AP Biology: Genetics Practice Questions
Test yourself on AP Biology: Genetics with 13 original AP Exams practice questions. Pick an answer to see instant feedback and a full explanation.
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1. In pea plants, purple flower color (P) is dominant to white (p). A heterozygous purple plant is crossed with a white plant. What fraction of offspring is expected to have white flowers?
Explanation. This is a test cross: Pp × pp. The Punnett square yields 1/2 Pp (purple) and 1/2 pp (white). So 1/2 of offspring are expected white. The 3/4 ratio applies only to a monohybrid Pp × Pp cross.
2. A dihybrid cross (RrYy × RrYy) involving two genes that assort independently produces offspring. What is the expected phenotypic ratio?
Explanation. For two independently assorting heterozygous genes, the classic dihybrid phenotypic ratio is 9:3:3:1 (9 both dominant : 3 dominant/recessive : 3 recessive/dominant : 1 both recessive). This arises from multiplying two 3:1 monohybrid ratios.
3. A color-blind man (X-linked recessive trait) marries a woman who is a carrier. What percentage of their SONS are expected to be color-blind?
Explanation. Father is XᶜY, mother is XᶜX. Sons receive Y from father and an X from mother. Mother contributes Xᶜ or X with equal probability, so half of sons (XᶜY) are color-blind. Note: 50% of all sons, not 50% of all children.
4. Two genes are located 12 map units apart on the same chromosome. In a test cross, what percentage of offspring is expected to be recombinant?
Explanation. Map units (centimorgans) equal recombination frequency as a percentage. 12 map units = 12% recombinant offspring, with the remaining 88% being parental types.
5. In a population in Hardy-Weinberg equilibrium, the frequency of the recessive phenotype is 0.16. What is the frequency of the dominant allele?
Explanation. q² = 0.16, so q = 0.4 (recessive allele). Since p + q = 1, p = 0.6. The dominant allele frequency is 0.6.
6. A woman with type AB blood has children with a man with type O blood. Which blood types are possible in their children?
Explanation. AB parent is IᴬIᴮ; O parent is ii. Children receive Iᴬ or Iᴮ from the mother and i from the father, giving Iᴬi (type A) or Iᴮi (type B). Neither AB nor O is possible.
7. Incomplete dominance is best illustrated by which of the following?
Explanation. Incomplete dominance produces an intermediate (blended) phenotype in heterozygotes, such as pink from red × white. Codominance (option B) shows both alleles fully and distinctly, as in AB blood type.
8. A trait that appears in every generation, affects males and females equally, and where affected individuals always have at least one affected parent, most likely shows what inheritance pattern?
Explanation. Appearing in every generation with affected individuals having affected parents (no skipping) and equal sex distribution indicates autosomal dominant inheritance. Recessive traits often skip generations; X-linked recessive disproportionately affects males.
9. During meiosis, nondisjunction in meiosis I results in:
Explanation. In meiosis I nondisjunction, homologous chromosomes fail to separate, so both products of meiosis I are abnormal. After meiosis II, two gametes have n+1 and two have n−1 chromosomes. Failure of sister chromatids describes meiosis II nondisjunction.
10. A cross between a black, short-haired animal and a brown, long-haired animal produces all black, short-haired offspring. This indicates that:
Explanation. If all F1 offspring show black and short, those traits are dominant (assuming homozygous parents). The masking of brown and long phenotypes in the F1 generation reveals which alleles are dominant.
11. In epistasis, the phenotypic ratio of a dihybrid cross may differ from 9:3:3:1 because:
Explanation. Epistasis occurs when one gene's product affects the expression of another gene at a different locus, altering the standard 9:3:3:1 ratio (e.g., to 9:3:4 or 9:7). It is a gene interaction, not linkage or blending.
12. A population of 500 individuals is in Hardy-Weinberg equilibrium with allele frequencies p = 0.7 and q = 0.3. How many individuals are expected to be heterozygous?
Explanation. Heterozygote frequency = 2pq = 2(0.7)(0.3) = 0.42. Multiply by 500: 0.42 × 500 = 210 individuals.
13. Which of the following is a condition that must be met for a population to remain in Hardy-Weinberg equilibrium?
Explanation. Hardy-Weinberg equilibrium requires random mating, no selection, no mutation, no gene flow (migration), and a very large population. Random mating is the only listed condition consistent with equilibrium; the others all disrupt it.
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FAQ
How much genetics is on the AP Biology exam?
Genetics (heredity, Mendelian and non-Mendelian inheritance, gene expression, and population genetics/Hardy-Weinberg) is a major unit, typically accounting for roughly 25-35% of the exam combined with molecular genetics. Expect both multiple-choice and free-response (often grid-in math) questions.
Do I need to memorize the Hardy-Weinberg equations?
The equations p + q = 1 and p² + 2pq + q² = 1 are provided on the AP Biology equation sheet, but you must know how to apply them. Practice identifying which term represents homozygous dominant (p²), heterozygous (2pq), and homozygous recessive (q²) genotypes, and remember q² equals the recessive phenotype frequency.
What's the best strategy for genetics problems on the exam?
Carefully define alleles and parental genotypes first, then use a Punnett square or probability multiplication for crosses. For pedigree problems, determine dominant vs. recessive and autosomal vs. sex-linked by checking generation patterns and sex ratios. Show your work on grid-in/FRQ math for partial credit.