Question 1 · 5.1 Meiosis
Why does a human zygote have 46 chromosomes, the same number as its parents' body cells?
- A The egg alone supplies all 46 chromosomes
- B Each gamete adds one set of 23 chromosomes
- C The zygote discards half after fertilization
- D Meiosis in the zygote restores the full 46
Show the answer
B — Meiosis gives each sperm and egg one set of 23 chromosomes, so fertilization combines two sets to restore 46. The egg is haploid too, so it supplies only half of the zygote's chromosomes.
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
Show the answer
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
Show the answer
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.