Some Operational Definitions
More often than not, when the term hereditary is applied to a particular trait, the natural inclination is to equate it with the term genetic. However, hereditary refers only to the sustained passage of a trait from generation to generation.
Surnames, property, and titles may be hereditary by law or custom, but they are not genetic. We may regard a hereditary trait as genetic only when the passage of that trait from generation to generation is determined, at least in part, by one or more genes. In much the same way, the term congenital has often been equated with genetic when, in fact, this term signifies only that a trait is present at birth. A congenital trait is not necessarily genetic or even hereditary. Thus, the purview of medical genetics may be said to include those traits, both congenital and of delayed onset, whose origins lie in defects in single genes, in groups of genes, or in the size or number of the chromosomes.A useful checklist for assessing whether a trait is genetically determined was provided by J. V. Neel and W. J. Schull (1954). A trait is genetically determined by the following:
1. It occurs in definite proportions among persons related by descent, when environmental causes have been ruled out.
2. It fails to appear in unrelated lines such as those of in-laws.
3. It displays a characteristic age of onset and course of development, in the absence of other known causes.
4. It shows greater concordance in monozygous (identical) twins than in dizygous (fraternal) twins.
When the role of chromosomal anomalies became fully appreciated in the 1960s, a fifth item was added to the checklist:
5. It consists of a characteristic suite of features (syndrome) coupled with a clearly demonstrated chromosomal abnormality (Thompson and Thompson 1980).
In the present era of molecular biology and recombinant DNA technology we must add yet another item to the list:
6. It is linked to a specific DNA sequence (probe) that has been mapped to a particular region of a chromosome.
Throughout the following discussions, traits that are regarded as genetic but not chromosomal, having met criteria 1 through 4 and 6 but not 5, are accompanied by a reference number. This number is the entry number of a given disorder in the catalog complied by V. A. McKusick (1986) of more than 3,000 Mendelian phenotypes in human beings. For example, the entry number for sickle cell anemia is 14190. An excellent reference for chromosomal conditions is E. Thurman (1986).