AP Biology: Unit 5, Meiosis & Genetic Diversity (Drill 17)

Question 1. Based on the passage, which of the following best explains why Genes P and Q are less likely to assort independently compared to two genes located on different chromosomes?

• A) Genes P and Q are both located on chromosome 1, so they are physically linked and tend to be inherited together unless separated by crossing over. ✓
• B) Genes P and Q cannot undergo crossing over at all because they are located on the very same chromosome rather than on homologous pairs in the scenario described.
• C) Independent assortment only applies to genes located on sex chromosomes, not autosomes.
• D) Genes near the centromere are always dominant over genes near the telomere, reducing variation.

Explanation: Independent assortment applies to genes on different (non-homologous) chromosomes, which orient randomly during metaphase I. Genes on the same chromosome are physically linked and tend to move together during meiosis I. They can be separated by crossing over, but the probability of separation decreases as physical proximity increases. B is incorrect -- crossing over can and does occur between linked genes, particularly those far apart on the same chromosome. C incorrectly restricts independent assortment to sex chromosomes. D introduces a dominance concept that has no bearing on chromosome movement.

Question 2. The researcher notes that Gene Q, located near the telomere of chromosome 1, shows a higher recombination frequency with Gene P than two other genes located adjacent to each other near the centromere. Which of the following best explains this observation?

• A) Telomeric regions of the chromosome replicate faster than centromeric regions, and this faster replication increases the chance of a crossover within the parameters described here.
• B) Genes located farther apart on the same chromosome are more likely to have a crossover event occur between them, increasing the probability of recombination. ✓
• C) Crossing over preferentially occurs at centromeres, so genes near the centromere recombine more frequently than genes near telomeres.
• D) Recombination frequency is determined by the number of alleles at each locus, not by physical distance.

Explanation: Recombination frequency is used as a measure of physical distance between genes on the same chromosome. The greater the physical distance between two loci, the more likely a crossover event is to occur between them during prophase I, increasing the probability that the alleles are separated. Gene P (centromere) and Gene Q (telomere) are far apart on chromosome 1, so recombination between them is more probable than between two closely spaced genes. A incorrectly invokes replication rate. C has the relationship reversed -- crossing over is actually suppressed near centromeres. D is incorrect; recombination frequency is a function of distance, not allele number.

Question 3. For the organism described in the scenario (2n = 8), how many possible chromosome combinations can be produced in the gametes through independent assortment alone?

• A) 8
• B) 16 ✓
• C) 32
• D) 64

Explanation: The passage states that for an organism with n pairs of homologous chromosomes, independent assortment produces 2^n possible combinations of homologous chromosomes in gametes. With 2n = 8, there are n = 4 homologous chromosome pairs, so 2^4 = 16 possible combinations. This calculation does not account for crossing over, which would increase diversity further. A (8) would correspond to n = 3. C (32) would correspond to n = 5. D (64) would correspond to n = 6.

Question 4. A student argues that crossing over and independent assortment are redundant mechanisms because both produce genetic variation in gametes. Which of the following best identifies the flaw in this argument?

• A) The argument is correct -- crossing over and independent assortment produce identical types of genetic variation, so just one of the two mechanisms is sufficient.
• B) The argument is flawed because independent assortment only occurs in meiosis II, while crossing over occurs in meiosis I, so they cannot both contribute to the same gamete.
• C) The argument is flawed because the two mechanisms operate at different levels: independent assortment randomly distributes homologous chromosomes (each consisting of sister chromatids), while crossing over generates new allele combinations within individual chromosomes, producing variation that independent assortment alone cannot generate. ✓
• D) The argument is flawed because crossing over is only significant in organisms with large numbers of chromosomes, making it irrelevant for the organism described in the scenario.

Explanation: Independent assortment and crossing over are complementary, not redundant. Independent assortment randomly distributes homologous chromosomes (each consisting of sister chromatids) into gametes, producing variation at the chromosomal level. Crossing over creates new combinations of alleles on a single chromosome by physically exchanging segments between homologous chromosomes during prophase I -- producing recombinant chromatids that could not exist through assortment alone. A directly contradicts biological reality. B incorrectly places independent assortment in meiosis II; it occurs during metaphase I. D is incorrect -- crossing over occurs in organisms regardless of chromosome number and is significant in the scenario organism.

Question 5. The researcher discovers a mutation that eliminates chiasmata formation entirely during prophase I. Which of the following best predicts the consequence of this mutation for genetic diversity in the organism's gametes?

• A) Genetic diversity will be unaffected because independent assortment alone is sufficient to generate all possible allele combinations.
• B) Genetic diversity will increase because homologous chromosomes will segregate more efficiently without the physical entanglement of chiasmata.
• C) Genetic diversity will be reduced because crossing over will no longer generate recombinant chromatids, eliminating one of the two major sources of genetic variation in meiosis. Independent assortment will still occur but cannot create new allele combinations within chromosomes. ✓
• D) Genetic diversity will be eliminated entirely because chiasmata are physically required for the independent assortment of homologous chromosomes to function, so chromosomes would be inherited in fixed combinations with no meaningful variation among the gametes produced in meiosis.

Explanation: Eliminating chiasmata removes crossing over as a source of genetic variation. Gametes will still vary due to independent assortment -- independent assortment (random orientation of homologous chromosome pairs during metaphase I) can still occur, although the absence of chiasmata may increase the risk of segregation errors. However, all chromosomes distributed to gametes will be nonrecombinant (parental-type) chromosomes, with no recombinant combinations. This reduces but does not eliminate genetic diversity. A overstates the capacity of independent assortment -- it cannot produce new intrachromosomal allele combinations. B is incorrect -- chiasmata contribute to proper chromosome alignment and tension, and their absence is associated with segregation errors, not efficiency gains. D overstates the consequence -- independent assortment does not require chiasmata.