Biology · Grade 12 · Molecular Genetics · Term 2

Non-Mendelian Inheritance Patterns

Students investigate complex inheritance patterns such as incomplete dominance, codominance, multiple alleles, and polygenic inheritance.

• Ontario Curriculum Expectations
• HS-LS3-3

About This Topic

Non-Mendelian inheritance patterns build on basic Mendelian genetics by introducing complexity in how alleles interact. Students examine incomplete dominance, where heterozygotes show an intermediate phenotype, such as pink snapdragon flowers from red and white parents. Codominance features both alleles expressing fully, as seen in roan cattle with red and white hairs or human AB blood type. Through examples and Punnett squares, students predict ratios and differentiate these from simple dominance.

Multiple alleles, like the three in the ABO blood group system, create more phenotypes and highlight diversity. Polygenic inheritance involves many genes contributing additively to traits such as human height, skin color, or intelligence, producing a bell-shaped curve of continuous variation. This topic aligns with Ontario's molecular genetics expectations, fostering analysis of genetic diversity and its role in populations.

Active learning suits this topic well. Students model patterns with beads or cards, simulate crosses in pairs, and graph class data for polygenic traits. These hands-on methods make abstract probabilities concrete, encourage peer discussion of ratios, and link concepts to observable human variation.

Key Questions

1. Differentiate between incomplete dominance and codominance using specific examples.
2. Analyze how multiple alleles can lead to a greater diversity of phenotypes within a population.
3. Explain how polygenic inheritance contributes to continuous variation in traits.

Learning Objectives

• Compare and contrast the phenotypic ratios resulting from incomplete dominance and codominance in diploid organisms.
• Analyze the impact of multiple alleles on the diversity of phenotypes within a given population, using the ABO blood group system as a model.
• Explain the genetic basis of polygenic inheritance and predict the distribution of phenotypes for traits controlled by multiple genes.
• Differentiate between Mendelian and non-Mendelian inheritance patterns by identifying key characteristics of each.

Before You Start

Mendelian Genetics: Laws of Inheritance

Why: Students must understand basic concepts of alleles, genotypes, phenotypes, and Punnett squares to build upon these for non-Mendelian patterns.

Genotype-Phenotype Relationships

Why: A clear understanding of how genotypes translate into observable traits is essential for differentiating between various dominance patterns.

Key Vocabulary

Incomplete Dominance	A form of inheritance where one allele is not completely dominant over another, resulting in a heterozygous phenotype that is an intermediate blend of the two homozygous phenotypes.
Codominance	A form of inheritance where both alleles in a heterozygote are fully and simultaneously expressed, leading to a phenotype that displays both parental traits distinctly.
Multiple Alleles	The existence of more than two alleles for a single gene within a population, allowing for a greater number of possible genotypes and phenotypes.
Polygenic Inheritance	A mode of inheritance in which a trait is controlled by two or more genes, with each gene contributing additively to the final phenotype.
Continuous Variation	A type of variation where individuals within a population exhibit a range of phenotypes that grade smoothly from one extreme to the other, typically resulting from polygenic inheritance.

Watch Out for These Misconceptions

Common Misconception

Incomplete dominance means one allele is partially dominant over the other.

What to Teach Instead

In incomplete dominance, neither allele dominates; they blend to create a new phenotype, like pink flowers. Active simulations with paint mixing or bead blending help students visualize the intermediate result and distinguish it from dominance through direct comparison.

Common Misconception

Codominance and incomplete dominance produce the same results.

What to Teach Instead

Codominance shows both traits fully, such as red and white hairs together, while incomplete blends them. Pair activities drawing Punnett squares and simulating with dual-colored beads clarify the difference as students observe and debate distinct outcomes.

Common Misconception

Polygenic inheritance involves multiple alleles at a single locus.

What to Teach Instead

Polygenic traits result from many genes, each with multiple alleles, interacting additively. Class surveys and graphing activities reveal the continuous variation curve, helping students shift from discrete Mendelian thinking to complex models.

Active Learning Ideas

Pairs: Incomplete Dominance Flower Crosses

Partners use colored beads for red (RR), white (WW), and pink (RW) alleles. They complete Punnett squares for RW x RW crosses, simulate 20 offspring by drawing beads randomly, and tally phenotypes. Groups compare simulated ratios to predicted 1:2:1.

25 min·Pairs

Small Groups: Codominance Blood Type Simulation

Each group assigns red beads for IA, white for IB, and blue for i. They perform crosses like IAi x IBi, use spinners to generate offspring, and classify into A, B, AB, or O phenotypes. Discuss how codominance produces novel types.

30 min·Small Groups

Whole Class: Polygenic Trait Height Survey

Students measure heights in cm, self-report family averages, and enter data into a shared spreadsheet. Class plots a histogram to show continuous distribution. Discuss how multiple genes and environment contribute to variation.

40 min·Whole Class

Individual: Multiple Alleles Pedigree Analysis

Provide ABO blood type pedigrees. Students assign genotypes, predict offspring possibilities, and identify patterns. Share findings in a brief gallery walk.

20 min·Individual

Real-World Connections

• Animal breeders select for specific traits like coat color in horses or cattle, utilizing knowledge of codominance and incomplete dominance to predict offspring phenotypes and achieve desired genetic outcomes.
• Medical professionals use blood typing, which involves multiple alleles for the ABO system, to ensure safe blood transfusions and to determine compatibility during organ transplants.
• Agricultural scientists study polygenic inheritance to develop crop varieties with improved yield, disease resistance, or nutritional content, as these traits are often influenced by numerous genes.

Assessment Ideas

Quick Check

Present students with scenarios describing crosses involving snapdragons or chickens. Ask them to determine if the inheritance pattern is incomplete dominance or codominance and to write the resulting genotypic and phenotypic ratios using Punnett squares.

Discussion Prompt

Pose the question: 'How does the existence of multiple alleles for a single gene, like in the ABO blood group, increase the genetic diversity of a human population compared to a gene with only two alleles?' Facilitate a class discussion where students share their reasoning.

Exit Ticket

Provide students with a brief description of a trait exhibiting polygenic inheritance (e.g., height). Ask them to explain in 2-3 sentences how multiple genes contribute to the observed range of heights and why this results in continuous variation.

Frequently Asked Questions

What is the difference between incomplete dominance and codominance?

Incomplete dominance blends parental traits in heterozygotes, producing pink flowers from red and white. Codominance expresses both traits fully, like roan fur with red and white hairs. Students grasp this best through Punnett squares showing 1:2:1 ratios for both, but visual models distinguish blending from co-expression. Real-world examples like blood types reinforce the concepts in population contexts.

How does polygenic inheritance explain continuous variation?

Polygenic inheritance involves multiple genes, each contributing small effects to a trait like height or skin color, plus environmental influences. This creates a range of phenotypes in a bell curve, unlike discrete Mendelian categories. Class data collection on traits like earwax type or tongue rolling plots this variation, showing students how genetics underlies diversity.

How can active learning help students understand non-Mendelian inheritance?

Active approaches like bead simulations for allele crosses or class surveys for polygenic traits make probabilities tangible. Pairs predicting and testing ratios with random draws reveal patterns firsthand, while graphing collective data highlights continuous variation. These methods build confidence in complex models, encourage error analysis, and connect abstract genetics to personal observations over lectures alone.

What are examples of multiple alleles in humans?

The ABO blood group uses three alleles: IA, IB (codominant), and i (recessive), yielding four phenotypes: A, B, AB, O. Another is the MN blood system. Punnett squares for parent crosses predict child types accurately. Activities simulating inheritance with cards help students explore how this increases diversity and relates to medical applications like transfusions.