In fruit flies (Drosophila melanogaster), eye color is controlled by a gene with two alleles: red (the wild type) and white (the mutant). A scientist crosses pure-breeding red-eyed females with white-eyed males. All F1 offspring (both sexes) have red eyes. The F1 flies are then crossed with one another. The F2 generation shows the following phenotype counts (rounded):
F2 phenotype
Approximate frequency
Red-eyed females
~50%
Red-eyed males
~25%
White-eyed males
~25%
White-eyed females
0%
A
Based on the data, determine whether the white allele is dominant or recessive, AND whether the gene is autosomal or X-linked. Justify your answer using the data.
✓ Model answer (earns the point)
The white allele is recessive, because all F1 offspring (which inherited one white allele from one parent) showed the red phenotype — meaning red masks white. The gene is X-linked, because the F2 phenotypes are different in males and females: white-eyed flies appear ONLY among males, never among females. If the trait were autosomal, both sexes would show the same ratios. The pattern (recessive trait appearing only in males in F2) is the classic signature of X-linked recessive inheritance.
Why it scores: Identifies BOTH dominance pattern AND chromosomal location, AND justifies each conclusion using specific features of the data (F1 all red → recessive; trait only in F2 males → X-linked).
B
Using standard genetic notation (X^R for the red allele and X^r for the white allele), write the genotypes of (i) the parental (P) flies, (ii) the F1 flies, and (iii) the F2 flies of each phenotype.
✓ Model answer (earns the point)
(i) Parents: Red-eyed female = X^R X^R; white-eyed male = X^r Y.
(ii) F1: Red-eyed female = X^R X^r (heterozygous carrier); red-eyed male = X^R Y.
(iii) F2: Red-eyed female = X^R X^R OR X^R X^r; red-eyed male = X^R Y; white-eyed male = X^r Y; white-eyed females = none (would require X^r X^r, but the F1 father only contributes X^R or Y — never X^r).
Why it scores: Uses the correct X^R / X^r notation, correctly identifies F1 females as heterozygous carriers, AND explains why white-eyed females don't appear in F2 (the F1 father has X^R Y — no X^r to give).
C
Predict the phenotype ratios of offspring from a cross between an F2 white-eyed male and an F1 red-eyed female. Show your work using a Punnett square or written reasoning.
✓ Model answer (earns the point)
The cross is X^R X^r (F1 red-eyed female) × X^r Y (F2 white-eyed male). The female produces X^R and X^r gametes (each 50%). The male produces X^r and Y gametes (each 50%).
Punnett square offspring (each cell ¼ of total):
X^R X^r — red female (carrier), 25%
X^r X^r — white female, 25%
X^R Y — red male, 25%
X^r Y — white male, 25%
So the expected phenotype ratio is 1 red female : 1 white female : 1 red male : 1 white male, or 1:1:1:1 overall. Half of all offspring are red-eyed and half are white-eyed, and the ratio is the same in both sexes.
Why it scores: Correctly identifies the parental genotypes, shows the cross (Punnett square or written), gives the genotypes AND phenotypes of all offspring, and states the final phenotype ratio (1:1:1:1 or "50% red : 50% white in both sexes"). Bonus: explicitly noting that white-eyed females ARE now possible because the father (X^r Y) contributes X^r.
How to score points on AP Biology FRQs
Use the data when it's provided. If there's a table or graph in the stimulus, reference specific values. The graders look for it.
Name specific biology, not vague concepts. "Thylakoid membrane" beats "inside the chloroplast." "Limiting factor" beats "something else slows it down." "Rubisco fixes CO₂" beats "an enzyme attaches CO₂."
Answer the verb in the prompt. "Describe" needs an observation. "Explain" needs a mechanism. "Predict and justify" needs a prediction AND the biology behind it.
Trace cause to effect. Don't just state outcomes — connect them. "Recessive allele on X → only one X in males → males show the trait" is a strong causal chain.
For limiting factor questions, identify WHICH factor limits at each end. AP graders love when students name the specific limiting factor (light vs. CO₂ vs. temperature vs. rubisco).