Mendelian Genetics Laws

Why This Matters

Mendelian genetics is the foundation of everything you'll encounter in heredity—and it shows up constantly on the AP Biology exam. The principles Gregor Mendel discovered through his pea plant experiments explain how alleles segregate during meiosis, why offspring show predictable trait ratios, and what mechanisms create genetic variation. These aren't just historical curiosities; they're the framework you need to analyze pedigrees, predict cross outcomes, and understand why populations change over time (hello, Unit 7!).

You're being tested on your ability to apply these laws, not just recite them. Can you predict the outcome of a dihybrid cross? Can you explain why the 3:1 ratio appears in a monohybrid cross? Can you connect segregation to what's physically happening during meiosis? The exam loves asking you to use Punnett squares, calculate probabilities, and distinguish between genotype and phenotype. Don't just memorize the law names—know what biological process each one describes and when to apply it.

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The Core Laws: What Alleles Do During Meiosis

These foundational principles describe the physical behavior of alleles during gamete formation. Meiosis is the mechanism that makes Mendel's laws work—alleles separate and sort because chromosomes do.

Law of Segregation

• Each organism carries two alleles per gene—one inherited from each parent, and these separate during gamete formation so each gamete carries only one allele
• Meiosis I is the mechanism—homologous chromosomes (carrying different alleles) are pulled to opposite poles, ensuring each gamete is haploid
• Produces the 3:1 ratio in monohybrid crosses; this ratio is your signal that a single gene with complete dominance is at work

Law of Independent Assortment

• Genes on different chromosomes sort independently—the allele a gamete receives for one gene doesn't affect which allele it gets for another
• Only applies to unlinked genes—genes on the same chromosome may be linked and inherited together (a key exception tested in Topic 5.4)
• Creates the 9:3:3:1 ratio in dihybrid crosses and generates new allele combinations, increasing genetic variation in offspring

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Allele Interactions: How Traits Get Expressed

These principles explain what happens when two alleles meet in a diploid organism. The relationship between alleles determines whether you see one phenotype, the other, or something in between.

Law of Dominance

• One allele can mask another—in a heterozygote, the dominant allele is expressed while the recessive allele is hidden (but still present in the genotype)
• Explains phenotype vs. genotype distinction—organisms with genotypes $AA$ and $Aa$ look identical but carry different genetic information
• Test crosses reveal hidden recessives—crossing an unknown dominant phenotype with a homozygous recessive ($aa$) reveals whether it's $AA$ or $Aa$

Concept of Alleles

• Alleles are variant forms of a gene—they arise through mutation and occupy the same locus on homologous chromosomes
• Homozygous vs. heterozygous—$AA$ or $aa$ (two identical alleles) versus $Aa$ (two different alleles), a distinction critical for predicting offspring ratios
• Allele combinations determine phenotype—but the specific outcome depends on dominance relationships, which can be complete, incomplete, or codominant

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Predicting Outcomes: Crosses and Probability

These tools let you calculate expected offspring ratios. Punnett squares are visual representations of probability rules applied to genetics.

Monohybrid Cross

• Examines one gene with two alleles—typically crossing two heterozygotes ($Aa \times Aa$) to observe segregation
• F2 phenotypic ratio is 3:1—three dominant phenotypes to one recessive, the classic Mendelian ratio that confirms complete dominance
• Genotypic ratio is 1:2:1—one $AA$, two $Aa$, one $aa$; knowing both ratios helps you answer different question types

Dihybrid Cross

• Examines two genes simultaneously—crossing $AaBb \times AaBb$ tests whether genes assort independently
• F2 phenotypic ratio is 9:3:3:1—this specific ratio only appears when both genes are unlinked and show complete dominance
• Demonstrates independent assortment—the four phenotype classes arise because each gene segregates without influencing the other

Principle of Probability

• Product rule for independent events—multiply probabilities when asking "what's the chance of A and B?" (e.g., $\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$)
• Sum rule for mutually exclusive events—add probabilities when asking "what's the chance of A or B?"
• Punnett squares apply these rules visually—each box represents an equally likely outcome, making probability calculations intuitive

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Foundational Concepts: The Framework Behind the Laws

These ideas establish why Mendelian genetics works the way it does. Understanding them helps you recognize when Mendelian rules apply—and when they don't.

Law of Unit Characters

• Genes are discrete hereditary units—traits don't blend; they're controlled by specific segments of DNA that maintain their identity across generations
• Each gene can have multiple alleles—though any individual carries only two (one per homologous chromosome)
• Foundation for all genetic analysis—this concept allows us to track inheritance patterns and predict outcomes mathematically

Law of Uniformity (First Filial Generation)

• Crossing homozygous parents yields uniform F1—when $AA \times aa$, all offspring are $Aa$ and show the dominant phenotype
• F1 uniformity confirms parental genotypes—if F1 offspring vary, at least one parent wasn't homozygous
• Sets up the F2 generation—crossing F1 heterozygotes ($Aa \times Aa$) produces the 3:1 ratio that reveals segregation

Law of Reciprocal Crosses

• Swapping parental sexes doesn't change autosomal ratios—if $AA$ female × $aa$ male gives the same result as $aa$ female × $AA$ male, the gene is autosomal
• Deviations indicate sex-linkage—when reciprocal crosses do differ, the gene is likely X-linked (covered in Topic 5.4)
• Useful for ruling out maternal effects—confirms that nuclear genes, not cytoplasmic factors, control the trait

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Quick Reference Table

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Self-Check Questions

1. Both the Law of Segregation and the Law of Independent Assortment describe events during meiosis. What specific cellular process does each law depend on, and how do the resulting offspring ratios differ?

2. You cross two heterozygous pea plants ($Aa$) and observe the offspring. What phenotypic and genotypic ratios would you expect, and which Mendelian law explains why the recessive phenotype reappears in the F2 generation?

3. A dihybrid cross yields a 9:3:3:1 ratio. What two conditions must be true about the genes involved for this ratio to appear? What ratio might you see instead if the genes were linked?

4. Compare and contrast a test cross with a monohybrid cross between two heterozygotes. When would you use each, and what information does each provide about genotype?

5. A researcher performs reciprocal crosses for a trait and gets different ratios depending on which parent contributes the dominant allele. What type of inheritance pattern does this suggest, and how does this differ from standard Mendelian autosomal inheritance?