ACT Science Genetic Inheritance: Use Punnett Squares to Predict Trait Outcomes

Punnett squares show how alleles combine in offspring. Set up: Dominant allele (capital letter, e.g., A), recessive allele (lowercase, e.g., a). Cross: Aa × Aa (heterozygous × heterozygous). Fill grid showing all allele combinations. Result: 25% AA (dominant), 50% Aa (dominant), 25% aa (recessive). Phenotype ratio: 3 dominant : 1 recessive. The square is mechanical: list parent alleles, fill grid with all combinations, count phenotypes. Questions ask you to predict offspring traits or identify parent genotypes from offspring. Process: (1) Identify parent alleles. (2) Fill Punnett square. (3) Count phenotypes. (4) Answer the question about inheritance pattern or probability.

Example: If both parents are Aa (heterozygous for height), 75% of offspring will be tall (A_) and 25% short (aa).

Mistake 1: Forgetting that capital = dominant, lowercase = recessive. This determines phenotype. Mistake 2: Misreading the Punnett square grid. Count carefully; one error cascades through the ratio. Mistake 3: Confusing probability with certainty. A 75% ratio means roughly 3 out of 4, not exactly 3 out of 4 in any small group. Understand that ratios describe probability, not guarantees.

During practice, draw Punnett squares carefully. Write parent genotypes at grid edges clearly. Fill every cell before counting.

Problem 1: Cross AA × aa. Punnett square shows all Aa offspring (100% heterozygous). Phenotype: 100% dominant trait. Problem 2: Cross Aa × Aa. Punnett square: 1 AA, 2 Aa, 1 aa. Phenotype: 75% dominant, 25% recessive. Problem 3: Cross aa × aa. All aa offspring. Phenotype: 100% recessive. Problem 4: Cross AA × Aa. Punnett square: 1 AA, 1 Aa. Phenotype: 100% dominant. Problem 5: Three-trait cross (AaBb × AaBb). 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb (9:3:3:1 ratio for independent traits). Create Punnett squares for each cross and identify phenotypic ratios.

Find five genetics problems from a practice test. For each, create or analyze a Punnett square and predict offspring phenotypes. By the fifth problem, grid-filling and counting will be automatic.

Genetics questions appear on some ACT Science sections. They test understanding of inheritance patterns. Students who systematically use Punnett squares pick up 1 point because the method is mechanical and predictable.

Drill Punnett square construction daily this week. For each cross, draw a grid, fill it completely, and count phenotypes. By test day, you should predict any single-trait or two-trait inheritance pattern within 90 seconds.