Ichthyosis Genetics - 1
Probably the question most commonly asked of people with ichthyosis is: "Is it contagious?” The answer is, of course, “ NO!” None of the ichthyoses or the related disorders in the family of skin conditions that are included in the umbrella of the FIRST is contagious. These are all genetic disorders and can be "caught" only through heredity. Although not all members of the FIRST family of disorders are an “ichthyosis” per se, for ease of discussion, we will use that term to denote any of these conditions.
Probably the most common question asked by people who have ichthyosis, or by parents of a child with ichthyosis is: "How is it passed on? I have one child, and that child has ichthyosis. Will all my children have it?" Or, "I have ichthyosis. If I have children, will I pass it on to them?"
The answers to these questions will depend on the specific type of ichthyosis present in the family. Although the ichthyoses are all inherited or genetic disorders, the pattern or mode of inheritance varies considerably among the FIRST family of disorders. Therefore, the very first step in genetic counseling is a firm and accurate diagnosis. 
 Diagnosis of the ichthyoses and its related disorders is usually achieved by consideration of the appearance of the skin in combination with the family history and sometimes other findings, such as skin histopathology or other laboratory testing. These diagnoses are based upon phenotype, i.e., on how the disorder manifests itself. Sometimes more than one gene (genotype) can cause a given phenotype. In other words, one’s genetic makeup is his/her genotype. How he/she appears, i.e., how the genes are expressed, is his/her phenotype. This is important because a given appearance (phenotype) can be caused by more than one genotype. For example, one of more than 6 genes can underlie the Lamellar Ichthyosis/ Non-Bullous Congenital Ichthyosiform Erythroderma phenotypes. Phenotypes are very complex because the result of a given genotype may be modified by other factors, such as other, modifying genes, and environmental factors. As an example of the former, co-existence of the gene for Ichthyosis Vulgaris, which is very common in the population, with the less common gene for X-Linked Ichthyosis, may result in more severe skin disease. Darier disease is an example of a genetic condition that can be modified by environmental factors, e.g., ultraviolet light exposure. Because phenotypes are complex and sometimes confusing, the most accurate diagnosis for genetic counseling is one that is based upon determination of the causative gene (i.e., the genotype) in that family.
Next Chapter: An Introduction to Basic Genetics
The information presented here is designed to be generally informative and is derived from sources believed reliable. The genetic concepts involved are extremely technical in many cases and have been simplified for clarity. The information presented is not, nor is it intended to be, sufficient for readers to make any decision about their personal genetic situations. Readers should always consult a professional geneticist or physician for individualized genetic counseling before making any decisions in their individual cases.