Traditional genetics has also been extended by the introduction of several new non-Mendelian concepts of importance in dermatology. These include the following.
1. Mosaicism. A mosaic is a single individual madeup of two or more genetically distinct cell lines. The concept is important in several skin disorders including incontinentia pigmenti and seg-mental neurofibromatosis. The mutation of a single cell in a fetus (a postzygotic mutation) may form a clone of abnormal cells. In the epidermis these often adopt a bizarre pattern of lines and whorlsaBlaschko’s lines, named after the dermato-logist who recorded them in linear epidermal naevi in 1901.
2. Contiguous gene deletions. Complex phenotypesoccur when several adjacent genes are lost. In this way, for example, X-linked ichthyosis may associate with hypogonadism or anosmia.
3. Genomic imprinting means that genes may differ intheir effect depending on the parent from which they are inherited. Genes from the father seem especially important in psoriasis, and from the mother in atopy.
4 Uniparental disomyoccurs when both pairs of genesare derived from the same parent so that an individual lacks either a maternal or a paternal copy. In this way a disorder usually inherited as a recessive trait can arise even though only one parent is a carrier.