Trihybrid Cross

A trihybrid cross extends the same reasoning to three gene pairs at once, and demonstrates that Mendel's Law of Segregation and Law of Independent Assortment both hold even when more than two characters are tracked simultaneously. A cross between homozygous parents differing in three characters at once - for example tall, yellow, round (TTYYRR) crossed with dwarf, green, wrinkled (ttyyrr) - produces a uniformly tall, yellow, round F1 (TtYyRr). Because a trihybrid has three independently assorting gene pairs, it forms 2 cubed, or 8, different kinds of gametes; when the F1 trihybrid is self-fertilised, these 8 gamete types combine to give 8 x 8, or 64, different zygote combinations, which sort by dominance into 8 distinct phenotype classes in the ratio 27:9:9:9:3:3:3:1 - simply the product of three independent 3:1 monohybrid ratios (3:1 x 3:1 x 3: …