AP Biology: Meiosis and Genetic Variation Practice Questions
Test yourself on AP Biology: Meiosis and Genetic Variation with 12 original AP Exams practice questions. Pick an answer to see instant feedback and a full explanation.
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1. A diploid organism carries two homologs of chromosome 3, one inherited from each parent. Before DNA replication, which statement best describes these two homologs?
Explanation. Homologous chromosomes carry corresponding genes at corresponding loci, but the maternal and paternal copies can contain different alleles. Sister chromatids are the replicated copies of a single chromosome formed during S phase; they are not the two parental homologs before replication.
2. A diploid germ cell completes DNA replication and enters meiosis I. During normal anaphase I, which structures separate toward opposite poles?
Explanation. Meiosis I separates the members of each homologous pair. Sister chromatids normally remain associated through this first division. Their separation occurs during anaphase II, so meiosis I reduces the number of homologous chromosome sets without first separating every sister-chromatid pair.
3. After a normal meiosis I, a cell begins meiosis II without another S phase. What event normally occurs during anaphase II?
Explanation. During anaphase II, sister chromatids separate and each becomes an individual chromosome. Homologous pairs were separated during meiosis I. There is normally no intervening round of DNA replication, allowing the two divisions to produce haploid products from a replicated diploid starting cell.
4. A species has a diploid chromosome number of 12. Count chromosomes by centromeres. Immediately after normal meiosis I and cytokinesis, how many chromosomes and chromatids are present in each daughter cell?
Explanation. The diploid number 12 means there are six homologous pairs. Meiosis I puts one chromosome from each pair into each daughter cell. Each of these six chromosomes still consists of two sister chromatids, giving six chromosomes and twelve chromatids per cell after cytokinesis.
5. During normal prophase I, a reciprocal crossover occurs between a maternal homolog and its paternal counterpart. Which participants exchange corresponding DNA segments?
Explanation. A meiotic crossover exchanges corresponding DNA regions between nonsister chromatids of homologous chromosomes. This can join alleles of maternal and paternal origin on one chromatid. The exchange rearranges existing allele combinations; it does not require a new allele to be created by mutation.
6. A diploid cell has four homologous chromosome pairs. Ignoring crossing over and mutation, how many different combinations of maternal versus paternal homologs can independent assortment generate across many gametes?
Explanation. Each of the four homologous pairs contributes one of two parental homologs to a gamete. Independent assortment therefore permits 2 raised to the fourth power, or 16 combinations, across many meiotic events. This is a count of possible combinations, not the number of products from one meiosis.
7. A diploid individual has genotype AaBb, with the A and B loci on different chromosome pairs. Assume normal segregation, independent assortment, and no selection among gametes. What fraction of a large gamete sample is expected to carry Ab?
Explanation. A gamete has a probability of one half of receiving A and one half of receiving b. Because the loci are on different chromosome pairs and assort independently under the stated assumptions, the joint probability is one half times one half, or one quarter.
8. An individual has linked alleles arranged as AB on one homolog and ab on the other. If exactly one crossover occurs between these loci and involves two of the four chromatids, which allele combinations are present among the four chromatids afterward?
Explanation. The two chromatids participating in the crossover become recombinant Ab and aB chromatids. The other two remain parental AB and ab chromatids. Thus a single such crossover produces two recombinant and two parental chromatids, rather than making all four chromatids recombinant.
9. Before premeiotic S phase, a diploid cell has DNA content 2C, where C is the DNA amount in one unreplicated haploid genome. What DNA content is expected in each product after normal meiosis II and cytokinesis?
Explanation. Premeiotic S phase increases the starting cell's DNA content from 2C to 4C. Meiosis I partitions this into cells with 2C each, and meiosis II partitions each again to give 1C per product. There is no second S phase between the two meiotic divisions in this normal sequence.
10. In a model meiotic cell, both homologs of one chromosome pair move to the same pole during meiosis I. All other pairs segregate normally, and meiosis II is normal. Relative to the normal haploid number n, what chromosome counts occur among the four final products?
Explanation. Nondisjunction in meiosis I sends both homologs to one daughter and neither to the other. Normal meiosis II then produces two products with an extra chromosome and two missing that chromosome. The pattern with two normal products instead fits a single meiosis II nondisjunction event.
11. Which observation specifically illustrates genetic variation produced by crossing over, rather than by independent assortment of whole homologs?
Explanation. Crossing over can create a chromosome that combines alleles from both parental homologs along its length. Independent assortment concerns which homolog from each different pair enters a gamete. Receiving a mix of intact maternal and paternal chromosomes does not by itself demonstrate a crossover.
12. In a model species, a testcross of an AB/ab individual with an ab/ab individual produces 420 AB, 430 ab, 75 Ab, and 75 aB offspring. Labels identify the gamete from the heterozygous parent. Assuming equal viability and reliable scoring, what is the observed recombination frequency?
Explanation. The parental combinations are AB and ab, while Ab and aB are recombinant. There are 75 plus 75, or 150 recombinant offspring among 1000 total, so the observed frequency is 15%. This measures recombinant gametes in the sample, not the percentage of meiotic cells in which any crossover occurred.
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FAQ
What separates in meiosis I and meiosis II?
Homologous chromosomes separate in meiosis I while sister chromatids remain associated. Sister chromatids separate in meiosis II. DNA normally replicates once before meiosis I, with no additional S phase between the two divisions.
How do crossing over and independent assortment differ?
Crossing over exchanges corresponding DNA segments between nonsister chromatids of homologous chromosomes and can create new allele combinations within a chromosome. Independent assortment distributes maternal and paternal homologs from different pairs in different combinations across gametes.
Does a haploid cell always have unreplicated chromosomes?
No. A cell after normal meiosis I has one homolog from each pair and is haploid, but its chromosomes still have two sister chromatids. After meiosis II, each product normally has one chromatid per chromosome. Ploidy counts chromosome sets, while DNA content also reflects replication.