ACT Science: Inheritance Patterns and Dominant/Recessive Traits—Predict Offspring Genotypes

An allele is a version of a gene. Dominant alleles (capital letter, e.g., A) mask recessive ones (lowercase, e.g., a). Genotype is the genetic makeup (AA, Aa, aa). Phenotype is the trait observed. Example: Eye color gene. A (brown) is dominant. a (blue) is recessive. AA = brown. Aa = brown (carrier). aa = blue. A carrier (Aa) has the dominant phenotype but carries a recessive allele. On the ACT, you'll: (1) Use Punnett squares to predict offspring genotypes, (2) Calculate probability of traits, (3) Identify carriers. Cross Aa×Aa: Offspring are 25% AA (brown), 50% Aa (brown carrier), 25% aa (blue). Phenotypic ratio: 3:1 (brown:blue). Carriers show dominant phenotype but can pass recessive alleles to offspring.

Why inheritance patterns matter: Understanding heredity explains trait distribution in populations and helps predict genetic outcomes for traits affecting health, agriculture, conservation.

Misconception 1: Recessive traits skip generations. True in some cases (carriers), but not universally. Misconception 2: Dominant alleles are "better." False. Dominance is about masking, not quality. Some recessive traits (sickle cell resistance in malaria regions) are beneficial. Misconception 3: All traits follow Mendelian inheritance. False. Many traits are polygenic (multiple genes) or influenced by environment. Misconception 4: Heterozygous individuals always show dominant phenotype. True for simple dominance, but incomplete dominance and codominance show blended or both phenotypes. Misconception 5: Punnett squares predict exact outcomes for one individual. False. They predict probabilities for large offspring populations, not specific individuals. Remember: Aa=carrier (phenotypically dominant but genotypically heterozygous). aa=expresses recessive phenotype. AA=homozygous dominant.

Checklist: (1) Identify parent genotypes. (2) Set up Punnett square. (3) Predict offspring genotypes. (4) Determine offspring phenotypes using dominance rules. (5) Calculate ratios or probabilities. (6) Consider carriers.

Problem 1: Cross AA×aa. Offspring: All Aa. Phenotype: All dominant (assuming A is dominant). Problem 2: Cross Aa×Aa. Offspring: 25% AA, 50% Aa, 25% aa. Phenotypic ratio: 3 dominant : 1 recessive. Problem 3: A woman is a carrier (Aa). A man is homozygous dominant (AA). Children: 50% AA, 50% Aa. All dominant phenotype. Problem 4: Two carriers (Aa×Aa) have a child. Probability child is aa (recessive phenotype): 25%. Problem 5: A trait is recessive. Both parents show dominant phenotype. Can they have a child with recessive phenotype? Yes, if both are Aa (carriers). Punnett square: 25% chance. For each, draw the Punnett square, identify genotypes and phenotypes, calculate ratios.

Daily drill: Solve one inheritance problem daily. Alternate between simple Mendelian crosses, carrier scenarios, and multi-trait problems. Practice Punnett square setup until automatic.

About 1-2 genetics questions per ACT Science section test Mendelian inheritance, Punnett squares, or carrier status. These are predictable: set up the square, apply dominance rules, calculate. If you master this method, you answer these quickly and correctly. Genetics questions are high-value: straightforward logic, consistent format, many students struggle with genetics, so correct answers stand out.

Spend 2 days on inheritance patterns. Memorize dominant/recessive rules, practice Punnett squares, and drill probability calculations. By test day, inheritance questions will feel routine and you'll gain reliable points.