📌Definition Table

Term	Definition
Incomplete dominance	Heterozygote shows intermediate phenotype between two alleles.
Codominance	Both alleles expressed equally in heterozygotes (e.g., ABO blood group).
Multiple alleles	More than two alleles exist for a gene within a population.
Polygenic inheritance	Traits controlled by many genes, producing continuous variation.
Epistasis	Interaction where one gene affects the expression of another.

📌Introduction

While Mendelian ratios explain many traits, real-world inheritance is often more complex. Non-Mendelian patterns account for cases where alleles interact differently, or multiple genes influence traits. These principles explain phenomena such as blood groups, skin colour, and quantitative traits

📌 Incomplete Dominance and Codominance

• In incomplete dominance, heterozygotes show blended traits (e.g., red × white flowers → pink).
• Codominance occurs when both alleles are expressed equally (e.g., AB blood group).
• Both patterns differ from simple dominance/recessiveness.
• Ratios in crosses differ from Mendelian predictions.
• Provide key examples for exam questions.

🧠 Examiner Tip: Always use ABO blood groups for codominance examples—it’s a standard IB marking point.

📌 Multiple Alleles and Polygenic Traits

• Multiple alleles extend beyond two variations (e.g., IA, IB, and i in ABO system).
• Polygenic traits result from many genes (e.g., skin colour, height).
• Show continuous variation instead of discrete categories.
• Often influenced by environmental factors.
• Analyzed statistically rather than with Punnett squares.

🧬 IA Tips & Guidance: An IA could investigate continuous variation (e.g., human height in a sample group), showing polygenic inheritance.

📌 Epistasis and Gene Interactions

• Epistasis occurs when one gene masks or modifies another’s expression.
• Example: coat colour in Labrador retrievers (E gene affects pigment deposition).
• Explains why ratios deviate from Mendelian predictions.
• Shows complexity of gene networks.
• Links genetics to biochemistry.

🌐 EE Focus: An EE could explore the genetics of polygenic traits like skin colour or the inheritance of epistatic traits in model organisms.

📌 Human Traits and Disorders

• Blood groups are codominant with multiple alleles.
• Polygenic traits explain wide diversity in human populations.
• Complex disorders like diabetes and heart disease involve many genes.
• Environmental influence further modifies expression.
• Research into these traits connects genetics to public health.

❤️ CAS Link: Students could survey traits (like tongue rolling, ear lobes) in classmates to demonstrate variation and inheritance patterns.

📌 Genetics Beyond Mendel

• Many traits are polygenic or influenced by environment.
• Gene–gene interactions modify expected outcomes.
• Modern genomics explores complex inheritance.
• Statistical analysis essential for quantitative traits.
• Mendelian genetics is a foundation, but real inheritance is more nuanced.

🔍 TOK Perspective: Classification of inheritance patterns simplifies reality. TOK reflection: Does reducing genetic complexity into categories help or hinder understanding of biology?

📝 Paper 2: Expect questions on codominance, incomplete dominance, and blood groups. Data questions may involve polygenic traits or inheritance ratios that deviate from Mendelian predictions.