Inheritance patterns describe how traits pass through families. AP Biology emphasises recognising autosomal versus sex-linked patterns and using pedigree evidence to infer likely genotypes and predict risk in offspring.

Core idea: tracking alleles through pedigrees

A pedigree is a family diagram showing phenotypes across generations; it helps infer whether a trait is autosomal or sex-linked, and whether it is likely dominant or recessive.

Standard pedigree symbols used to represent sex (square = male, circle = female), affected status (filled symbols), carrier status (partially filled or marked symbols), and other common annotations (e.g., deceased individuals). Using a consistent symbol key is essential because pedigree analysis relies on recognizing transmission patterns across generations rather than on individual cases. Source

When analysing pedigrees, focus on:

• Which sexes are affected and in what proportions

• Whether the trait appears in every generation or skips generations

• Whether affected fathers pass the trait to sons (male-to-male transmission)

• Whether unaffected parents can produce affected offspring (suggests recessive inheritance)

Autosomal inheritance patterns

Autosomal dominant (often “vertical” pattern)

Typical clues:

• Appears in every generation if fully penetrant

• Affected individuals usually have an affected parent

• Males and females affected at similar rates

• Father-to-son transmission can occur (strong evidence it is not X-linked)

Genotype logic:

• Many affected individuals are heterozygous (Aa), because homozygous dominant may be rare or severe.

• Unaffected individuals are typically aa and do not transmit the dominant phenotype.

Autosomal recessive (often “horizontal” pattern)

Typical clues:

• Can skip generations

• Unaffected parents may have affected children (both parents often carriers)

• Males and females affected at similar rates

• Increased frequency with consanguinity (shared ancestry raises the chance of shared recessive alleles)

Sex-linked inheritance patterns (primarily X-linked)

Because males are XY, they are hemizygous for most X-linked genes: a single recessive allele on the X chromosome can be expressed in the phenotype.

X-linked recessive

Common pedigree clues:

Conceptual family diagram illustrating X-linked recessive inheritance, emphasizing that males (XY) express a recessive X-linked allele with just one affected X, while females usually require two affected X chromosomes to express the trait. The figure also clarifies why affected fathers do not pass X-linked traits to sons (sons inherit the Y from their father) but can pass the affected X to all daughters. Source

• More males affected than females

• Affected males are often born to unaffected (carrier) mothers

• No father-to-son transmission (fathers give sons the Y chromosome)

• Daughters of affected fathers receive the father’s X and are at least carriers if the mother provides a normal allele

Key transmission ideas:

• Carrier mothers can pass the recessive allele to sons (affected) or daughters (carriers/affected depending on father)

• Female expression typically requires two recessive alleles, so affected females are less common and often indicate an affected father plus a carrier/affected mother

X-linked dominant

Common pedigree clues:

• Often appears in every generation

• No father-to-son transmission

• Affected fathers pass the trait to all daughters (they receive his only X) and no sons

• Affected mothers can pass the trait to sons or daughters

Y-linked (rare)

Typical clues:

• Only males affected

• Father-to-son transmission in every generation This pattern is uncommon because the Y chromosome contains relatively few genes.

Genetically linked inheritance (pattern-level recognition)

Genes on the same chromosome are genetically linked, so allele combinations may be inherited together more often than expected by chance.

Diagram comparing unlinked genes (independent assortment) with linked genes at different distances, showing how recombination produces recombinant (nonparental) offspring. It highlights the key quantitative idea that recombination frequency ranges from 0% (complete linkage) up to 50% (effectively unlinked), which is what creates deviations from expected independent-assortment ratios. Source

In family data, linkage can look like:

• Two traits co-segregating (appearing together in offspring) across generations

• Deviations from expected independent-assortment ratios, especially in larger datasets Pedigrees can suggest linkage, but confirming linkage typically requires more offspring data and recombination analysis.