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.

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, chromosomes assorting independently) is what lets you solve any inheritance problem rather than just plugging numbers into a memorized template.

Monohybrid Crosses

• Tracks a single gene with two alleles. This is the simplest cross type and the best way to see the Law of Segregation at work.
• Produces a 3:1 phenotypic ratio in the F2 generation under complete dominance. That ratio reflects a 1:2:1 genotypic ratio underneath (1 $AA$ : 2 $Aa$ : 1 $aa$).
• Law of Segregation in action: each parent contributes one allele per gamete because homologous chromosomes separate during meiosis I. The two alleles for a gene don't travel together into the same gamete.

Dihybrid Crosses

• Tracks two genes simultaneously. This tests whether you understand that genes on different chromosomes behave independently of each other.
• Produces the classic 9:3:3:1 ratio in F2. Each trait still follows a 3:1 ratio on its own (9+3 = 12 vs. 3+1 = 4 for one trait; 9+3 = 12 vs. 3+1 = 4 for the other).
• Law of Independent Assortment applies when the two genes are on different chromosomes, or far enough apart on the same chromosome that they aren't linked.

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Dominance Relationships: How Alleles Interact

The type of dominance determines what phenotype the heterozygote displays. This is about protein function and expression, not about which allele gets inherited more often. Both alleles are still passed on with equal probability. Recognizing the 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, so you can't distinguish $Aa$ from $AA$ by phenotype alone.
• Classic 3:1 phenotypic ratio in F2 monohybrid crosses.
• Mendel's pea plant traits (tall/short, purple/white flowers) all showed complete dominance, which is why his ratios came out so cleanly.

Incomplete Dominance

• The heterozygote shows an intermediate phenotype. Neither allele is fully dominant, so you get a blend. For example, crossing a red snapdragon ($C^R C^R$) with a white snapdragon ($C^W C^W$) produces pink F1 offspring ($C^R C^W$).
• Phenotypic ratio shifts to 1:2:1 in F2 because now you can visually distinguish all three genotypes.
• The genotypic and phenotypic ratios match. If you notice a 1:2:1 phenotypic ratio with an intermediate heterozygote, that's your clue that incomplete dominance is operating.

Codominance

• Both alleles are fully expressed simultaneously. This is not blending; both phenotypes are visible at once in the heterozygote.
• AB blood type is the classic example: the $I^A$ and $I^B$ alleles each produce their own distinct surface glycoprotein on red blood cells, and both are detectable.
• Phenotypic ratio is also 1:2:1, but the heterozygote shows both parental traits distinctly rather than 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 single individual still carries only two (one per homologous chromosome).
• ABO blood group is the go-to example: three alleles exist ($I^A$, $I^B$, and $i$). $I^A$ and $I^B$ are codominant with each other, and both are dominant over $i$.
• This creates a dominance hierarchy. You need to know which alleles dominate which to predict phenotypes. For instance, genotype $I^A i$ gives type A blood, $I^B i$ gives type B, $I^A I^B$ gives type AB, and $ii$ gives type O.

Sex-Linked Inheritance

• Genes located on the X (or Y) chromosome follow inheritance patterns that differ between males (XY) and females (XX).
• Males express X-linked recessive traits more frequently because they're hemizygous for X-linked genes: with only one X chromosome, there's no second copy to mask a recessive allele. A female needs two copies of the recessive allele to express the trait, but a male needs only one.
• Classic examples: red-green color blindness, hemophilia A. In pedigrees, look for traits that appear mainly in males and can skip generations (carrier mothers pass the allele to affected sons).

Autosomal Inheritance

• Genes on chromosomes 1–22 (the non-sex chromosomes) affect both sexes equally.
• Can be dominant or recessive. Use pedigree analysis to determine which pattern fits the data. Autosomal dominant traits appear in every generation; autosomal recessive traits can skip generations and often appear when two carriers have children.
• No sex bias in expression. If males and females are affected at roughly equal rates in a pedigree, the trait is likely autosomal.

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Tools for Solving Genetics Problems

These are the methods you'll use on every genetics question. Master the logic behind each one, not just the mechanics.

Punnett Squares

• A visual tool for predicting offspring ratios. It organizes all possible allele combinations from both parents into a grid.
• Works for any cross type: monohybrid ($2 \times 2$ grid), dihybrid ($4 \times 4$), or even trihybrid ($8 \times 8$).
• Shows both genotypic and phenotypic outcomes. To use one correctly: list all possible gamete types from each parent along the top and side, then fill in each box by combining the row and column gametes.

Test Crosses

A test cross answers a specific question: does this dominant-phenotype individual have genotype $AA$ or $Aa$?

1. Cross the unknown individual with a homozygous recessive ($aa$) individual.
2. Observe the offspring phenotypes.
3. If any offspring show the recessive phenotype, the unknown parent must be $Aa$ (heterozygous).
4. If all offspring show the dominant phenotype (with a large enough sample), the unknown parent is likely $AA$.

A 1:1 phenotypic ratio in the offspring confirms the parent is $Aa$.

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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?