1. Likelihood of fragile X–associated mental retardation syndrome in an artificial pedigree. The percentages shown indicate the likelihood of clinical manifestation according to position in the pedigree. Because individuals carrying the abnormal X chromosome have a 50% chance of passing it to their offspring, penetrance is twice that of the values depicted. Penetrance increases with each successive generation owing to the progressive expansion of a triplet repeat element (see text). Expansion is dependent on maternal inheritance of the abnormal allele; thus, daughters of normal transmitting males (indicated with a T in II-4) are nonpenetrant. Obligate carrier females are indicated with a central dot. (Reproduced, with permission, from Nussbaum and Ledbetter. Fragile X syndrome: a unique mutation in man. Annu Rev Genet. 1986;20:109.
Source: Genetic Disease, Pathophysiology of Disease: An Introduction to Clinical Medicine, 7e
Citation: Hammer GD, McPhee SJ. Pathophysiology of Disease: An Introduction to Clinical Medicine, 7e; 2013 Available at: Accessed: December 29, 2017
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