Biol 222 - review points - week 2. Progeny from parental cross (dominant phenotype is observed) Progeny from f1 self-cross (dominant and recessive phenotypes observed) Test cross - crossing an unknown genotype to a homozygous recessive individual. If unknown is aa: only dominant phenotype observed. If unknown is aa: half of offspring will show the dominant phenotype, and half will show the recessive phenotype. A a aa a aa a aa aa. Mendel"s first law - equal segregation of alleles of a gene pair into gametes. Alleles of a gene pair assort equally into gametes in meiosis. Dihybrid cross (2 gene pairs: 9:3:3:1 f2 phenotypic ratio. Mendels 2 nd law independent assortment of gene pairs into gametes. Punnett square vs. branch diagram vs. multiplication rule: branch diagrams (for phenotypic ratios) Bb = 1/16 aabb: multiplication rule (use for anything above a dihybrid cross) Aa bb cc dd ee x aa bb cc dd ee.