Teach & case
Help a near-peer reason through the idea — when you explain it, you understand it best — then read the real decision it comes from.
Help Mara read the cross
What you've shown you can teach
A real case · you decide
Mara and the 3:1 that "means three TT"
Mara — a student breeding pea plants who keeps mixing up what the 3:1 ratio actually counts
Mara crosses two tall pea plants, both Tt. The offspring come out about 3 tall : 1 short, and Mara writes: "So the genotypes are 3 TT : 1 tt." A classmate crossing the same plants got 1 TT : 2 Tt : 1 tt and says Mara's wrong. They can't both be right — what does the 3:1 actually count?
The Punnett square for Tt × Tt gives four equally-likely genotype boxes: TT, Tt, Tt, tt — that's the GENOTYPE ratio 1 TT : 2 Tt : 1 tt. It counts the allele combinations.
The PHENOTYPE ratio counts what you SEE: TT, Tt, and Tt all show the dominant trait (tall), and only tt is short — so 3 tall : 1 short. The 3:1 and the 1:2:1 describe the SAME cross from two different levels: appearance vs. genes.
Mara reads the 3:1 as "3 TT : 1 tt." Is that what 3:1 counts?
Mara picks a tall offspring and writes "genotype = TT." Safe conclusion?
Mara's fix is to label the level before writing a ratio: is this counting what the plants LOOK like (phenotype, 3:1) or what alleles they CARRY (genotype, 1:2:1)? Same cross — but never the same number for the same question.