A pedigree is a diagram that records family relationships together with the presence or absence of a phenotype across generations. Pedigrees are especially useful when controlled crosses are impossible or inappropriate, as in human genetics.

Common conventions include:

• a square for a male individual;
• a circle for a female individual;
• a filled symbol for an individual showing the phenotype being tracked;
• a horizontal line connecting reproductive partners;
• vertical descent lines leading to offspring;
• generations arranged from older to younger.

The symbols record observations. The genetic task is to infer which genotypes and inheritance models are consistent with those observations.

A model is fully penetrant when every individual carrying a phenotype-producing genotype is assumed to show the tracked phenotype. The simple examples below make that assumption.

Autosomal recessive reasoning

Suppose two unaffected parents have an affected child under a fully penetrant single-locus recessive model.

An affected child must have genotype

$$aa.$$

Each parent therefore had to contribute an $a$ allele. Since the parents are unaffected under complete recessiveness, each is inferred to be

$$Aa.$$

Thus

unaffected Aa × unaffected Aa
             ↓
possible affected aa child

For each subsequent child of these same parents, the probability of genotype $aa$ is $1/4$ under the simple Mendelian model.

Autosomal dominant reasoning

For a fully penetrant autosomal dominant phenotype, an unaffected individual normally lacks the phenotype-producing dominant allele. If an affected heterozygous parent $Aa$ and an unaffected parent $aa$ have children, each child has probability

$$\frac12$$

of inheriting $A$.

An affected child with an unaffected parent is therefore consistent with transmission from the affected parent under this simple model. Other biological possibilities, such as a newly arising DNA sequence change, require additional evidence and are not part of the basic Mendelian model.

X-linked clues

Sex-chromosome transmission adds distinctive constraints. For example, an X-linked allele is not transmitted directly from father to son in the ordinary human XY system, because a son receives the father's Y chromosome rather than his X.

An affected father's transmission to daughters versus sons can therefore help distinguish an X-linked model from an autosomal one.

Pedigrees constrain hypotheses; they do not prove them automatically

A small pedigree can be compatible with more than one inheritance model simply by chance. Real traits can also depart from the simple assumptions—for example, a phenotype-producing genotype may fail to produce the phenotype in every carrier, or several genes may contribute.

A sound analysis therefore proceeds by testing models:

1. propose an inheritance mode;
2. assign only genotypes required by observed phenotypes and transmission rules;
3. propagate allele constraints through parent-offspring relationships;
4. reject any model that requires an impossible transmission;
5. calculate offspring probabilities only after consistent parental genotypes are identified.

Pedigree analysis is therefore an exercise in constraint-based genetic inference, not merely visual pattern matching.