Patterns of Inheritance

Core Concepts

The vocabulary of inheritance

Each human cell (except gametes) has 46 chromosomes in 23 pairs: 22 pairs of autosomes plus one sex pair. A gene is a DNA segment coding for a trait; its location is the locus. Most genes come in versions called alleles: identical alleles mean homozygous; different alleles mean heterozygous. The genotype is the allele pair; the phenotype is the observable result. A dominant allele is expressed even paired with a different allele; a recessive allele needs two copies.

Mendel's laws and Punnett squares

Two of Mendel's principles organize simple inheritance. Segregation: each parent has two alleles per trait but passes one to each gamete, so offspring get one from each parent. Independent assortment: alleles of genes on different chromosomes pass independently. A Punnett square grids possible offspring genotypes from parental gametes. Two heterozygotes (Aa × Aa) yield a 3:1 phenotype ratio and 1:2:1 genotypes; a dihybrid cross (AaBb × AaBb) yields 9:3:3:1.

Autosomal dominant inheritance

One copy of the mutant allele produces the trait. Affected people usually have an affected parent; both sexes are affected equally; an affected heterozygote has roughly a 50% chance of passing the allele to each child; the trait generally does not skip generations. Commonly taught examples: Huntington disease and achondroplasia.

Autosomal recessive inheritance

Two copies of the mutant allele are required. Carriers — people with one copy — are typically unaffected but can pass the allele on. Two carriers have, per child, a 25% chance of an affected child, 50% of a carrier, and 25% of no copy. The trait often seems to skip generations and may cluster in siblings. Commonly taught examples: cystic fibrosis, sickle cell disease, Tay-Sachs disease.

X-linked inheritance

Genes on the X chromosome behave differently because males have one X and females two. In X-linked recessive inheritance, males are affected far more often: a male needs one copy, a female two. A carrier mother passes the allele to half her sons (affected) and half her daughters (carriers); an affected father passes his X to all daughters (carriers) but never to sons. Examples: hemophilia A and red-green color blindness. X-linked dominant conditions affect both sexes; Y-linked traits pass father-to-son.

Beyond simple dominance

Beyond simple dominance, patterns get richer. Incomplete dominance: the heterozygote shows an intermediate phenotype (some flower colors). Codominance: both alleles are fully expressed — the ABO blood system is the classic example: Iᴬ and Iᴮ are codominant while i is recessive, so three alleles yield four phenotypes (A, B, AB, O). Polygenic inheritance: many genes plus environment produce continuous variation (height, skin color). Pleiotropy: one gene affects multiple traits (Marfan syndrome, a commonly taught example). Penetrance/expressivity: some genotypes do not always produce the expected phenotype. Mitochondrial inheritance: mitochondria come almost entirely from the egg, so mitochondrial disorders pass from mothers to all children.

Chromosomal abnormalities

Nondisjunction — failure of chromosomes to separate during meiosis — yields gametes with an extra or missing chromosome; fertilization produces aneuploidy. Trisomy 21 (Down syndrome) is the most common survivable autosomal trisomy, with risk commonly taught to increase with maternal age. Sex-chromosome aneuploidies include Turner syndrome (45, X) and Klinefelter syndrome (47, XXY).

Pedigrees and genetic counseling

A pedigree is a family tree recording who is affected across generations; reading one is how clinicians identify the mode of inheritance. Genetic counseling translates patterns into recurrence risks and discusses testing options. This guide explains the biology; specific risk figures and testing guidance belong to genetics professionals.

Common Confusions