genetics has also been extended by the introduction of several new
non-Mendelian concepts of importance in dermatology. These include the
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 whorlsaBlaschkoâ€™s lines, named after the dermato-logist who recorded them
in linear epidermal naevi in 1901.
gene deletions. Complex phenotypesoccur when several adjacent genes are lost. In
this way, for example, X-linked ichthyosis may associate with hypogonadism or
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.
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.