Meiosis & Genetic Diversity (College Board AP® Biology): Exam Questions

Both mitosis and meiosis are forms of cell division that produce daughter cells containing genetic information from the parent cell.

Describe TWO events that are common to both mitosis and meiosis that ensure the resulting daughter cells inherit the appropriate number of chromosomes. 

The genetic composition of daughter cells produced by mitosis differs from that of the daughter cells produced by meiosis. Describe TWO features of the cell division processes that lead to these differences. 

Identify THREE ways that sexual reproduction increases genetic variability. For each, explain how it increases genetic diversity among the offspring. 

A research team has genetically engineered a strain of fruit flies to eliminate errors during DNA replication. The team claims that this will eliminate genetic variation in the engineered flies. A second research team claims that eliminating errors during DNA replication will not entirely eliminate genetic variation in the engineered flies.

Provide ONE piece of evidence that would indicate new genetic variation has occurred in the engineered flies.

Describe ONE mechanism that could lead to genetic variation in the engineered strain of flies.

During meiosis, double-strand breaks occur in chromatids. The breaks are either repaired by the exchange of genetic material between homologous nonsister chromatids, which is the process known as crossing over (Figure 1A), or they are simply repaired without any crossing over (Figure 1B). Plant breeders developing new varieties of corn are interested in determining whether, in corn, a correlation exists between the number of meiotic double-strand chromatid breaks and the number of crossovers.

Figure 1 Double-strand breaks in chromatids are repaired with crossing over (A) or without crossing over (B)

Using specialized staining and microscopy techniques, scientists counted the number of double-strand chromatid breaks and the number of crossovers in the same number of meiotic gamete-forming cells of six inbred strains of corn (Table 1).

TABLE 1. NUMBER OF CHROMATID DOUBLE-STRAND BREAKS AND AVERAGE NUMBER OF CROSSOVERS IN INBRED STRAINS OF CORN

The double-strand breaks occur along the DNA backbone. Describe the process by which the breaks occur.

Using the template in the space provided for your response, construct an appropriately labeled graph that represents the data in Table 1 and allows examination of a possible correlation between double-strand breaks and crossovers. Based on the data, determine whether corn strains I, II, and Ill differ in their average number of crossovers.

Based on the data, describe the relationship between the average number of double-strand breaks and the average number of crossovers in the strains of corn analyzed in the experiment.

Crossing over (Figure 1A) creates physical connections that are required for proper separation of homologous chromosomes during meiosis. A diploid cell with four pairs of homologous chromosomes undergoes meiosis to produce four haploid cells. Crossing over occurs between only three of the pairs. Predict the number of chromosomes most likely present in each of the four haploid cells. Provide reasoning to justify your prediction. Explain how plant breeders can use the information in Table 1 to help develop new varieties of corn.

In the flowering plant Arabidopsis thaliana, the purple flower color trait (P) is dominant over white (p), and tall height (T) is dominant over short (t). A plant that is heterozygous for both traits was self-fertilized.

The resulting seeds were grown and analyzed, and the offspring phenotypes are shown in Table 1.

Table 1. Distribution of offspring phenotypes from a self-cross of the parent plants

(i) Describe the process of allele separation during meiosis that leads to gamete formation.

(ii) Explain how independent assortment contributes to genetic diversity in this cross.

Construct a genetic diagram of the self-cross described in part (a)

(i) Analyze the data in Table 1 to determine whether the observed results are consistent with independent assortment.

(ii) Identify one biological reason why observed phenotypic ratios may differ from predicted ratios.

The genes P and T are located on different chromosomes.

(i) Explain how the principle of independent assortment applies to these genes during meiosis.

(ii) Predict how the results might differ if the two genes were located close together on the same chromosome.

(iii) Explain your prediction using a biological mechanism.

Researchers studied the effect of a mutation on the synaptonemal complex in a population of mice. The synaptonemal complex is a multi-protein structure required for alignment and crossing over of homologous chromosomes during prophase I of meiosis. The different alleles are notated as follows:

• Wild-type alleles (functional synaptonemal complex) = SC⁺

• Mutated alleles (non-functional synaptonemal complex) = SC⁻

Researchers bred heterozygous mice (SC⁺/SC⁻) and collected data on meiotic outcomes in the offspring.

(i) During meiosis I homologous chromosomes pair and physically exchange segments of DNA. Describe the importance of these events for proper gamete formation.

(ii) Explain how this process increases genetic diversity in offspring.

Analyze the data in Figure 1 to determine the relationship between SC genotype and crossover frequency.

Researchers wanted to determine whether changes to the frequency of crossing over in SC⁻/SC⁻ individuals affected the accuracy of chromosome segregation.

(i) Describe an appropriate control group in an experiment investigating the effect of the SC⁻/SC⁻ genotype on accuracy of chromosome segregation.

(ii) Identify the dependent variable in the experiment described in (i).

(iii) Justify why this variable is relevant for evaluating chromosome separation.

(i) Describe a chromosomal abnormality that is likely to occur in SC⁻/SC⁻ gametes.

(ii) Explain how such an abnormality could be caused by the SC⁻ mutation.

Nondisjunction during meiosis can result in gametes with abnormal numbers of chromosomes. Trisomy 21 is a genetic condition that arises when an individual inherits three copies of chromosome 21.

(i) Describe the process of chromosome separation during meiosis I.

(ii) Explain how errors in this process can lead to trisomy 21.

Researchers investigated how maternal age affects the frequency of trisomy 21 using karyotype analysis.

The following data shows the percentage of offspring with trisomy 21 in mothers of different ages.

Table 1. Frequency of trisomy 21 based on maternal age

(i) Use the template to construct an appropriately labeled graph to illustrate the effect of maternal age on the frequency of trisomy 21 in offspring.

(iii) Use the graph to identify the maternal age at which the risk of trisomy 21 reaches 4 %.

(i) Analyze the data presented in Table 1 to determine the relationship between maternal age and the frequency of trisomy 21.

(ii) Explain how the incidence of trisomy 21 in the human population might change if the average maternal age increases in future generations.

A researcher investigated whether errors in chromosome separation occur more often during meiosis I than meiosis II. The researcher uses a model organism to express fluorescence-tagged chromosomes to visualize segregation errors.

(i) State a null hypothesis for the experiment.

(ii) Identify the independent and dependent variables.

A researcher studied the inheritance of two linked genes in the fruit fly Drosophila melanogaster. The genes are known to affect wing and body shape in flies.

The researcher crossed flies that were heterozygous for the genes b+ and vg+ and analyzed the phenotypes of 1000 offspring. The expected outcome with normal recombination is a 1:1:1:1 ratio.

Table 1. The number of offspring with different genotypes and phenotypes from a heterozygous cross.

(i) Describe the process of crossing over during meiosis. Include the timing of this event.

(ii) Explain how crossing over can result in the observed phenotypes in Table 1.

(i) Calculate the percentage of offspring that displays trait combinations different from either parent (recombinant frequency).

(ii) Explain what this result suggests about the process of crossing over during meiosis in this organism.

The researcher compared the observed phenotypic values in Table 1 to the expected 1:1:1:1 ratio using a chi-square test.

(i) Calculate the chi-square value for the data in Table 1 using the formula:

χ² = Σ((O−E)² / E)

(ii) State the null hypothesis for this test.

(iii) The critical value, at df = 3 and p = 0.05, is 7.815. Justify whether the null hypothesis should be accepted or rejected.

(iv) Explain what the result of this test indicate about the two genes in this cross.

Scientists discovered a deletion mutation in a meiosis-specific gene that reduces crossing over between homologous chromosomes.

(i) Predict how this mutation would affect the offspring ratios observed in part (a).

(ii) Explain how this change could affect genetic diversity in the population.