Inheritance Patterns and Pedigrees

Use allele notation, genetic crosses, and pedigree patterns to infer how traits are inherited across generations.

Lesson progress

Distinguish genotype and phenotypeRecognize autosomal dominant, autosomal recessive, and X-linked pedigree cluesUse probability in monohybrid crossesExplain how linkage can alter expected ratios

Inheritance problems connect alleles to phenotypes and then use probability to predict offspring. Pedigrees extend the same logic across several generations.

Definition

Genotype

The alleles an individual carries for a gene or set of genes.

Definition

Phenotype

The observable characteristic produced by genotype together with environmental influences.

Autosomal dominant inheritance

A dominant phenotype generally appears when at least one dominant allele is present. In a typical autosomal dominant pedigree, affected people often appear in successive generations and both sexes can be affected.

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Autosomal dominant pedigree

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Autosomal recessive inheritance

A recessive phenotype usually requires two recessive alleles. Unaffected heterozygous carriers can pass the allele on, so affected children can be born to two unaffected parents.

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Autosomal recessive pedigree with carriers

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X-linked inheritance

For X-linked traits, inheritance depends on both allele dominance and the sex chromosomes inherited. Males have one X chromosome, so a recessive allele on that X is expressed without a second copy to mask it. Father-to-son transmission does not occur for X-linked alleles because fathers pass a Y chromosome to sons.

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X-linked dominant inheritance pattern

Reference asset tracked internally.

Pedigree clues

Pattern	Useful clues
Autosomal dominant	Often vertical across generations; affected individual often has an affected parent; both sexes affected.
Autosomal recessive	Can skip generations; unaffected parents may have affected children; both sexes affected.
X-linked recessive	Often more males affected; no father-to-son transmission; affected father passes the allele to all daughters.
X-linked dominant	No father-to-son transmission; affected father passes the dominant allele to all daughters.

Worked example

Monohybrid probability workflow

Write clear allele symbols and state which allele is dominant when known.
Identify parental genotypes from the information given.
List possible gametes from each parent.
Combine gametes in a Punnett square or use probability rules.
Report genotype and phenotype probabilities separately.

Linkage

Genes on the same chromosome can be inherited together more often than genes on different chromosomes. Crossing over can separate linked alleles, and the recombination frequency generally increases as the distance between loci increases.

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