Mendelian Inheritance Patterns

Why This Matters

Mendelian inheritance is the foundation of everything you'll encounter in genetics—from predicting offspring ratios to understanding why certain diseases run in families. When you're working through crosses on an exam, you're really being tested on whether you understand how alleles segregate, how genes on different chromosomes behave independently, and how different dominance relationships change the phenotypic outcomes. These principles don't just apply to pea plants; they're the framework for analyzing inheritance in any organism.

Here's the key insight: the specific ratios you calculate (3:1, 9:3:3:1, 1:2:1) aren't random numbers to memorize—they're predictable outcomes of meiosis. Every cross you solve is really asking you to apply the Law of Segregation or Independent Assortment. Don't just memorize the ratios; know which mechanism produces each one and which type of dominance you're dealing with. That's what separates a student who can handle any genetics problem from one who freezes when the numbers look unfamiliar.

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Mendel's Laws: The Core Mechanisms

These two laws explain why genetic crosses produce predictable ratios. Understanding the cellular basis—alleles separating during meiosis I and chromosomes assorting independently—is essential for solving any inheritance problem.

Monohybrid Crosses

• Tracks a single gene with two alleles—the simplest cross type, perfect for demonstrating the Law of Segregation
• Produces a 3:1 phenotypic ratio in the F2 generation when complete dominance is operating; this ratio reflects the 1:2:1 genotypic ratio underneath
• Law of Segregation in action: each parent contributes one allele per gamete because homologous chromosomes separate during meiosis I

Dihybrid Crosses

• Tracks two genes simultaneously—tests whether you understand that genes on different chromosomes behave independently
• Produces the classic 9:3:3:1 ratio in F2; each trait still follows a 3:1 ratio individually (9+3=12 vs. 3+1=4 for each trait)
• Law of Independent Assortment applies when genes are on different chromosomes or far apart on the same chromosome

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Dominance Relationships: How Alleles Interact

The type of dominance determines what phenotype the heterozygote displays. This is purely about protein function and expression—not about which allele gets inherited more often. Recognizing dominance type is critical because it changes the expected ratios.

Complete Dominance

• One allele completely masks the other—the heterozygote looks identical to the homozygous dominant
• Classic 3:1 phenotypic ratio in F2 monohybrid crosses; you can't distinguish $Aa$ from $AA$ by looking
• Mendel's pea plant traits (tall/short, purple/white flowers) all showed complete dominance, which is why his ratios were so clean

Incomplete Dominance

• Heterozygote shows an intermediate phenotype—neither allele fully functional alone, so you get blending (red × white = pink)
• Phenotypic ratio shifts to 1:2:1 in F2 because now you can visually distinguish all three genotypes
• The genotypic and phenotypic ratios match—this is your clue that incomplete dominance is operating

Codominance

• Both alleles fully expressed simultaneously—not blending, but both phenotypes visible at once
• AB blood type is the classic example: $I^A$ and $I^B$ alleles both produce functional surface proteins
• Phenotypic ratio is also 1:2:1 but the heterozygote shows both traits distinctly, not an intermediate

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Beyond Simple Dominance: Complex Allele Patterns

Real inheritance often involves more than two alleles or genes located on sex chromosomes. These patterns build on Mendel's laws but add complexity you need to recognize.

Multiple Alleles

• More than two alleles exist in the population—though any individual still carries only two (one per homolog)
• ABO blood group has three alleles: $I^A$, $I^B$ (codominant to each other), and $i$ (recessive to both)
• Creates a dominance hierarchy—you must know which alleles dominate which to predict phenotypes correctly

Sex-Linked Inheritance

• Genes located on X or Y chromosomes—inheritance patterns differ between males (XY) and females (XX)
• Males express X-linked recessive traits more frequently because they have no second X to mask a recessive allele
• Classic examples: color blindness, hemophilia—look for traits that skip generations and appear mainly in males

Autosomal Inheritance

• Genes on chromosomes 1-22 (non-sex chromosomes)—affects both sexes equally
• Can be dominant or recessive—use pedigree analysis to determine which pattern fits the data
• No sex bias in expression—if males and females are equally affected, the trait is likely autosomal

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Tools for Solving Genetics Problems

These aren't just techniques—they're the methods you'll use on every genetics question. Master the logic behind each tool.

Punnett Squares

• Visual tool for predicting offspring ratios—organizes all possible allele combinations from both parents
• Works for any cross type: monohybrid ($2 \times 2$), dihybrid ($4 \times 4$), or even trihybrid ($8 \times 8$)
• Shows both genotypic and phenotypic outcomes—essential for calculating probabilities on exams

Test Crosses

• Determines unknown genotype of an individual with dominant phenotype—is it $AA$ or $Aa$?
• Cross with homozygous recessive ($aa$)—if any offspring show recessive phenotype, the unknown parent must be $Aa$
• Results are diagnostic: all dominant offspring suggests $AA$; 1:1 ratio confirms $Aa$

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Quick Reference Table

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Self-Check Questions

1. Both incomplete dominance and codominance produce a 1:2:1 phenotypic ratio in F2. How would you distinguish between them if given only phenotype descriptions?

2. A dihybrid cross yields a 9:3:3:1 ratio. Which of Mendel's laws does this demonstrate, and what must be true about the chromosomal location of the two genes?

3. Why are males more likely than females to express X-linked recessive traits? Explain using the concept of hemizygosity.

4. You cross a purple-flowered plant with a white-flowered plant and all F1 offspring are purple. In the F2, you observe 3:1 purple to white. What type of dominance is this, and what would the F2 ratio be if it were incomplete dominance instead?

5. Compare and contrast a test cross and a standard Punnett square: when would you use each, and what question does each one answer?