Mendelian Concepts

Genetics connects molecular biology to whole-organism and population biology, and Mendelian concepts are its grammar. Master these ideas and genetics passages — from pedigrees to population genetics — become far easier to decode.

Priority labels: Must know = cold; Know the logic = mechanism not names; Passage-level = recognize, don't memorize; Optional = skippable.

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The Gene as the Unit of Heredity

Must know

A gene is a discrete, heritable unit of information in DNA that influences one or more traits. The key Mendelian insight: inheritance is particulate — traits pass as discrete packages, not blended fluids (which is why a trait can disappear in one generation and reappear in the next).

Each gene sits at a specific physical address on a chromosome, its locus (plural loci). Every diploid organism carries two copies of each autosomal locus, one per homologous chromosome.

Mendel's Laws — The Foundation

Must know

Law of Segregation: The two alleles at a locus separate during gamete formation, so each gamete carries exactly one allele per gene (this is homologs pulling apart in meiosis I).

Law of Independent Assortment: Alleles at different loci assort into gametes independently — provided the loci are on different (non-homologous) chromosomes. Linked genes (same chromosome) violate this. For the MCAT, independent assortment gives the 9:3:3:1 dihybrid ratio.

Quick check: A plant is $Aa$. What fraction of its gametes carry the $A$ allele?
Answer: 1/2. Segregation gives each gamete exactly one of the two alleles, so half carry $A$ and half carry $a$.

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Phenotype and Genotype

Must know

Genotype = the specific alleles an organism carries at one or more loci. Phenotype = the observable characteristics resulting from genotype interacting with the environment (phenotype = genotype × environment). Two identical genotypes can give different phenotypes in different environments — the MCAT tests this under penetrance and expressivity (below). For most classical problems, treat genotype → phenotype as a reliable mapping.

Standard notation: dominant = uppercase ($A$), recessive = lowercase ($a$), genotype as a pair ($AA$, $Aa$, $aa$), multiple genes written consecutively ($AaBb$).

Quick check: Two organisms share the same phenotype (both have brown eyes). Does this mean they have the same genotype?
Answer: Not necessarily. One might be $BB$ and the other $Bb$. Only a test cross can distinguish them.

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Alleles: Single and Multiple

Must know

An allele is one of two or more alternative versions of a gene at a locus. Many loci have multiple alleles — more than two versions exist in the population, even though any individual carries only two (one per homolog).

The canonical example is the ABO blood group, with three alleles ($I^A$, $I^B$, $i$). Three alleles create six genotypes and four blood-type phenotypes, and the system illustrates co-dominance plus recessiveness (below).

Quick check: If a locus has 4 alleles in a population, how many alleles does any one diploid individual carry at that locus?
Answer: Still exactly 2 — one per homolog. "Multiple alleles" describes population diversity, not individual genotype.

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Homozygosity and Heterozygosity

Must know

• Homozygous: Both alleles identical ($AA$ or $aa$); breeds true.
• Heterozygous: The two alleles differ ($Aa$); produces 50% $A$ and 50% $a$ gametes.
• Hemizygous: Only a single copy of a gene is present, so there is no partner allele to mask it. The classic case is X-linked genes in males ($XY$) — whatever allele is on his single X is expressed. (This is why X-linked recessive traits appear far more often in males; details in the Variability guide.)

A monohybrid cross between two heterozygotes ($Aa \times Aa$) gives the classic 3:1 phenotypic ratio and 1:2:1 genotypic ratio ($AA : Aa : aa$).

The test cross determines an unknown genotype: cross a dominant-phenotype individual of unknown genotype with a homozygous recessive ($aa$). All dominant offspring → parent was $AA$; half recessive → parent was $Aa$. Because the $aa$ parent contributes only $a$ gametes, the offspring are a direct readout of the unknown parent's gametes.

Quick check: A dominant-phenotype organism is test-crossed with $aa$. Offspring are 50% dominant, 50% recessive. What was the unknown parent's genotype?
Answer: $Aa$. It produces 50% $A$ gametes (→ $Aa$, dominant) and 50% $a$ gametes (→ $aa$, recessive).

Predicting Offspring: Punnett Squares and the Probability Rules

Must know

A Punnett square lists one parent's gametes across the top, the other's down the side, and fills each cell with the combined genotype. For $Aa \times Aa$ the four cells give the 1:2:1 genotypic ratio.

For two or more genes, use the probability rules instead of a large grid:

• Product rule (AND): probability that two independent events both happen = product of their probabilities. Treat each gene as its own monohybrid cross and multiply. In $AaBb \times AaBb$, $P(aabb) = \tfrac{1}{4} \times \tfrac{1}{4} = \tfrac{1}{16}$.
• Sum rule (OR): probability of either of two mutually exclusive outcomes = sum of probabilities. $P(AA \text{ or } aa) = \tfrac{1}{4} + \tfrac{1}{4} = \tfrac{1}{2}$.

The product rule is far faster than a big Punnett square: an $Aa \times Aa$ couple having three $aa$ children in a row is $\left(\tfrac{1}{4}\right)^3 = \tfrac{1}{64}$.

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Wild-Type Allele and Phenotype

Must know

Wild-type = the most common allele or phenotype in a natural population — the "standard" form. It is not necessarily dominant; it just means most prevalent. Notation: $w^+$ or simply $+$.

Know the logic

A loss-of-function mutation is usually recessive (one functional copy suffices); a gain-of-function mutation is often dominant (an aberrant product acts even with a normal allele present). This logic lets you reason about dominance for unfamiliar examples.

Quick check: A new allele causes disease even in heterozygotes. Loss-of-function or gain-of-function?
Answer: Gain-of-function. Loss-of-function is typically recessive because one working copy suffices; a gain-of-function allele dominates even with a normal allele present.

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Recessiveness and Complete Dominance

Must know

Recessiveness: A recessive allele shows its phenotype only in homozygotes ($aa$); in heterozygotes its effect is masked (one functional $A$ copy is enough).

Complete dominance: A dominant allele is fully expressed with only one copy, so the heterozygote looks identical to the dominant homozygote ($AA$ and $Aa$ → dominant; $aa$ → recessive).

Classic monohybrid ratios (complete dominance):

Cross	Phenotypic Ratio	Genotypic Ratio
$Aa \times Aa$	3:1	1:2:1
$Aa \times aa$	1:1	1:1
$AA \times aa$	All dominant	All $Aa$

Quick check: In $Aa \times Aa$, what fraction of offspring show the recessive phenotype?
Answer: 1/4. Only $aa$ shows it, at probability $\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$.

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Co-dominance

Must know

Co-dominance: both alleles are simultaneously and fully expressed in the heterozygote — neither masks the other, and you can see both products.

The prototype is ABO blood type: $I^A$ makes A antigens, $I^B$ makes B antigens, and an $I^A I^B$ individual displays both (type AB) — not an intermediate. (Sickle-cell trait, $Hb^A Hb^S$, is sometimes called co-dominant because both hemoglobins are detectable.)

Quick check: A red-flowered plant ($R^R R^R$) and a white-flowered plant ($R^W R^W$) are crossed. If co-dominant, what do the $F_1$ look like?
Answer: Both pigments fully expressed — patches of both red and white (e.g., "roan"), each color distinct, not blended.

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Incomplete Dominance

Must know

Incomplete dominance: the heterozygote is intermediate between the two homozygotes. The appearance blends, but the alleles stay particulate and reappear unchanged later.

Classic example: snapdragon color. Red ($R^1 R^1$) × white ($R^2 R^2$) → pink $F_1$ ($R^1 R^2$). Crossing two pinks:

$R^1 R^2 \times R^1 R^2 \rightarrow 1\ R^1 R^1 : 2\ R^1 R^2 : 1\ R^2 R^2$

Phenotypically 1 red : 2 pink : 1 white — the 3:1 of complete dominance becomes 1:2:1 because all three genotypes are now distinguishable. (Mechanistically, one functional copy makes some pigment but not as much as two — gene dosage.)

Quick check: $F_1$ pink snapdragons are self-fertilized. What fraction of $F_2$ are pink?
Answer: 1/2. $R^1 R^2 \times R^1 R^2$ gives 1/4 red + 1/2 pink + 1/4 white.

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Pleiotropy and Polygenic Inheritance

Must know

Two departures from one-gene/one-trait Mendelism:

• Pleiotropy: a single gene affects multiple, seemingly unrelated traits. Sickle cell disease is the classic example — one $\beta$-globin mutation causes anemia, pain crises, splenic damage, and stroke risk.
• Polygenic inheritance: a single trait is controlled by many additive genes, producing a continuous (quantitative) distribution rather than discrete classes (e.g., height, skin color). No clean Mendelian ratios — population distributions instead.

Don't confuse them: pleiotropy = one gene → many traits; polygenic = many genes → one trait. (Epistasis — one gene masking another — is a third departure, covered in the genetic-diversity unit.)

Quick check: Human skin color shows a smooth gradient rather than distinct categories. Which pattern explains this?
Answer: Polygenic inheritance. Many genes contribute additively, giving a continuous distribution.

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Leakage, Penetrance, and Expressivity

These explain why real genetics is messier than a Punnett square — among the trickiest topics in this unit.

Penetrance

Must know

Penetrance = the proportion of individuals with a genotype who actually display the phenotype (a population statistic, in %).

• Complete: 100% show the phenotype.
• Incomplete: <100% show it (e.g., BRCA1 — not every carrier develops breast cancer). A dominant allele with 70% penetrance means 30% of heterozygotes look normal, so the trait can appear to skip generations in a pedigree.

Expressivity

Must know

Expressivity = the range of severity among individuals who do show the phenotype. Penetrance asks "did you get it?"; expressivity asks "how severely?"

• Variable expressivity: same genotype, different degrees (e.g., neurofibromatosis type 1 — all affected show symptoms, but severity ranges widely).

Mnemonic: Penetrance = whether; Expressivity = how much.

Leakage

Must know

Leakage: a supposedly recessive allele produces some detectable effect even in heterozygotes — the dominant allele doesn't fully mask it, blurring the line between complete and incomplete dominance.

Quick check: A pedigree shows two unaffected parents with an affected child; next generation, an individual with the disease genotype appears completely normal. Penetrance or expressivity?
Answer: Penetrance. The genotype is present but the phenotype is entirely absent — incomplete penetrance. Expressivity would explain variation in degree, not complete absence.

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Hybridization and Viability

Must know

Hybridization = crossing individuals differing in one or more heritable traits.

• Monohybrid cross: $Aa \times Aa$ → 3:1 phenotypic.
• Dihybrid cross: $AaBb \times AaBb$ → 9:3:3:1 phenotypic = 9 (A_B_) : 3 (A_bb) : 3 (aaB_) : 1 (aabb). Deviations signal interactions between loci (epistasis).

Viability: not all genetic combinations survive.

• Lethal alleles: some homozygotes are embryonic lethal. Classic example: yellow mouse coat ($A^y$) — $A^y A^y$ dies in utero, so two yellow mice ($A^y a \times A^y a$) give an apparent 2:1 ratio instead of 3:1. A 2:1 monohybrid ratio → suspect a homozygous lethal.
• Interspecies hybrids (e.g., mule) may be viable but sterile because differing chromosome numbers prevent meiotic pairing.

Quick check: A cross between two mice yields 2:1 instead of 3:1. Most likely explanation?
Answer: A homozygous lethal allele. One expected class ($AA$) is inviable, eliminating 1/4 of offspring and skewing 3:1 to 2:1 among survivors.

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The Gene Pool of a Population

Hardy-Weinberg Equilibrium — The Baseline

Must know

The gene pool = all alleles across all individuals in a population. Allele frequency = the proportion of a given allele at that locus.

The Hardy-Weinberg principle: in a large, randomly mating population free from evolutionary forces, allele frequencies stay constant. Genotype frequencies follow:

$p + q = 1$

$p^2 + 2pq + q^2 = 1$

where $p$ = freq of $A$, $q$ = freq of $a$; $p^2 = AA$, $2pq = Aa$, $q^2 = aa$.

Five conditions (violating any means evolution is occurring): large population, random mating, no mutation, no gene flow, no natural selection.

Hardy-Weinberg is the standard tool for MCAT problems calculating carrier frequency from disease prevalence.

Worked Example: Carrier Frequency

Must know

PKU is autosomal recessive with incidence 1 in 10,000. Carrier frequency?

• $q^2 = \frac{1}{10{,}000} = 0.0001 \Rightarrow q = 0.01$
• $p = 1 - q = 0.99$
• Carriers $= 2pq = 2(0.99)(0.01) \approx 0.0198 \approx 1/50$

About 1 in 50 are carriers — far more than the 1 in 10,000 affected. High-yield result.

Quick check: For an autosomal recessive condition, which HW term represents affected individuals ($aa$)?
Answer: $q^2$. Start there, then take the square root to find $q$.

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Common Confusions & Tricks

Co-dominance vs. incomplete dominance: Co-dominance = both phenotypes fully present (A and B antigens). Incomplete dominance = a blended intermediate (pink flowers). "Both simultaneously and distinctly" → co-dominance; "intermediate/blended" → incomplete. ABO = co-dominant; snapdragons = incomplete.

Penetrance vs. expressivity: Penetrance = on/off switch (did it show at all?); expressivity = volume knob (how intensely?). "Skipped generations" despite a dominant allele = incomplete penetrance, not incomplete dominance.

Multiple alleles ≠ polyploidy: Multiple alleles = >2 variants in the population; any diploid individual still carries only two.

3:1 vs. 1:2:1: With complete dominance, $Aa \times Aa$ = 3:1 phenotypic, 1:2:1 genotypic. With incomplete dominance, it's 1:2:1 for both. The genotypic ratio is always 1:2:1; only the phenotypic ratio changes with dominance.

The 2:1 lethal allele tip: A 2:1 monohybrid ratio → suspect a homozygous lethal allele.

Test cross logic: Always cross to homozygous recessive ($aa$); offspring ratio directly reveals the unknown parent's gametes and genotype.

Hardy-Weinberg: always start with $q^2$: For autosomal recessive disease, affected = $q^2$. Find $q$, then $p$, then carriers $2pq$. Don't confuse $2pq$ (carriers, normal) with $q^2$ (affected).

Wild-type ≠ dominant: Wild-type is most common in nature, but can be recessive. Don't assume wild-type = dominant.

Locus ≠ allele ≠ gene: Locus = chromosomal address; gene = functional unit there; allele = a specific version of that gene.

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Key Takeaways

• Gene: discrete hereditary unit; locus: chromosomal address; allele: version of a gene at a locus.
• Every diploid individual carries two alleles per locus regardless of how many exist in the population; hemizygous = only one copy (e.g., X-linked genes in $XY$ males), always expressed.
• Punnett squares predict offspring ratios; for multi-gene crosses use the product rule (AND, multiply) and sum rule (OR, add).
• Segregation: alleles separate during gamete formation. Independent assortment: genes on different chromosomes assort independently → 9:3:3:1 dihybrid ratio.
• Complete dominance: $Aa$ looks like $AA$; $Aa \times Aa$ → 3:1 phenotypic, 1:2:1 genotypic.
• Incomplete dominance: $Aa$ is intermediate; $Aa \times Aa$ → 1:2:1 phenotypic AND genotypic.
• Co-dominance: both alleles fully expressed (both products detectable); ABO is the prototype.
• Multiple alleles: >2 versions in population; ABO has $I^A$, $I^B$, $i$; individuals still carry two.
• Penetrance: % of genotype-positive who show phenotype; incomplete → apparent skipping in pedigrees.
• Expressivity: degree of severity; variable expressivity → same genotype, different severity.
• Lethal alleles: homozygous lethal → 2:1 ratio instead of 3:1.
• Test cross ($\times\ aa$): distinguishes $AA$ from $Aa$; all dominant → $AA$; 1:1 → $Aa$.
• Wild-type: most common in wild; not necessarily dominant.
• Hardy-Weinberg: $p + q = 1$; $p^2 + 2pq + q^2 = 1$; for autosomal recessive disease, $q^2$ = disease frequency → carrier frequency $2pq$.
• HWE requires: large population, random mating, no mutation, no migration, no selection.