Biology · Unit 5: Heredity · 25 min read · Updated 2026-05-13
Non-Mendelian Genetics — AP Biology
AP Biology · Unit 5: Heredity · 25 min read
1. Incomplete Dominance vs Codominance★★☆☆☆⏱ 5 min
Incomplete dominance produces a blended intermediate phenotype in heterozygotes, while codominance results in both parental traits being fully expressed simultaneously in heterozygotes. Both produce a 1:2:1 phenotypic ratio (same as genotypic ratio) in crosses of two heterozygotes.
Exam tip: AP multiple choice questions almost always test the difference between these two patterns: remember blended = incomplete, both traits expressed = codominance.
2. Multiple Alleles and Sex-Linked Inheritance★★★☆☆⏱ 6 min
Multiple alleles means there are more than two alleles for a gene in a population (though each individual only inherits two). Sex-linked traits are carried on the X chromosome, so inheritance patterns differ between XY males and XX females.
Exam tip: X-linked recessive traits are much more common in males because they cannot be carriers. Always note this in FRQ explanations.
3. Polygenic and Multifactorial Inheritance★★★☆☆⏱ 5 min
Polygenic traits are controlled by multiple independent genes, leading to a continuous range of phenotypes rather than discrete categories. Multifactorial traits are polygenic traits also influenced by environmental factors. Common examples include human height, skin color, and risk for many diseases.
Exam tip: On FRQs, always mention continuous variation as the key indicator of polygenic inheritance, versus discrete traits from single-gene Mendelian inheritance.
4. Pleiotropy and Epistasis★★★★☆⏱ 6 min
Pleiotropy occurs when one gene affects multiple unrelated traits (e.g., the sickle cell anemia gene affects red blood cell shape, resistance to malaria, and organ function). Epistasis is an interaction where one gene masks or modifies the expression of another gene at a separate locus, modifying the standard 9:3:3:1 dihybrid ratio.
Exam tip: If an exam question gives you a dihybrid cross ratio that is not 9:3:3:1, it is almost certainly epistasis.
Common Pitfalls
Why: Assuming any non-Mendelian heterozygote phenotype is the same for both patterns
Why: Mixing up the number of genes vs number of alleles
Why: Forgetting males only have one X chromosome
Why: Confusing gene interaction with physical arrangement on chromosomes
Why: Assuming all genetic crosses follow Mendelian rules