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AP Biology : 5.2 Meiosis and Genetic Diversity – Exam Style questions with Answer- MCQ

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Question

R. C. Punnett conducted experiments on the inheritance of traits in the sweet pea, Lathyrus odoratus. In one experiment, he crossed two different true-breeding sweet pea plant strains, one with erect petals and long pollen, and the other with hooded petals and round pollen. All the offspring (\(F_1\) generation) had erect petals and long pollen (Figure 1).

The figure presents the crossing of two different strains of sweet pea plants. In the Parental Generation, a strain with Erect Petals and Long Pollen is crossed with a strain with Hooded Petals and Round Pollen. The cross produces an F 1 generation with Erect Petals and Long Pollen.
Next, Punnett allowed the \(F_1\) generation to self-fertilize and recorded the phenotypes of their offspring. The data are shown in T
Which of the following questions would be most useful to researchers trying to determine the role of meiosis in the \(F_2\) phenotypic frequencies?
A. What is the molecular mechanism underlying the dominance of erect petals and long pollen?
B. Which phenotypes give pea plants the highest level of fitness: erect or hooded petals and long or round pollen?
C. How do the phases of meiosis differ between sweet pea plants and other organisms?
D. What is the recombination frequency between the genes for petal shape and pollen shape?
▶️Answer/Explanation

Ans: D
The \(F_2\) generation in this experiment showed a greater-than-expected frequency of phenotype combinations from the parental generation. The erect petals, round pollen, and hooded petals, long pollen phenotype combinations are the result of crossing over of homologous chromosomes during meiosis I. The recombination frequency in this experiment was \(\frac{(62+71)}{1118}=12%\) The closer genes are to each other, the lower the chance of them being separated from each other by crossing over and the lower the recombination frequency.

Question

In anaphase I of meiosis, cohesion between the centromeres of sister chromatids is maintained while homologous chromosomes migrate to opposite poles of the cell along the meiotic spindle as represented in Figure 1.

The figure presents the migration of the members of two pairs of homologous chromosomes away from each other during anaphase 1 of meiosis. The centromeres of each chromosome are pointed toward the poles of the cell, while the arms of the chromosomes follow along behind. Differential shading indicates that crossing over occurred between chromatids of each pair of homologous chromosomes.

Figure 1. Migration of homologous chromosomes during anaphase I of meiosis

A compound that prevents the separation of the homologous chromosomes in anaphase I is being studied. Which of the following questions can be best answered during this study?

A. Will the cells produced at the end of meiosis still be genetically identical to each other in the presence of this compound?

B. Will the long-term development of the individual be affected by this meiotic error?

C. When do the centrosomes start to move apart during meiosis I as compared to meiosis II?

D. Is there a pattern to the movement of homologous chromosomes in the presence of this compound?

▶️Answer/Explanation

Ans: D
The compound prevents separation of homologous chromosomes, meaning that both the maternal and paternal chromosomes will migrate along the spindle fibers together. Patterns and other aspects of the movement of these homologous chromosomes in the presence of this compound could be recognized effectively during this study.

Question

A model of crossing over during gamete formation is shown in Figure 1.

The figure presents one pair of homologous chromosomes, with each chromosome in the pair represented as two chromatids. The chromatids of one chromosome are unshaded, and the chromatids of the other chromosome are shaded. There are three parts to the figure. In the first part, the chromosomes are straight and parallel to each other. In the second part of the figure, the bottom portion of one unshaded chromatid is crossed over the bottom portion of one shaded chromatid. In the third part of the figure, there are four individual chromosomes, one that is unshaded, one with the top three fourths unshaded and the bottom one fourth shaded, one with the top three fourths shaded and the bottom one fourth unshaded, and one that is all shaded.

Figure 1. Model of crossing over during meiosis

Based on Figure 1, which of the following questions could best be addressed?

A. Does synapsis of homologous chromosomes in the parent cell contribute to an increase in genetic diversity in the daughter cells?

B. Do sister chromatids separate and form diploid daughter cells?

C. Do chromatids from nonhomologous chromosomes rearrange to produce identical daughter cells?

D. Does synapsis of nonhomologous chromosomes produce daughter cells that are identical to the parent cell?

▶️Answer/Explanation

Ans: A
Based on the model, synapsis of homologous chromosomes allows the opportunity for crossing over to occur, which can result in chromosomal rearrangement and increased genetic variation in the daughter cells.

Question

The figure presents a model of crossing over between a pair of homologous chromosomes and represents the chromosomes before and after crossing over has occurred. Each chromosome is represented as a pair of chromatids, with uppercase or lowercase letters representing genetic loci on the chromatids. In one chromosome of the homologous pair, the loci are all represented as uppercase letters in the following order: L, M, N, O, P, and Q above the centromere and R, S, T, U, V, and W below the centromere. In the other chromosome of the homologous pair, the loci are all represented as lowercase letters in the following order: l, m, n, o, p, and q above the centromere and r, s, t, u, v, and w below the centromere. One of the chromatids with lowercase letters is shown overlapping a chromatid with uppercase letters at an approximate position immediately below uppercase T and lowercase t. A note indicates that “crossing between loci s and u will happen only during 2% of meiotic divisions” and a second note indicates that “crossing between loci r and v will happen only during 40% of meiotic divisions.” In the second part of the figure, the pair of homologous chromosomes is again represented, with no change to the order or case of the loci above the centromeres of the chromatids. Below the centromeres, one chromatid has loci that are all uppercase, R, S, T, U, V, and W; the next chromatid has uppercase R, S, and T loci, then lowercase u, v, and w loci; the next chromatid has lowercase r, s, and t loci, then uppercase U, V, and W loci; the fourth chromatid has loci that are all lowercase, r, s, t, u, v, and w.

Figure 1. Model of crossing over between homologous chromosomes, indicating crossing over rate of selected loci.

During prophase

I

replicated homologous chromosomes pair up and undergo synapsis. What testable question is generated regarding synapsis and genetic variability by Figure 1 ?

A. Is the distance between two gene loci related to crossover rate?

B. Does crossing over occur more often in some chromosomes than in others?

C. Is crossing over inhibited by methylation?

D. Is crossing over promoted by methylation?

▶️Answer/Explanation

Ans: A
During meiosis I, homologous, non-sister chromatids exchange genetic material via a process called crossing over (recombination). The likelihood that a recombination event will separate two genes on the same chromosome increases as the distance between the two gene loci increases.

Question

Scientists have found that DNA methylation suppresses crossing-over in the fungus Ascobolus immersus. Which of the following questions is most appropriately raised by this specific observation?

A. Is the level of genetic variation in the gametes related to the amount of DNA methylation observed?

B. Without crossing-over, will gametes be viable and be able to produce zygotes?

C. Does DNA methylation result in shorter chromosomes?

D. Is this species of fungus a diploid organism?

▶️Answer/Explanation

Ans: A
DNA methylation is shown to suppress crossing-over, which is an important source of genetic variation. Thus, it is reasonable to investigate how the level of genetic variation in gametes being produced is related to how much DNA has been methylated.

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