5.3 Mendelian Genetics

Genotype = the actual alleles an organism carries for a gene (its genetic makeup). It can be homozygous (AA or aa) or heterozygous (Aa). Phenotype = the observable trait or physical expression that results from that genotype plus any environmental effects (for example, flower color, height, or blood type). On AP Bio, you’ll use genotypes in Punnett squares and probability problems to predict phenotypes (EK 5.3.A.2.i–iv). Remember: two different genotypes can give the same phenotype if an allele is dominant (e.g., AA and Aa both show the dominant trait), and environment can modify phenotype (see Topic 5.5). For more practice linking genotype → phenotype with monohybrid/ dihybrid and test crosses, check the Mendelian Genetics study guide (https://library.fiveable.me/ap-biology/unit-5/mendelian-genetics/study-guide/SdlMbZYAD4sxuXuRygPv), the Unit 5 overview (https://library.fiveable.me/ap-biology/unit-5), and extra practice problems (https://library.fiveable.me/practice/ap-biology).

Mendel’s laws describe what happens to alleles during meiosis and fertilization. Law of segregation: each individual has two alleles per gene (homozygous or heterozygous). During gamete formation the two alleles separate so each gamete gets one allele—that’s why a monohybrid cross gives a 3:1 phenotypic ratio when one allele is dominant. Law of independent assortment: alleles of genes on different chromosomes sort into gametes independently because homologous pairs orient randomly at metaphase I of meiosis. That’s why dihybrid crosses (unlinked genes) give the 9:3:3:1 ratio and you multiply probabilities (P(A and B)=P(A)×P(B)). Exceptions: linked genes don’t assort independently. On the AP exam you’ll apply these with Punnett squares, probability rules, monohybrid/dihybrid/testcrosses, and pedigrees (see the Topic 5.3 study guide for examples: https://library.fiveable.me/ap-biology/unit-5/mendelian-genetics/study-guide/SdlMbZYAD4sxuXuRygPv). For broader unit review: https://library.fiveable.me/ap-biology/unit-5 and for lots of practice problems: https://library.fiveable.me/practice/ap-biology.

We use Punnett squares to predict the genotypes and phenotypes of offspring from known parental genotypes by applying Mendel’s laws (segregation and independent assortment) and basic probability. They’re handy for monohybrid, dihybrid, and test crosses on the AP exam (EK 5.3.A.1–2; LO 5.3.A). How to set one up correctly: 1. Determine each parent’s genotype (homozygous or heterozygous). 2. List the gametes each parent can produce (for a dihybrid, list combinations like RrYy → RY, Ry, rY, ry). 3. Draw a grid: 2×2 for monohybrid, 4×4 for dihybrid, etc. Put one parent’s gametes across the top and the other’s down the side. 4. Fill boxes by combining gamete alleles to get offspring genotypes. 5. Count genotypes → convert to phenotype ratios. Use probability rules if events are independent (multiply probabilities for combined traits). Note limits: linked or sex-linked genes don’t follow simple independent-assortment ratios. For review and practice problems, see the Topic 5.3 study guide (https://library.fiveable.me/ap-biology/unit-5/mendelian-genetics/study-guide/SdlMbZYAD4sxuXuRygPv), the unit page (https://library.fiveable.me/ap-biology/unit-5), and Fiveable practice questions (https://library.fiveable.me/practice/ap-biology).

Homozygous vs. heterozygous is about the alleles an organism carries for a gene. If you’re homozygous, both alleles at that locus are the same (e.g., AA or aa). If you’re heterozygous, the two alleles are different (e.g., Aa). The CED defines genotype as the set of alleles an individual inherits and notes genotypes can be homozygous or heterozygous (EK 5.3.A.2.iii). Phenotype is the trait you actually see; a dominant allele in a heterozygote (Aa) usually determines the phenotype, while a recessive trait shows only in homozygous recessive (aa). On the AP exam you’ll use this language when doing Punnett squares, monohybrid/dihybrid crosses, and test crosses to predict genotype and phenotype probabilities (apply probability rules: P(A and B) = P(A)×P(B)). For targeted review, see the Topic 5.3 study guide (https://library.fiveable.me/ap-biology/unit-5/mendelian-genetics/study-guide/SdlMbZYAD4sxuXuRygPv) and practice questions (https://library.fiveable.me/practice/ap-biology).

A dominant allele is one whose trait shows up in the phenotype when an organism has just one copy (heterozygous). A recessive allele only affects the phenotype when an organism has two copies (homozygous recessive). Example: if A = dominant (purple flowers) and a = recessive (white), AA and Aa are purple, aa is white. Dominant doesn’t mean more common or “stronger”—it just means it masks the recessive allele’s effect in heterozygotes. Mendel’s law of segregation explains why each gamete gets one allele, and Punnett squares + probability let you predict genotype and phenotype ratios (monohybrid, dihybrid, test crosses; LO 5.3.A). Pedigrees and pattern type (autosomal vs. sex-linked) help you infer dominance from data on the AP exam. For a quick refresher, see the Topic 5.3 study guide (https://library.fiveable.me/ap-biology/unit-5/mendelian-genetics/study-guide/SdlMbZYAD4sxuXuRygPv) and practice problems (https://library.fiveable.me/practice/ap-biology).

Look for these pedigree patterns (use Punnett squares/test crosses to confirm): - Autosomal dominant: trait usually appears in every generation (no skipping). Every affected individual has at least one affected parent. Males and females affected equally. An affected heterozygote typically passes the trait to ~50% of offspring. If two unaffected parents have an affected child, dominance is unlikely. - Autosomal recessive: trait can skip generations (appears in siblings whose parents are both carriers). Unaffected parents can produce affected offspring. Males and females affected equally. If two affected parents always produce affected children, that fits recessive inheritance. On the AP exam you should state these patterns, justify with Mendel’s laws (segregation, Punnett squares) and, when possible, calculate probabilities (LO 5.3.A; EK 5.3.A.2). For a quick review, see the Topic 5.3 study guide (https://library.fiveable.me/ap-biology/unit-5/mendelian-genetics/study-guide/SdlMbZYAD4sxuXuRygPv). For more practice with pedigrees and crosses, try Fiveable’s unit resources (https://library.fiveable.me/ap-biology/unit-5) and the practice question bank (https://library.fiveable.me/practice/ap-biology).

A monohybrid cross follows one gene (one pair of alleles) between parents—think Rr × Rr for seed shape. Use a 2×2 Punnett square: F2 genotypes are 1 RR : 2 Rr : 1 rr and the typical dominant:recessive phenotype ratio is 3:1. A dihybrid cross follows two independent genes (two pairs of alleles) at once—e.g., RrYy × RrYy. Use a 4×4 Punnett square or apply the product rule (P(A and B) = P(A)×P(B)). For unlinked, independently assorting genes the F2 phenotype ratio is classically 9:3:3:1 (dominant/dominant : dominant/recessive : recessive/dominant : recessive/recessive). Both cross types illustrate Mendel’s law of segregation; dihybrids also show independent assortment (EK 5.3.A.1). You’ll use these for test crosses and probability problems on the AP (see Topic 5.3 study guide for examples: https://library.fiveable.me/ap-biology/unit-5/mendelian-genetics/study-guide/SdlMbZYAD4sxuXuRygPv). For extra practice, try the AP-style problems at (https://library.fiveable.me/practice/ap-biology).

You need probability rules because inheritance is random. Mendel’s laws (segregation and independent assortment) describe how alleles separate into gametes and then combine at fertilization—but to predict offspring genotypes/phenotypes you must quantify those random events. Rules you use on the AP: add probabilities when outcomes are mutually exclusive (A or B) and multiply when independent (A and B). That’s how you get monohybrid 3:1 or dihybrid 9:3:3:1 ratios from Punnett-square logic without drawing every box. Probability also helps with test crosses, pedigrees, and deciding expected counts for chi-square on FRQs and multiple choice (CED EK 5.3.A.1–2; LO 5.3.A). Practice applying these rules on Mendelian problems so you can quickly set up multiplies/adds during the exam. For a targeted review see the Topic 5.3 study guide (https://library.fiveable.me/ap-biology/unit-5/mendelian-genetics/study-guide/SdlMbZYAD4sxuXuRygPv) and extra practice (https://library.fiveable.me/practice/ap-biology).

Check whether offspring fit independent assortment or not. Cross parents to get F1 heterozygotes for both genes, then do a dihybrid testcross (F1 x double recessive) or self the F1. If the genes are on different chromosomes, expect Mendelian dihybrid ratios (9:3:3:1 in an F2 or ~1:1:1:1 from a testcross) because alleles assort independently (CED EK 5.3.A.1). If most offspring are the two parental phenotypes and far fewer recombinants, the genes are linked. Quantify linkage: calculate recombination frequency = (number of recombinant offspring) / (total offspring) × 100%. <50% indicates linkage; ~50% means unlinked (different chromosomes or far apart). Use recombination % as map distance in centiMorgans (1% = 1 cM). You can also run a chi-square test to see if observed counts deviate significantly from expected independent-assortment ratios. For step-by-step examples and practice problems, see the AP Topic 5.3 study guide (https://library.fiveable.me/ap-biology/unit-5/mendelian-genetics/study-guide/SdlMbZYAD4sxuXuRygPv) and lots of practice questions at (https://library.fiveable.me/practice/ap-biology).

A test cross is when you mate an individual showing a dominant phenotype but unknown genotype with a homozygous recessive individual for the same gene. Because the recessive parent can only contribute the recessive allele, the offspring’s phenotypes reveal the unknown parent’s genotype. If all offspring show the dominant trait, the mystery parent was likely homozygous dominant (AA). If about half show dominant and half recessive, the mystery parent was heterozygous (Aa). You use a test cross whenever you need to determine whether a dominant-looking organism is AA or Aa (monohybrid or extended to dihybrid cases), and you analyze results with Punnett squares and probability rules (EK 5.3.A.2; LO 5.3.A). Practice test-cross problems to get comfortable predicting expected ratios—see the Topic 5.3 study guide (https://library.fiveable.me/ap-biology/unit-5/mendelian-genetics/study-guide/SdlMbZYAD4sxuXuRygPv) and more practice questions (https://library.fiveable.me/practice/ap-biology).

Fertilization increases genetic variation because it randomly combines two haploid gametes (each carrying one set of chromosomes and different allele combos) into a diploid zygote, creating new allele combinations that neither parent had alone. Mendel’s law of segregation ensures gametes get one allele per gene; independent assortment (for genes on different chromosomes) means each gamete gets a different mix of maternal and paternal chromosomes. Because which sperm meets which egg is random, probability multiplies the number of possible genotypes—even two heterozygous parents (Aa × Aa) can produce AA, Aa, or aa in predictable ratios you can work out with Punnett squares. This is exactly why fertilization + meiosis boosts population-level variation, fueling evolution and affecting phenotype distributions on AP-style questions (LO 5.3.A; EK 5.3.A.2). Review the Topic 5.3 study guide for examples and practice problems (https://library.fiveable.me/ap-biology/unit-5/mendelian-genetics/study-guide/SdlMbZYAD4sxuXuRygPv) and try extra practice questions (https://library.fiveable.me/practice/ap-biology).

Start by reading the symbols: squares = males, circles = females, shaded = affected, unshaded = unaffected, and horizontal lines = mating; generations are usually numbered (I, II, III) and individuals numbered left → right. To infer inheritance: - Autosomal dominant: affected appears in every generation; an affected parent usually has affected children; unaffected parents don’t produce affected offspring. - Autosomal recessive: trait can skip generations; unaffected parents can have affected children (both parents can be carriers); look for consanguinity increasing risk. - X-linked recessive: more males affected; affected males often have carrier mothers; father→son transmission does not occur. - X-linked dominant: affected fathers pass trait to all daughters but no sons; both sexes can be affected. - Use patterns + Punnett squares and rules of probability (EK 5.3.A.2, LO 5.3.A) to test genotypes; do test crosses when possible. For AP prep, practice pedigree problems and Punnett squares on the Mendelian Genetics study guide (https://library.fiveable.me/ap-biology/unit-5/mendelian-genetics/study-guide/SdlMbZYAD4sxuXuRygPv) and more problems at (https://library.fiveable.me/practice/ap-biology).

Autosomal inheritance involves genes on nonsex chromosomes (autosomes). Traits follow Mendel’s laws (segregation, independent assortment), so males and females are equally likely to show the trait; use Punnett squares, monohybrid/dihybrid crosses, and pedigrees to tell dominant vs. recessive autosomal patterns (e.g., two affected parents → affected kids for dominant; two unaffected parents can have affected kids for recessive). Sex-linked inheritance usually means genes on the X (or rarely Y) chromosome. X-linked recessive traits show up much more often in males (they’re hemizygous), affected fathers don’t pass X-linked traits to sons but do to all daughters (who can be carriers), and carrier mothers can pass the trait to sons. On the AP exam you’ll predict patterns with Punnett squares and analyze pedigrees (LO 5.3.A, EK 5.3.A.1, EK 5.3.A.2.v). For a focused review, see the Topic 5.3 study guide (https://library.fiveable.me/ap-biology/unit-5/mendelian-genetics/study-guide/SdlMbZYAD4sxuXuRygPv), unit overview (https://library.fiveable.me/ap-biology/unit-5) and practice questions (https://library.fiveable.me/practice/ap-biology).

Traits often “skip” generations because the allele that causes the trait is recessive or not always expressed. If a trait is caused by a recessive allele, an individual must inherit two copies (homozygous recessive) to show the phenotype. Parents can be heterozygous carriers (genotype Aa) who look normal but pass the recessive allele to kids—so the trait can reappear in grandchildren by probability (Punnett squares, LO 5.3.A). Skipping can also happen with X-linked recessive traits (more common in males) or when penetrance/expressivity vary so some carriers don’t show the trait. On the AP exam you’ll use pedigrees and probability rules to predict these patterns and distinguish autosomal vs. sex-linked inheritance (EK 5.3.A.1, EK 5.3.A.2.v). For a quick review, check the Topic 5.3 study guide (https://library.fiveable.me/ap-biology/unit-5/mendelian-genetics/study-guide/SdlMbZYAD4sxuXuRygPv) and practice pedigree/Punnett problems (https://library.fiveable.me/practice/ap-biology).

Start by writing the parents’ genotypes and the alleles each can put into gametes. Use a Punnett square for clarity or apply the probability rules from the CED: - “And” (both events happen): multiply probabilities (P(A and B) = P(A) × P(B)). - “Or” (either event): add mutually exclusive probabilities (P(A or B) = P(A) + P(B)). Quick steps: 1. Determine parental genotypes (e.g., Aa × Aa). 2. List gametes (each parent makes A or a with given probs). 3. Use a Punnett square or multiply gamete probs to get genotype probs. 4. Convert genotype → phenotype using dominance. Examples: - Monohybrid Aa × Aa → P(aa) = 1/4 (Punnett) or (1/2 × 1/2). - Dihybrid AaBb × AaBb, probability offspring shows both dominant traits = P(A_) × P(B_) = (3/4) × (3/4) = 9/16 (independent assortment). For AP-style practice and to match EK 5.3.A.1/ii and the CED probability rules, try problems in the Topic 5.3 study guide (https://library.fiveable.me/ap-biology/unit-5/mendelian-genetics/study-guide/SdlMbZYAD4sxuXuRygPv) and hundreds of practice questions (https://library.fiveable.me/practice/ap-biology).