Mendelian Genetics — AP Biology
1. Core Concepts & Mendel's Laws of Inheritance ★★☆☆☆ ⏱ 3 min
Mendelian genetics is the study of discrete single-gene trait inheritance, derived from Gregor Mendel’s pea plant experiments. It is the foundational framework for all modern genetics, making up 8-10% of AP Biology Unit 5 exam weight, appearing regularly in both MCQs and FRQs.
Mendel derived two core laws, both directly rooted in homologous chromosome behavior during anaphase I of meiosis:
- **Law of Segregation**: Two alleles for a single trait separate during gamete formation, so each gamete receives only one allele.
- **Law of Independent Assortment**: Alleles of different genes assort independently of one another during gamete formation, only true for unlinked genes on separate non-homologous chromosomes.
Two core probability rules are used to apply Mendel's laws: the product rule for independent events, and the sum rule for mutually exclusive events.
Exam tip: On FRQ questions asking why segregation or independent assortment occur, always connect the process to anaphase I of meiosis (separation of homologous chromosomes) — this is required for full credit.
2. Punnett Square Analysis ★★☆☆☆ ⏱ 3 min
A Punnett square is a visual tool to organize all possible gamete combinations from two parents and calculate expected genotype and phenotype ratios of offspring. For a monohybrid cross (one gene, two alleles), the 2×2 Punnett square produces 4 equally likely outcomes, each with 25% probability.
3. Dihybrid Cross Calculations ★★★☆☆ ⏱ 4 min
For dihybrid crosses (two unlinked genes), a double heterozygote (AaBb) produces four gamete types (AB, Ab, aB, ab) each at 25% frequency, which can be organized into a 4×4 Punnett square. A faster, more error-resistant shortcut is to split the cross into two separate monohybrid crosses, calculate the desired probability for each gene, then multiply using the product rule.
The well-known 9:3:3:1 phenotypic ratio only applies to dihybrid crosses between two double heterozygotes (AaBb × AaBb) with complete dominance and unlinked genes. Never assume this ratio applies to all dihybrid crosses.
Exam tip: Never assume a 9:3:3:1 ratio applies to any dihybrid cross. Always calculate ratios from scratch for any parental combination other than AaBb × AaBb.
4. Test Cross Design & Interpretation ★★★☆☆ ⏱ 4 min
A test cross is a diagnostic cross used to determine the genotype of an individual with a dominant phenotype. A dominant phenotype can come from either a homozygous dominant (AA) or heterozygous (Aa) genotype, so phenotype alone cannot reveal genotype.
The homozygous recessive tester can only pass a recessive allele to offspring, so offspring phenotype depends entirely on the allele inherited from the unknown parent. If any offspring show the recessive phenotype, the unknown parent must be heterozygous. If all offspring show the dominant phenotype, the unknown parent is almost certainly homozygous dominant. For dihybrid crosses, a test cross (AaBb × aabb) can test for independent assortment: a 1:1:1:1 ratio confirms independent assortment, while deviation indicates linkage.
Exam tip: To get full credit for test cross reasoning on FRQs, always explicitly state that the tester parent is homozygous recessive and only contributes recessive alleles to offspring.
5. AP-Style Practice Check ★★★☆☆ ⏱ 3 min
Common Pitfalls
Why: Students memorize this ratio for dihybrid crosses and forget it only arises from one specific parental combination (AaBb × AaBb, unlinked, complete dominance).
Why: Students confuse the product rule and sum rule, and forget heterozygous genotype can form two different ways (A from mom/a from dad, or a from mom/A from dad).
Why: Students mix up homologous chromosome separation (anaphase I) and sister chromatid separation (anaphase II).
Why: Students remember test crosses determine unknown genotypes but forget the required tester genotype is homozygous recessive.
Why: Students forget the core condition required for independent assortment to hold.