AP Biology: Heredity Practice Questions
Test yourself on AP Biology: Heredity with 12 original AP Exams practice questions. Pick an answer to see instant feedback and a full explanation.
Free original practice questions for study purposes. Open Exam Prep is an independent study resource and is not affiliated with, endorsed by, or sponsored by the makers of AP Exams.
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1. A diploid organism has genotype AaBb, and the two genes assort independently. Which gamete genotype is possible after meiosis?
Explanation. Gametes are haploid, so they receive one allele for each gene. Independent assortment can produce AB, Ab, aB, or ab gametes from an AaBb parent. Aa and AaBb are diploid combinations, not single gametes.
2. In a monohybrid cross between two heterozygotes, Aa x Aa, what fraction of offspring are expected to show the recessive phenotype if A is completely dominant?
Explanation. The genotypes from Aa x Aa are expected in a 1 AA : 2 Aa : 1 aa ratio. Only aa shows the recessive phenotype under complete dominance, so the expected fraction is 1/4.
3. Which event during meiosis most directly explains Mendel's law of segregation?
Explanation. Alleles of a gene usually reside on homologous chromosomes. When homologous chromosomes separate during meiosis I, the two alleles segregate into different gametes, matching Mendel's law of segregation.
4. Two genes are located very close together on the same chromosome. Which result is most likely?
Explanation. Genes close together on the same chromosome are linked. Crossing over can still separate them, but close genes recombine less often, so they are inherited together more frequently than independently assorting genes.
5. A testcross between an individual with a dominant phenotype and a homozygous recessive individual produces both dominant and recessive offspring. What can be inferred about the dominant-phenotype parent?
Explanation. A homozygous recessive tester contributes only recessive alleles. If recessive offspring appear, the dominant-phenotype parent must have contributed a recessive allele, so it is heterozygous.
6. In a pedigree, a trait appears in every generation and affected individuals usually have an affected parent. Both males and females are affected at similar rates. Which inheritance pattern is most consistent with this description?
Explanation. Autosomal dominant traits often appear in each generation because one dominant allele can produce the phenotype. Similar rates in males and females point away from sex-linked patterns. Recessive traits can skip generations.
7. A couple are both carriers for an autosomal recessive condition. What is the probability that their child will be affected?
Explanation. Two carriers have genotype Aa x Aa. The expected offspring genotypes are AA, Aa, Aa, and aa. Only aa is affected by an autosomal recessive condition, so the probability is 1 out of 4, or 25%.
8. A dihybrid cross AaBb x AaBb produces phenotypes in a ratio close to 9:3:3:1. Which assumption best supports this expected ratio?
Explanation. The classic 9:3:3:1 phenotypic ratio depends on two heterozygous genes that assort independently and show complete dominance. Complete linkage, maternal-only inheritance, or polygenic inheritance would produce different patterns.
9. A chi-square test is used to compare observed offspring counts with an expected genetic ratio. What does a high p-value generally indicate?
Explanation. A high p-value means the differences between observed and expected counts are not unusually large under the model being tested. It does not prove the model, but it suggests the data do not strongly reject the expected ratio.
10. Which situation best illustrates incomplete dominance?
Explanation. Incomplete dominance occurs when the heterozygous phenotype is intermediate between the two homozygous phenotypes. Both alleles fully visible at the same time is codominance, not incomplete dominance.
11. Why can crossing over increase genetic variation among offspring?
Explanation. Crossing over swaps corresponding DNA segments between homologous chromosomes during prophase I. This recombination creates chromosomes with new combinations of maternal and paternal alleles.
12. A female carrier for an X-linked recessive trait has children with an unaffected male. What is the expected probability that a son will show the trait?
Explanation. A son receives his X chromosome from his mother and his Y chromosome from his father. If the mother is a carrier, each son has a 1/2 chance of receiving the recessive allele on her X chromosome and showing the trait.
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FAQ
What heredity ideas are most important for AP Biology?
Know meiosis, segregation, independent assortment, linked genes, crossing over, common inheritance patterns, probability, pedigrees, and how chi-square tests evaluate expected genetic ratios.
How should I approach AP Biology genetics problems?
Start by identifying the inheritance pattern and the parent genotypes. Then track gametes, build a Punnett square or probability setup, and check whether the question asks for genotype, phenotype, or a statistical conclusion.
What mistakes should I avoid on heredity questions?
Do not confuse genotype ratios with phenotype ratios, and do not assume every two-gene cross gives 9:3:3:1. Linkage, sex-linked inheritance, incomplete dominance, codominance, and small sample sizes can change the pattern.