AP Biology Unit 5: Heredity MCQs – 30 Practice Questions with Answers (College Board CED)
Free AP Biology practice for Unit 5: Heredity, covering meiosis, Mendelian and non-Mendelian genetics, linkage, pedigrees, and chi-square analysis, with a short explanation after every question.
Meiosis and Chromosome Behaviour
Q1. A diploid organism has 2n = 6. After meiosis I, each daughter cell contains how many chromosomes, and in what state?
- (A) 3 chromosomes, each made of one chromatid
- (B) 6 chromosomes, each made of one chromatid
- (C) 3 chromosomes, each made of two chromatids
- (D) 6 chromosomes, each made of two chromatids
Show Answer
Answer: C – Meiosis I separates homologous chromosomes, so chromosome number halves, but each chromosome still consists of two sister chromatids until meiosis II.
Q2. Crossing over between non-sister chromatids of homologous chromosomes takes place during which stage?
- (A) Prophase I
- (B) Anaphase II
- (C) Prophase of mitosis
- (D) Metaphase II
Show Answer
Answer: A – Crossing over occurs in prophase I, when homologous chromosomes pair up and exchange segments.
Q3. A researcher labels the two copies of chromosome 3 in different colours. In metaphase I, the two copies line up independently of how chromosome 5 lines up. Which principle does this illustrate?
- (A) Segregation only
- (B) Independent assortment
- (C) Codominance
- (D) Linkage
Show Answer
Answer: B – Random alignment of different homologous pairs at metaphase I produces independent assortment of the genes they carry.
Q4. Which combination of events most increases genetic variation among offspring of sexually reproducing organisms?
- (A) Crossing over, independent assortment, and random fertilisation
- (B) Mitosis and binary fission
- (C) Chromosome duplication without division
- (D) DNA replication and cytokinesis only
Show Answer
Answer: A – Crossing over reshuffles alleles, independent assortment mixes chromosome combinations, and random fertilisation combines gametes in new ways.
Q5. How do the daughter cells of meiosis differ from those of mitosis in the same organism?
- (A) Both produce two varied haploid cells
- (B) Meiosis produces genetically identical diploid cells; mitosis produces varied haploid cells
- (C) Meiosis produces genetically varied haploid cells; mitosis produces identical diploid cells
- (D) Both produce four identical diploid cells
Show Answer
Answer: C – Meiosis halves the chromosome number and shuffles alleles, while mitosis copies the genome faithfully into two diploid cells.
Q6. Nondisjunction of a homologous pair during anaphase I produces which set of gametes?
- (A) One gamete with an extra chromosome, one missing it, and two normal gametes
- (B) Four gametes each missing one chromosome
- (C) Four normal haploid gametes
- (D) Two gametes with an extra chromosome and two missing that chromosome
Show Answer
Answer: D – If homologues fail to separate in meiosis I, both go to one pole, giving two gametes with an extra chromosome and two lacking it.
Q7. In a species, gene A and gene B lie on different chromosomes. A cross between two AaBb individuals is made. What fraction of offspring is expected to be aabb?
- (A) 1/8
- (B) 1/16
- (C) 9/16
- (D) 1/4
Show Answer
Answer: B – Each parent produces ab gametes with probability 1/4, so aabb offspring occur with probability 1/4 x 1/4 = 1/16.
Q8. A plant showing a dominant trait is crossed with a homozygous recessive plant, and half the offspring show the recessive trait. What was the genotype of the dominant parent?
- (A) Homozygous recessive
- (B) Cannot be determined from this cross
- (C) Homozygous dominant
- (D) Heterozygous
Show Answer
Answer: D – This is a test cross: a 1:1 ratio from a cross with a recessive parent shows the dominant parent carried one recessive allele.
Mendelian Genetics and Probability
Q9. Mendel’s law of segregation states that the two alleles of a gene do what during gamete formation?
- (A) Exchange places with alleles of other genes
- (B) Fuse together permanently
- (C) Separate so each gamete receives one allele
- (D) Move to the same pole together
Show Answer
Answer: C – Segregation means the two alleles of a gene separate into different gametes during meiosis.
Q10. A pedigree shows two unaffected parents with an affected child. Assuming a single-gene trait, which inheritance pattern fits best?
- (A) Autosomal dominant
- (B) X-linked dominant
- (C) Autosomal recessive
- (D) Y-linked
Show Answer
Answer: C – Unaffected parents can both be carriers of a recessive allele and pass it to an affected child; a dominant trait would normally appear in a parent.
Q11. A woman who is a carrier for an X-linked recessive condition has children with an unaffected man. Which child is most likely to show the condition?
- (A) Their daughters, because they receive two X chromosomes
- (B) Their sons, because a son receives his only X chromosome from his mother
- (C) Their sons, because fathers pass the condition to sons
- (D) No child, because the mother is unaffected
Show Answer
Answer: B – A son inherits his single X chromosome from his mother, so a carrier mother has a 1 in 2 chance of passing the affected allele to each son.
Q12. In snapdragons, flower colour shows incomplete dominance: red (RR) crossed with white (WW) gives pink (RW) offspring. What phenotypic ratio is expected in the F2 generation from self-pollinating pink plants?
- (A) All pink
- (B) 1 red : 1 white
- (C) 3 red : 1 white
- (D) 1 red : 2 pink : 1 white
Show Answer
Answer: D – The F2 genotypes are 1 RR : 2 RW : 1 WW, and because heterozygotes are intermediate, the phenotypes follow the same 1:2:1 ratio.
Q13. A person with blood group AB carries both the IA and IB alleles. Which pattern does this illustrate?
- (A) Incomplete dominance
- (B) Codominance
- (C) Polygenic inheritance
- (D) Epistasis
Show Answer
Answer: B – In codominance both alleles are fully expressed in the heterozygote, so both A and B antigens appear.
Q14. Two genes assort independently. A parent of genotype AaBb is crossed with aabb. What fraction of offspring is expected to show both recessive traits?
- (A) 1/2
- (B) 1/4
- (C) 1/8
- (D) 3/4
Show Answer
Answer: B – The AaBb parent makes ab gametes with probability 1/4, and the aabb parent always contributes ab, giving 1/4 aabb offspring.
Q15. In human ABO blood groups, which genotype gives blood group O?
- (A) ii
- (B) IAIA
- (C) IAi
- (D) IBIB
Show Answer
Answer: A – Group O results only when neither the IA nor the IB allele is present, so the genotype is ii.
Q16. A dihybrid cross between two AaBb individuals gives offspring in which 9 show both dominant traits, 3 show one dominant trait, 3 show the other, and 1 shows both recessive traits. What does the 9:3:3:1 pattern support?
- (A) Both genes are sex-linked
- (B) The two genes are linked on one chromosome
- (C) The two genes assort independently
- (D) One gene masks the other
Show Answer
Answer: C – A 9:3:3:1 phenotypic ratio is the classic result of independent assortment of two heterozygous gene pairs.
Non-Mendelian Patterns, Linkage and Pedigrees
Q17. Human height varies continuously across a population, with most people near the middle of the range. Which inheritance pattern best explains this?
- (A) A single dominant allele
- (B) A single recessive allele
- (C) Polygenic inheritance with environmental influence
- (D) Y-linked inheritance
Show Answer
Answer: C – Continuous traits such as height are influenced by many genes acting together, plus environmental factors, producing a bell-shaped distribution.
Q18. A single gene affects both red blood cell shape and resistance to a particular infection in the same individuals. Which term describes this?
- (A) Codominance
- (B) Linkage
- (C) Polygeny
- (D) Pleiotropy
Show Answer
Answer: D – Pleiotropy occurs when one gene influences more than one phenotypic trait.
Q19. In a cross, the expected 9:3:3:1 dihybrid ratio is altered because one gene prevents expression of another gene. Which phenomenon is this?
- (A) Incomplete dominance
- (B) Epistasis
- (C) Independent assortment
- (D) Sex linkage
Show Answer
Answer: B – In epistasis, the allele of one gene masks or modifies the expression of a different gene.
Q20. A researcher counts offspring from a cross involving two genes on the same chromosome: 180 and 180 of the parental types, 20 and 20 of the recombinant types. What is the recombinant frequency?
- (A) 5 percent
- (B) 10 percent
- (C) 20 percent
- (D) 50 percent
Show Answer
Answer: B – Recombinant offspring total 40 out of 400, giving 40/400 = 10 percent; recombinant frequency counts both recombinant classes together.
Q21. An affected father has an affected daughter, and the trait appears in every generation of the pedigree. Which pattern is most consistent with this?
- (A) Mitochondrial only
- (B) Y-linked
- (C) Autosomal recessive
- (D) Autosomal dominant
Show Answer
Answer: D – A dominant trait typically appears in every generation and is passed by an affected parent to offspring, unlike a recessive trait that can skip generations.
Q22. Two heterozygous parents (Aa x Aa) have a child. What is the probability that the child is heterozygous Aa?
- (A) 3/4
- (B) 1
- (C) 1/4
- (D) 1/2
Show Answer
Answer: D – The Punnett square for Aa x Aa gives genotypes 1 AA : 2 Aa : 1 aa, so the probability of Aa is 2/4 = 1/2.
Q23. A gene for flower colour and a gene for seed shape lie very close together on the same chromosome. What is expected in a test cross?
- (A) A 3:1 ratio
- (B) A 1:1:1:1 ratio of all combinations
- (C) Mostly parental combinations, with few recombinants
- (D) Only recombinant combinations
Show Answer
Answer: C – Linked genes tend to be inherited together, so parental combinations predominate and recombinants are rare, with frequency rising as distance between genes increases.
Environment, Errors and Testing Hypotheses
Q24. Identical seedlings grown in bright light grow tall and green, while genetically identical seedlings grown in deep shade grow pale and spindly. What does this best show?
- (A) Phenotype results from genotype interacting with environment
- (B) Mutations occurred in the shade plants
- (C) Light changes the DNA sequence
- (D) The shade plants have different genes
Show Answer
Answer: A – Because the genotypes are identical, the phenotypic difference must come from environmental influence on gene expression and growth.
Q25. A karyotype shows three copies of chromosome 21 in a child’s cells. Which error most likely produced this?
- (A) A failure of DNA replication in mitosis
- (B) A point mutation in a single gene
- (C) Nondisjunction during gamete formation
- (D) Crossing over between sister chromatids
Show Answer
Answer: C – An extra whole chromosome arises when chromosomes fail to separate during meiosis, producing a gamete with an extra copy.
Q26. In humans, which parent determines the sex of a child, and why?
- (A) The father, because sperm carry either an X or a Y chromosome
- (B) Either parent equally, because both carry X and Y
- (C) Neither, because sex is set by temperature
- (D) The mother, because she contributes the larger gamete
Show Answer
Answer: A – Human females are XX and males are XY, so it is the sperm’s sex chromosome that determines whether the child is XX or XY.
Q27. A student crosses two heterozygous plants and counts 84 round seeds and 36 wrinkled seeds, expecting a 3:1 ratio (90 round : 30 wrinkled). The chi-square value is 1.6 with 1 degree of freedom, where the critical value at p = 0.05 is 3.84. What is the correct conclusion?
- (A) Fail to reject the null hypothesis, because 1.6 is less than 3.84
- (B) Reject the null hypothesis, because the counts differ from expected
- (C) Accept the null hypothesis as proven
- (D) Reject the null hypothesis, because 1.6 is less than 3.84
Show Answer
Answer: A – A chi-square value below the critical value means the deviation from expected is not statistically significant, so the null hypothesis is not rejected; this does not prove it true.
Q28. Two populations of the same species are separated by a new mountain range for many generations. If they later cannot produce fertile offspring together, which process has most likely occurred?
- (A) Speciation by geographic isolation
- (B) Independent assortment
- (C) Genetic drift within one generation
- (D) Artificial selection
Show Answer
Answer: A – Long-term geographic separation can allow populations to diverge until reproductive isolation develops, which is the basis of allopatric speciation.
Q29. A plant breeder crosses two AaBb plants and asks for the probability of an AaBb offspring. What is the correct value?
- (A) 1/8
- (B) 1/4
- (C) 9/16
- (D) 1/16
Show Answer
Answer: B – The probability of Aa from Aa x Aa is 1/2, and likewise for Bb, so AaBb occurs with probability 1/2 x 1/2 = 1/4.
Q30. A researcher repeats a genetics cross three times and pools the data before running a chi-square test. Why is pooling the repeats justified here?
- (A) It guarantees the expected ratio
- (B) It changes the degrees of freedom to zero
- (C) It removes the need for a null hypothesis
- (D) It increases sample size and reduces the effect of chance variation
Show Answer
Answer: D – Larger pooled samples give a more reliable test, because random fluctuations have less influence on the overall counts.
Work through any questions you missed again, then revise those areas before moving on to gene expression and regulation.