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 Marisela read the cross
In a minute you'll be the teacher. Get ready to teach Marisela: distinguishing the phenotype ratio.
A phenotype ratio counts how the offspring look; a genotype ratio counts their allele pairs. In a Tt × Tt cross the tall group hides two different allele pairs.
What you've shown you can teach
A case · you decide
Marisela and the 3:1 that "means three TT"
Marisela — a student breeding pea plants who keeps mixing up what the 3:1 ratio actually counts
Marisela crosses two tall pea plants, both Tt. The offspring come out about 3 tall : 1 short, and Marisela writes: "So the genotypes are 3 TT : 1 tt." A classmate crossing the same plants got 1 TT : 2 Tt : 1 tt and says Marisela'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.
Marisela reads the 3:1 as "3 TT : 1 tt." Is that what 3:1 counts?
Marisela picks a tall offspring and writes "genotype = TT." Safe conclusion?
Marisela'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.
When two ideas meet: 2 cases with two people
A case · you decide
Marisela and Lorcan: where the quarter comes from
Marisela & Lorcan — a student who draws Punnett squares, and a friend who knows what meiosis does to alleles
Two parents both carry one copy of a recessive allele (Aa). Neither shows the trait.
Lorcan explains that each parent's sex cells get either A or a, half and half.
Marisela draws the square: AA, Aa, aA, aa.
What is the chance a child of theirs shows the recessive trait?
Lorcan's meiosis makes the gametes. Marisela's square counts what they can make.
A case · you decide
Peregrine and Sunniva: who survives, who breeds
Peregrine & Sunniva — a student of natural selection, and a friend who tracks how populations change
A farmer sprays a field. Most beetles die. A few carried an allele that let them survive the spray.
Sunniva samples the field the next year. Far more beetles now survive the spray.
Why are this year's beetles harder to kill?
Peregrine saw the selection. Sunniva saw the population it made.