Question 1 · 5.1 Meiosis
Paired chromosomes from a fungus were scored for the position of visible exchange points, with the results shown. The data most strongly support which statement?
- A Exchange points are equally likely at any position along a chromosome
- B The chance of an exchange rises with distance from the centromere
- C Exchange points form only at the outer ends, where the arms swing freely
- D Exchange points concentrate where genes are packed most densely
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B — The three regions differ more than tenfold, from 4 per 100 beside the centromere to 46 per 100 near the outer end, and the counts climb steadily as the scored region moves outward. That pattern rules out an even spread, and it also stops short of an absolute rule, since the region beside the centromere still records 4 exchanges per 100. This uneven distribution matters downstream: two gene positions on the same chromosome are separated by an exchange more often when they sit far out on an arm than when they sit near the centromere.
Question 2 · 5.2 Meiosis and Genetic Diversity
A biologist compares the four products of one meiosis in a fruit fly with the two products of one mitosis in the same fly. Which statement about the genetic content of these cells is accurate?
- A Both sets of products are genetically identical within their own set
- B Both sets of products differ within their own set, since replication introduces new alleles
- C The meiotic products match, while the mitotic products differ from crossing over
- D The mitotic products match each other, while the meiotic products can differ from one another
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D — Mitosis copies each chromosome and hands one identical chromatid of every pair to each daughter cell, so the two products carry the same alleles as each other and as the original cell. Meiosis inserts two shuffling steps ahead of that kind of separation: homologs pair and exchange segments in prophase I, and each pair orients independently at metaphase I. Both steps act before the products are sealed off, so the four cells that emerge can carry different combinations of alleles. That difference is the reason meiosis, not mitosis, is the source of gamete-to-gamete variation in a sexually reproducing organism.
Question 3 · 5.3 Mendelian Genetics
An organism has the genotype AaBBCc, and the three genes lie on three different chromosome pairs. How many genetically different kinds of gamete can this organism produce with respect to these three genes?
- A Two
- B Three
- C Four
- D Eight
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C — Only heterozygous genes add variety. Gene A can send A or a and gene C can send C or c, while gene B sends B either way, so the number of combinations is 2 × 1 × 2 = 4: ABC, ABc, aBC, and aBc. The familiar shortcut of 2 raised to a power counts the heterozygous genes, not the total number of genes an organism carries.