Discuss why some skin disorders can show a mosaic pattern of expression and others do not?
In May 1998, an international symposium was held in Marburg, Germany, to discuss the growing research interest in Mosaicism in Human Skin. At this symposium, the work of Alfred Blaschko and Rudolf Happle was recognized for its contribution to the field . In 1901, Blaschko wrote his seminal paper on what became known as Blaschko lines, a set of fountain-like patterns on the back, whorls on the abdomen and linear stripes on the limbs. He proposed that these lines had their origins in embryonic development and in 1977, Happle linked these lines to a dateable embryonic event; X-inactivation.  The last 30 years has seen an increasing interest in this field, with proposed theories of X-linked skin diseases and the definition of the term mosaicism to categorize them.
A genetic mosaic is an organism composed of two or more genetically different populations of cells that originate from one genetically homogenous zygote. The incredible diversity of cutaneous patterns have been classified into four categories; the line of Blaschko, the checkerboard pattern, the phylloid pattern and a patchy pattern without midline separation. The causes of these forms of mosaic expression have two major classes indicative of their genetic mechanism. Functional mosaicism results from the Lyon effect of X-inactivation. Genomic mosaicism is a product of autosomal mutations that may be lethal or non-lethal.  Table 1 from Moss summarises many of the mosaic linear dermatoses. 
Some X-linked skin diseases are found predominantly in females as the mutation exerts a lethal effect on hemizygous male embryos.  Such diseases, like incontinentia pigmenti, focal dermal hypoplasia, X-linked dominant chondrodysplasia punctata, oral-facial-digital syndrome type I and CHILD syndrome are a few examples of X-linked male-lethal mutations. Females are usually carriers…