The term genetic linkage frequently proves misleading. It is often supposed that when two genes are linked they tend to stick together in hereditary transmission. What is actually observed is that, instead of the random distribution seen when the genes are situated on different chromosome pairs, they either tend to stick together unduly often, or, just as frequently, tend to separate unduly often.The consequences of genetic linkage are explained for the case of two rare dominant traits.An examination is made of a pedigree recently recorded by Penfold and Lipscomb, in which elliptocytosis and multiple telangiectasia occurred together in the same family group. The pedigree is a very small one, but it nevertheless provides strong evidence against very close linkage (a cross-over value of five per cent. or less) and good evidence against somewhat looser linkage (a cross-over value of 10 per cent. or less).Observers who encounter the simultaneous occurrence of two inherited conditions in the same family group should on no account omit to place on record as full a description as possible. It may easily happen that a single small pedigree may be decisive in regard to whether or not there is close or moderate linkage between the genes, thus making possible a solid contribution to the task of mapping the human chromosomes, and, if the chance is missed, so rare an opportunity may not recur for centuries.
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