Pedigree Analysis

Pedigree analysis is the essential, and in practice indispensable, method used to study exactly how a particular genetic trait or an inherited disorder has been transmitted through the successive generations of a real human family, a method made necessary by the simple fact that controlled, deliberately planned breeding experiments of the precise kind Mendel carried out on garden peas are, for entirely obvious ethical reasons, never possible with human subjects. A pedigree chart records a family's own documented history of a given trait or disorder using a standard, internationally agreed set of graphical symbols, so that any trained geneticist reading the chart can immediately understand the pattern being shown without needing any further explanation: an unaffected male family member is conventionally represented by an empty, unfilled square; an unaffected female family member is represented by an empty, unfilled circle; an AFFECTED male family member is represented by a completely filled-in (solidly shaded) square; and an affected female family member is represented by a completely filled-in (solidly shaded) circle. A horizontal line drawn directly connecting one square symbol to one circle symbol represents a marriage or mating relationship between those two particular individuals, and a further vertical line dropping straight down from the midpoint of this horizontal marriage line, which then typically branches out to connect with further symbols arranged along a lower horizontal line, represents the couple's own offspring — these offspring symbols being conventionally arranged from left to right in their actual birth order, eldest child first. A small solid dot placed inside an otherwise unshaded circle or square symbol, or alternatively a circle or square that is only HALF shaded, is commonly used to represent a phenotypically unaffected CARRIER — an individual who is heterozygous for a particular recessive disease allele but who does not personally show any symptoms of the disorder themselves.

By long-standing convention, the different generations represented within any single pedigree chart are labelled using successive Roman numerals — Generation I at the very top of the chart (representing the family's oldest, most senior generation shown), Generation II immediately below it, Generation III below that, and so on downward through as many generations as the chart records, reading from oldest at the top to youngest at the bottom. Within each individual generation, every single person shown is further numbered using ordinary Arabic numerals, counting from left to right along that generation's row — so that any single individual appearing anywhere within a completed, correctly labelled pedigree chart can always be uniquely and unambiguously identified and referred to using a simple combined reference such as "individual III-4," meaning the fourth person counted from the left within Generation III of that particular family's chart. …