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Genetic Variation & Change · Part 2 of 4
12 exam-style questions with model answers, plus 16 quick multi-choice questions — every question on this part of the standard, grouped by the 4 pages of notes they come from.
Write a full answer before you reveal the model one. That comparison is where the learning happens.
In pea plants, round seed (R) is dominant to wrinkled (r). Draw a Punnett square for the cross Rr × Rr and state the phenotype ratio of the offspring.
A farmer has a black sheep and wants to know whether it is homozygous or heterozygous for coat colour, where black (B) is dominant to white (b). Explain how a test cross would answer this, and explain what each possible result would mean.
A breeder crosses two heterozygous plants and obtains 100 offspring: 68 tall and 32 short. A colleague argues this disproves the expected 3:1 ratio. Discuss whether the data support or undermine the predicted ratio, and explain what would make the conclusion more secure.
Define incomplete dominance and co-dominance, and give one example of each.
Explain why a cross between two heterozygous individuals gives a 3:1 phenotype ratio under complete dominance but a 1:2:1 phenotype ratio under incomplete dominance.
A researcher studying a NZ plant finds that crossing two flowering plants of the same intermediate colour gives offspring in a 1:2:1 ratio of dark, intermediate and pale flowers. Discuss how the researcher could determine whether this is incomplete dominance or co-dominance, and explain why the distinction matters biologically.
Explain what is meant by multiple alleles, and list the possible genotypes for human blood group A.
Two Manx cats are crossed. Explain why the kittens show a 2:1 ratio of Manx to normal tail rather than the 3:1 ratio a Punnett square would normally predict.
A lethal recessive allele kills homozygous individuals before birth, yet it persists in populations for many generations. Discuss why natural selection does not eliminate it, and compare this with a lethal dominant allele that kills late in life.
List the four types of gamete produced by a plant with the genotype AaBb, and state how many boxes a Punnett square for AaBb × AaBb would contain.
Explain why a cross between two individuals heterozygous for two unlinked genes gives a 9:3:3:1 phenotype ratio.
A researcher crosses two plants heterozygous for two genes and expects a 9:3:3:1 ratio. Instead she obtains 152 offspring in the proportions 9 : 3 : 4 (three phenotypes, not four). Discuss the possible explanations and how she could distinguish between them.