Mendelian Genetics: Ratios, Laws and When Dominance Breaks Down

In the paper

Unit 5 carries 6 of the 40 botany marks; with five chapters sharing them, this one averages just over one (MEC publishes weights by unit, not by chapter). The MEC scope line: General terminology; Laws of inheritance; Incomplete dominance; Co-dominance. Every ratio used here is also on one page in the Genetics ratios cheat sheet. Three ways it is asked:

• Recall: the F2 ratio of a dihybrid cross; which of Mendel's pea traits is recessive; which plant shows pink heterozygotes.
• Understanding: why a test cross uses the recessive parent; why the Mirabilis F2 gives the same ratio for phenotype and genotype; why AB blood is co-dominance and not incomplete dominance.
• Application: tall × dwarf giving 1 : 1, so what the tall parent was; the fraction of RrYy offspring from two dihybrids; the blood groups possible from AB × O.