Heterozygous individuals carrying a "paradominant" mutation, as a rule, are phenotypically normal. Therefore, the mutation can be transmitted unperceived through many generations. The trait becomes manifest when a somatic mutation occurs during embryogenesis giving rise to loss of heterozygosity and forming a mutant cell population, being either homozygous or hemizygous for the mutation. This concept explains the occasional familial occurrence of usually sporadic traits like the Klippel-Trenaunay syndrome and others.
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