AP Biology Unit 5 Heredity: Beyond Mendel (Patterns, Environment, and Chromosomes)

Non-Mendelian Genetics

Mendel’s original pea-plant traits were carefully chosen because they behave in a “simple” way: one gene, two alleles, complete dominance, and independent assortment. Non-Mendelian genetics is the umbrella term for inheritance patterns that do not follow those simple rules. The key idea is not that Mendel was “wrong,” but that his rules are special cases within a much bigger framework of how genes produce traits.

Why this matters in biology (and on the AP exam): most real traits involve gene interactions, multiple alleles, or nuanced genotype-to-phenotype relationships. If you only memorize Punnett squares for complete dominance, you’ll be confused when a heterozygote looks “in between,” when two genes interact, or when phenotype ratios change.

Incomplete dominance

In incomplete dominance, neither allele fully masks the other. The heterozygote has a phenotype intermediate between the two homozygotes.

How it works: alleles often encode proteins (or affect protein expression). If having only one functional copy of an allele produces less pigment/enzyme than two copies, the heterozygote may show a “partial” phenotype.

What to expect: in a cross of two heterozygotes, the phenotypic ratio often matches the genotypic ratio (1:2:1), because each genotype looks different.

Example (worked): Suppose flower color shows incomplete dominance:

  • RR = red
  • Rr = pink
  • rr = white

Cross Rr × Rr.

  • Genotypes: 1 RR : 2 Rr : 1 rr
  • Phenotypes: 1 red : 2 pink : 1 white

Common misconception: students sometimes label this as “blending inheritance” (as if alleles permanently mix). It’s not blending in that sense—alleles remain discrete and can reappear unchanged in later generations.

Codominance

In codominance, both alleles are fully expressed in the heterozygote, so you see both phenotypes at the same time.

How it works: each allele may produce a distinct protein product, and in heterozygotes both products are made.

Classic example: the ABO blood group system. The alleles (often written as I with superscripts in some textbooks) include A, B, and O:

  • A and B are codominant with each other.
  • O is typically recessive to both A and B.

So an AB individual expresses both A and B antigens on red blood cells.

Codominance vs incomplete dominance: both involve heterozygotes that differ from either homozygote, but:

  • Incomplete dominance: heterozygote is intermediate.
  • Codominance: heterozygote shows both distinctly.

Multiple alleles (more than two options in a population)

Multiple alleles means a gene has more than two allele forms in the population (even though each individual still carries only two alleles—one per homologous chromosome).

Why it matters: multiple alleles create more possible genotypes and phenotypes than a two-allele model.

Example: ABO blood type has three alleles in the population (A, B, O), producing six common genotypes.

Lethal alleles and distorted ratios

A lethal allele causes death when present in a particular genotype (often homozygous). This can “remove” a genotype class from observed offspring and distort typical Mendelian ratios.

How it works: if one genotype dies before birth (or before being counted), you will not observe it among living offspring. The surviving offspring ratios are therefore calculated among the living.

Example concept: If a dominant coat-color allele in mice is lethal when homozygous, then a cross of two heterozygotes may yield a 2:1 ratio among living offspring instead of 3:1.

Common mistake: treating a missing genotype as “impossible by inheritance.” It may be produced but not survive.

Pleiotropy (one gene, multiple effects)

Pleiotropy occurs when one gene influences multiple traits.

How it works: a single gene product may be used in multiple tissues or pathways, so changing that gene can have wide-ranging effects.

Why it matters: pleiotropy helps explain why a single mutation can produce a syndrome with seemingly unrelated symptoms.

Epistasis (genes interacting)

Epistasis is an interaction in which one gene affects (masks, modifies, or enables) the expression of another gene.

How it works: many traits depend on pathways. If Gene A is needed to make a pigment precursor and Gene B converts it to the final pigment, then a nonfunctional Gene A can prevent pigment regardless of Gene B’s genotype.

Why it matters: epistasis is a major reason that dihybrid crosses do not always give the classic 9:3:3:1 phenotypic ratio.

Example (conceptual): Labrador retriever coat color is often used to illustrate epistasis. One gene influences pigment color (black vs brown), while another affects whether pigment is deposited in fur at all. When deposition fails, the coat appears yellow regardless of the color gene.

What goes wrong on exams: students often memorize “weird ratios” without explaining the biological mechanism. AP questions frequently reward connecting the ratio shift to a pathway explanation.

Polygenic inheritance (many genes, continuous variation)

In polygenic inheritance, many genes contribute to one trait. These traits often show continuous variation (a range), not discrete categories.

How it works: each gene may contribute a small additive effect to the phenotype. Environmental factors often also contribute, so you see a bell-shaped distribution in a population.

Examples: height and skin pigmentation are commonly described as polygenic.

Important connection: Polygenic traits are also where environmental effects are especially noticeable—you’re often seeing genotype, environment, and their interaction.

Penetrance and expressivity (how consistently a genotype shows)

These terms are useful when the same genotype does not always produce the same observable outcome.

  • Penetrance: the proportion of individuals with a genotype who actually show the phenotype.
  • Expressivity: how strongly the phenotype is expressed among those who show it.

Why it matters: they help explain why pedigrees sometimes look “messy,” even when a trait is genetic.

Worked practice: identifying the pattern from offspring ratios

A common AP-style skill is: given a cross and offspring phenotypes, infer the inheritance pattern.

Practice scenario: A red flower crossed with a white flower produces all pink offspring.

  • This strongly suggests incomplete dominance because the heterozygote is intermediate (pink).

If those pink offspring are crossed and you observe about 1 red : 2 pink : 1 white, that supports incomplete dominance.

Exam Focus
  • Typical question patterns:
    • You’re given a description of heterozygotes (intermediate vs both traits) and asked to name the pattern (incomplete dominance vs codominance).
    • You’re given offspring ratios that deviate from 3:1 or 9:3:3:1 and asked to explain gene interaction (epistasis, lethal alleles, polygenic traits).
    • You’re asked to justify a pattern using a biological mechanism (protein function, pathway steps), not just a label.
  • Common mistakes:
    • Assuming any “different-looking heterozygote” is incomplete dominance; AB blood type is codominance, not intermediate blending.
    • Treating polygenic traits as single-gene traits with dominance; continuous variation is a clue that many genes (and often environment) are involved.
    • Ignoring that lethal genotypes can be produced but not observed among living offspring.

Environmental Effects on Phenotype

A phenotype is the observable expression of traits (what you can measure or categorize), while a genotype is the underlying allele combination. In real organisms, phenotype is almost never determined by genotype alone. Environmental effects on phenotype means that external conditions can influence gene expression and biological processes, changing the trait you observe.

Why this matters: AP Biology emphasizes that DNA information flows through gene expression to proteins and traits. The environment can affect that flow at many points—by influencing transcription, translation, protein function, and development. Understanding this helps you interpret experiments, explain variation, and avoid the trap of genetic determinism (“genes are destiny”).

The big idea: genotype sets a range, environment helps pick the outcome

A helpful way to think about many traits is that genotype can define a range of possible phenotypes, and the environment influences where within that range an individual ends up.

This is especially visible for:

  • Polygenic traits (height, skin pigmentation)
  • Traits influenced by nutrition, temperature, light, stress, or toxins

Gene expression is environment-sensitive

The environment can influence phenotype because gene expression is regulated. Cells turn genes on/off and adjust how much mRNA/protein is produced.

Environmental factors can act by:

  • Activating signaling pathways that turn transcription factors on/off
  • Changing hormone levels that alter gene expression across tissues
  • Affecting protein folding and enzyme activity (temperature, pH)

Even if the DNA sequence does not change, the phenotype can.

Temperature-sensitive phenotypes (protein function)

Some phenotypes depend on enzymes whose activity changes with temperature. If an enzyme involved in pigment production works better at cooler temperatures, pigment may form more in cooler body regions.

Why it matters: this connects environment to protein structure and function—a core AP theme.

Nutrition and metabolism: classic gene-environment interactions

A genotype may create a vulnerability that only becomes apparent under certain diets.

Example (conceptual): Phenylketonuria (PKU) is often taught as a gene-diet interaction. Individuals with certain genotypes have difficulty metabolizing phenylalanine; a low-phenylalanine diet can prevent severe outcomes.

Key lesson: the presence of an allele associated with a disorder does not always mean the phenotype is unavoidable; environment and intervention can matter.

Hydrangea color and soil chemistry (environment affecting pigment)

Hydrangea flower color is often cited as a trait affected by soil conditions (such as pH and mineral availability), which influence pigment chemistry.

Key lesson: when the environment changes the chemical context in which pigments form, phenotype can shift without any genetic change.

Distinguishing environmental effects from genetic inheritance in data

AP questions often test whether you can reason from patterns:

  • If identical genotypes show different phenotypes in different environments, that supports environmental influence.
  • If phenotype differences track with genetic relatedness (and controls rule out environment), that supports genetic influence.

Example: designing an experiment (AP skill)

Suppose you suspect temperature affects pigment in a species of insect.

A strong design would:

  • Use individuals with the same genotype (or as genetically similar as possible)
  • Split them into groups raised at different temperatures
  • Measure pigment phenotype quantitatively
  • Control other variables (diet, light, humidity)

Common misconception: thinking environmental effects are “not genetic.” The trait can still be genetic if genes enable sensitivity to environment. What changes is expression, not necessarily inheritance.

Epigenetics (careful, but useful)

In AP Biology, you may see epigenetic regulation described as heritable changes in gene expression that do not involve changes to the DNA sequence (for example, DNA methylation patterns). Environmental factors can influence epigenetic marks, which can change gene expression.

What to keep straight:

  • Epigenetic changes affect expression.
  • They do not change the nucleotide sequence.
  • Not all environmentally induced changes are epigenetic; some are direct physiological effects on proteins or development.
Exam Focus
  • Typical question patterns:
    • You’re shown organisms with the same genotype raised in different environments and asked to explain phenotype differences using gene expression/protein activity.
    • You’re asked to predict how changing an environmental variable (temperature, diet, pH) changes phenotype and justify the mechanism.
    • You’re asked to evaluate an experimental design for testing environmental effects (controls, variables, sample size, and what is being measured).
  • Common mistakes:
    • Claiming the environment “changes the genes” when the prompt is about gene expression (DNA sequence usually stays the same).
    • Forgetting controls: if multiple environmental variables change at once, you cannot attribute phenotype differences to one factor.
    • Treating genotype and environment as competing explanations; many traits are best explained by their interaction.

Chromosomal Inheritance

Mendelian genetics tracks alleles as abstract units, but in real cells alleles live on chromosomes. Chromosomal inheritance explains patterns of inheritance by considering where genes are located on chromosomes and how chromosomes behave during meiosis.

Why this matters: many “non-Mendelian” outcomes actually come from chromosome behavior—especially genes on the same chromosome (linkage), genes on sex chromosomes (sex linkage), and errors during meiosis (nondisjunction). These topics connect heredity directly to meiosis and fertilization, which are central to Unit 5.

Linked genes and why independent assortment sometimes fails

Mendel’s law of independent assortment applies when genes are on different chromosomes (or far apart on the same chromosome). But if two genes are close together on the same chromosome, they are linked and tend to be inherited together.

How it works:

  • During meiosis I, homologous chromosomes separate.
  • If Gene A and Gene B are on the same chromosome, the alleles on that chromosome can travel together into the same gamete.
  • Crossing over (recombination) during prophase I can swap segments between homologs, producing new allele combinations.

So linkage is not “absolute.” The closer two genes are, the less likely a crossover will separate them, and the fewer recombinant gametes you get.

Recombination frequency and gene mapping (basic idea)

A useful way to quantify linkage is recombination frequency: the percentage of offspring that are recombinant types.

\text{recombination frequency} = \frac{\text{number of recombinant offspring}}{\text{total offspring}} \times 100\%

How to interpret it:

  • Higher recombination frequency suggests genes are farther apart.
  • Lower recombination frequency suggests genes are closer together.

In many AP contexts, you’ll also see the idea that recombination frequency can be used to build a genetic map (relative distances along a chromosome).

Common misconception: thinking crossing over happens between sister chromatids. In standard meiotic recombination, crossing over occurs between non-sister chromatids of homologous chromosomes.

Worked problem: detecting linkage from a testcross

A classic way to test linkage is a testcross: cross an individual heterozygous for two genes with an individual homozygous recessive for both.

Scenario: An organism has two genes, A/a and B/b. You testcross an AaBb individual with aabb. You observe these offspring:

  • 420 A B phenotype
  • 380 a b phenotype
  • 110 A b phenotype
  • 90 a B phenotype

First, identify parental vs recombinant classes. The two most common categories are usually the parental types (because linkage keeps them together):

  • Parental: AB (420) and ab (380)
  • Recombinant: Ab (110) and aB (90)

Compute recombination frequency:

  • Recombinants = 110 + 90 = 200
  • Total = 420 + 380 + 110 + 90 = 1000

\text{recombination frequency} = \frac{200}{1000} \times 100\% = 20\%

Interpretation: the genes are linked, and about 20% recombination suggests a moderate distance between them.

What goes wrong: students sometimes assume any deviation from 1:1:1:1 is linkage, but small sample sizes can deviate by chance. In AP free-response, you’re often expected to use reasoning plus the magnitude of deviation.

Sex-linked inheritance (genes on sex chromosomes)

In humans and many other organisms, sex is determined by sex chromosomes (often X and Y). Sex-linked traits usually refer to genes on the X chromosome because the X carries many more genes than the Y.

Key term: X-linked inheritance. Males (XY) have only one X, so they are hemizygous for X-linked genes—whatever allele is on their single X is expressed.

How it changes inheritance patterns:

  • Recessive X-linked traits appear more often in males because they do not need two recessive copies.
  • An affected male passes his X chromosome to all daughters and to none of his sons (sons get the Y).

Important caution: Do not say “sex-linked means only males are affected.” Females can be affected; it’s just less likely for recessive X-linked traits because females need two recessive alleles.

Example: reasoning through an X-linked recessive cross

Let X with allele N be normal and X with allele n be recessive affected. Consider:

  • Mother is a carrier: XN Xn
  • Father is unaffected: XN Y

Possible children:

  • Sons get Y from father and one X from mother:
    • 50% XN Y unaffected
    • 50% Xn Y affected
  • Daughters get XN from father and one X from mother:
    • 50% XN XN unaffected
    • 50% XN Xn carriers (typically unaffected for recessive trait)

This kind of reasoning is often tested in pedigree questions.

Sex chromosome inactivation (dosage compensation)

In mammals, females have two X chromosomes while males have one. Many mammals use X-inactivation (forming a Barr body) to balance gene dosage between sexes.

Why it matters: it helps explain mosaic phenotypes in heterozygous females for X-linked traits (different cells may inactivate different X chromosomes).

Nondisjunction and chromosomal abnormalities

Nondisjunction is an error in meiosis where homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate properly.

How it works:

  • If chromosomes do not separate, some gametes receive an extra chromosome (or chromatid) and some receive none.
  • After fertilization, the zygote may have an abnormal chromosome number.

Two major categories:

  • Aneuploidy: having an abnormal number of a particular chromosome (for example, a trisomy).
  • Polyploidy: having extra full sets of chromosomes (more common in plants).

Why it matters for inheritance patterns: nondisjunction can cause traits and disorders that do not follow allele-based Mendelian ratios because the chromosome number itself is altered.

Common misconception: believing nondisjunction happens during mitosis in gamete formation. Gametes are produced by meiosis; nondisjunction relevant to inherited aneuploidies occurs during meiosis in one parent.

Cytoplasmic (mitochondrial) inheritance

Not all genes are in the nucleus. Mitochondria have their own DNA, and in many animals mitochondrial inheritance is primarily maternal because the egg contributes most of the cytoplasm (and thus most mitochondria) to the zygote.

How it shows up in pedigrees:

  • Affected mothers can pass mitochondrial traits to all offspring.
  • Affected fathers typically do not pass mitochondrial traits to offspring.

Why it matters: it’s a clear exception to classic Mendelian nuclear inheritance and a favorite AP reasoning prompt when pedigrees don’t match autosomal or X-linked patterns.

Exam Focus
  • Typical question patterns:
    • You’re given offspring counts from a dihybrid testcross and asked whether genes are linked, then calculate recombination frequency and interpret it.
    • You’re given a pedigree and asked to determine whether a trait is autosomal, X-linked, or mitochondrial, with justification.
    • You’re asked to explain how nondisjunction during meiosis could produce a gamete leading to aneuploid offspring.
  • Common mistakes:
    • Mixing up meiosis I vs meiosis II nondisjunction (homologs vs sister chromatids). Focus on what fails to separate.
    • Assuming linked genes never recombine; crossing over can create recombinants, just at reduced frequency.
    • Confusing maternal inheritance (mitochondrial) with X-linked inheritance; mitochondrial patterns show transmission from mothers to all children, not the crisscross pattern typical of X-linked traits.