Pedigree Analysis

What a pedigree actually is

A pedigree is a diagram of biological relationships drawn with a fixed alphabet of symbols, so that any geneticist in the world can read it the same way. Males are squares, females are circles, and an individual whose sex is unrecorded is a diamond. A symbol is shaded solid if the person shows the trait (is affected) and left open if they do not. A horizontal line joining a square and a circle marks a mating; a short vertical line drops from the middle of that mating line to a horizontal sibship line, and the children hang from it in birth order, oldest on the left. Twins branch from a single point, with a connecting bar for identical twins. A diagonal slash through a symbol means the individual has died, and a small dot inside an open symbol, or a half-shaded symbol, flags a known carrier.

Generations are stacked top to bottom and labeled with Roman numerals on the left margin — generation I at the top, then II, III, and so on. Within a generation, individuals are numbered left to right with Arabic numerals, so "II-3" unambiguously names the third person in the second generation. The person who brought the family to clinical attention — the proband or index case — is marked with an arrow. None of this is decoration: the standardized notation (formalized by Bennett and colleagues in 1995 for human genetics) is what lets the diagram function as data rather than a sketch.

How you deduce the pattern

The whole exercise is a logic puzzle constrained by Mendelian segregation. Every diploid individual carries two alleles at a locus; each parent passes exactly one, chosen at random, to each child. Whether a single inherited allele is enough to produce the phenotype (dominant) or whether two are required (recessive), and whether the gene sits on an autosome or on a sex chromosome, leaves distinct fingerprints in the family tree. Reading a pedigree means matching the observed pattern of filled and open symbols to the inheritance mode that could have produced it — and ruling out the modes that could not.

Three questions do most of the work. Does the trait skip generations? If unaffected parents repeatedly produce affected children, the allele is hiding in heterozygotes, which means it is recessive. Does it appear in every generation with an affected parent always present? That is the signature of a dominant allele, which expresses itself even in a single dose. Does it favor one sex? A strong male bias points to the X chromosome, because males are hemizygous — they have only one X, so a single recessive allele has no second copy to mask it.

Once the mode is identified, the pedigree becomes quantitative. You assign genotypes where they are forced — an affected child of two unaffected parents must be aa, so both parents must be Aa — and then propagate probabilities forward. A child of two Aa carriers has a 1/4 chance of being affected (aa), a 1/2 chance of being an unaffected carrier (Aa), and a 1/4 chance of being homozygous normal (AA); an unaffected such child therefore has a 2/3 conditional probability of being a carrier, since the aa outcome has been excluded by observation. Stacking these conditional probabilities, often with Bayes' theorem, is how a counselor turns a hand-drawn tree into a recurrence risk for the next pregnancy.

The five inheritance patterns side by side

Each mode of inheritance produces a recognizable pedigree silhouette. The table below lists the diagnostic features used to tell them apart; in practice a clinician looks for several of these clues at once, because any single family is small enough that chance can mimic the wrong pattern.

Pattern	Skips generations?	Sex bias	Key diagnostic clue	Example disorder
Autosomal dominant	No — appears each generation	None (M = F)	Every affected child has an affected parent; ~50% of children affected	Huntington's disease, achondroplasia
Autosomal recessive	Yes — often skips	None (M = F)	Two unaffected (carrier) parents → ~25% affected; consanguinity common	Cystic fibrosis, sickle-cell anemia
X-linked recessive	Yes — via carrier females	Strong male excess	No father-to-son transmission; affected males via carrier mothers (criss-cross)	Hemophilia A, red-green color blindness, Duchenne muscular dystrophy
X-linked dominant	No	Female excess (2:1)	Affected fathers pass it to all daughters, no sons; no father-to-son transmission	Hypophosphatemic rickets, Rett syndrome
Y-linked (holandric)	No	Males only	Father-to-son transmission to every son; no females ever affected	Y-chromosome male infertility factors

The two recessive patterns share the skip-a-generation behavior but split on the sex ratio. In autosomal recessive conditions the affected children are split roughly evenly between sons and daughters, and the tell-tale that elevates suspicion is a mating between relatives — first cousins share about 1/8 of their genome, so a rare allele carried by a common grandparent has a much better chance of meeting itself. In X-linked recessive conditions the affected individuals are overwhelmingly male, and the females who tie affected males together across the tree are unaffected carriers. The single sharpest discriminator is father-to-son transmission: because a father gives his Y chromosome (not his X) to every son, any X-linked trait cannot pass directly from a father to his sons, so even one clear instance of an affected father with an affected son rules X-linkage out.

Why the numbers matter: carriers and risk

The arithmetic of pedigrees is the arithmetic of carrier frequency. Cystic fibrosis is autosomal recessive with a disease incidence of roughly 1 in 2,500 births in populations of Northern European ancestry. Working backward through the Hardy–Weinberg relationship, an incidence of q² ≈ 1/2,500 gives an allele frequency q ≈ 1/50 and a carrier frequency 2pq ≈ 1/25 — about 4% of that population carries one copy without knowing it. That is why a recessive disease can appear in a family with no prior history: two unsuspecting carriers simply had to meet, each with a 1/2 chance of transmitting the allele, giving a 1/4 chance per child of an affected (aa) offspring.

Sickle-cell anemia shows how selection shapes those frequencies. In parts of West Africa the carrier (heterozygous, HbA/HbS) frequency reaches 20–25% because carriers are partially protected against Plasmodium falciparum malaria — a textbook case of heterozygote advantage that keeps a damaging recessive allele common. A pedigree from such a region will therefore show recessive inheritance against a high background carrier rate, and a counselor must use the local allele frequency, not a global average, when computing risk. The lesson is that a pedigree is read in the context of a population: the same pattern of symbols implies a very different recurrence risk in Lagos than in Reykjavík.

Hemophilia and the royal pedigree

The most famous pedigree in history is the descent of hemophilia B through the royal houses of Europe from Queen Victoria. Victoria was an unaffected carrier of an X-linked recessive clotting-factor mutation; her son Leopold was affected and died at 30 from a brain hemorrhage after a fall, while at least two of her daughters were carriers who married into the Spanish, German, and Russian royal families. The criss-cross signature is textbook: the allele passed from carrier mothers to affected sons and carrier daughters, never father to son, surfacing in Tsarevich Alexei of Russia three generations later. The pedigree was the only diagnostic tool available — the clotting factors themselves were not identified until the twentieth century — yet it correctly traced the path of a gene no one could see, and DNA analysis of the Romanov remains in 2009 finally confirmed the specific F9 mutation responsible.

Clinical and evolutionary significance

• Genetic counseling. A three-generation family history is the standard intake for any inherited-disease consult; the pedigree converts that history into a recurrence risk for future children.
• Carrier screening. Identifying the inheritance mode tells the clinician who to test — for X-linked conditions, the at-risk individuals are the proband's sisters and maternal aunts.
• Hereditary cancer. Pedigrees flag dominant cancer-predisposition syndromes such as BRCA1/BRCA2 breast-ovarian cancer and Lynch syndrome, where a striking vertical pattern of early-onset cancers prompts genetic testing.
• Gene mapping. Before DNA sequencing, linkage analysis across large pedigrees located disease genes by tracking co-inheritance with marker loci — how the Huntington's and cystic-fibrosis genes were first found.
• Population genetics. Patterns of consanguinity and founder effects visible in extended pedigrees explain why specific recessive alleles cluster in particular communities.

Common mistakes when reading pedigrees

• Confusing "rare" with "recessive." Many dominant disorders are also rare; rarity raises suspicion of recessiveness but does not prove it.
• Forgetting incomplete penetrance. Some dominant alleles fail to express in a carrier, making the trait appear to skip and mimicking recessive inheritance.
• Ignoring new mutations. A dominant condition can appear with no affected parent if the allele arose de novo in the egg or sperm — common in achondroplasia.
• Over-reading small families. With only one or two children, a 1/4 or 1/2 ratio cannot be distinguished from chance; pedigree inference needs enough individuals.
• Missing the father-to-son test. A single affected father–affected son pair is enough to exclude X-linkage, yet it is easy to overlook.