Ichthyosis Genetics - 3
How Genes are Passed and Traced
The traits of the dominant gene are the easiest to trace through a family, since their effects are manifested in the offspring. An individual exhibiting a dominant trait can have one of two genetic make-ups:
The capital letter is used to designate the dominant gene, the lower case letter the recessive gene for the same trait. A commonly used example is eye color, although it is important to point out that it is, in fact, an oversimplification because the inheritance of eye color depends upon more than one genetic factor.  With this in mind, however, we will use eye color here as an example of patterns of inheritance because it is something that everyone is familiar with, easy to conceptualize, and not terribly "clinical."
We'll say capital “B” is the gene for brown eyes and lower case “b” is the gene for blue eyes. The capital “B” indicates that brown eyes are dominant over blue eyes. A brown-eyed person could have two “B” genes (i.e., “BB”) or he could have a “B” gene and a “b” gene (“Bb”); either way he would have brown eyes because the ”B” (brown) gene dominates the “b” (blue) gene, giving the person the trait represented by the “B” - in this case, brown eyes.
“BB” people pass on only “B” genes - it's all they have to give. “Bb” people, however, can pass on either a “B” or a “b” gene to each of the sperm or eggs they produce; that is, some eggs or spermatozoa will receive chromosome bearing “B” genes and others will get ones with “b” genes.
Thus, if two “BB” people marry and have children, there's no doubt what genes they will pass on or what color eyes their children will have. And all their children will also have a “BB” genetic makeup for their brown eyes.
If one parent is “BB” and one is “Bb”, both parents will have brown eyes and all of their children will also have only brown eyes, as shown here:
No matter which gene the child receives from the father, it will always be paired with a gene from the mother. Since any gene from the mother will be a dominant “B” gene for brown eyes, any child from this pairing will have brown eyes, despite the father's one recessive gene for blue eyes. This father would be called a carrier for blue eyes because he "carries" a recessive “b” gene for blue eyes, even though that gene cannot express itself in his eye color. It can, however, be passed to his offspring.
What if two brown-eyed parents have the genetic makeup of the father in the above example, however? If both parents are “Bb” they both will have brown eyes, but they will each pass on either “B” or “b” genes. The child who gets the “bb” gene combination will have blue eyes, not brown, because he has no “B” gene to bully his “b” gene. Thus the "weaker," or recessive trait is able to manifest itself - blue eyes.
In a later section (Autosomal Dominant Inheritance) we will further describe how the dominant inheritance pattern operates in these disorders. We will now describe the major patterns of inheritance. For simplicity of discussion we will use one type of ichthyosis as the example or paradigm. But the choice is arbitrary and the reader should bear in mind that the same genetics apply to all the other disorders that follow the same mode of inheritance.
 This type of inheritance is termed multigenetic inheritance. Many common traits, like eye color or skin or hair color, as well as many common diseases, such as diabetes or psoriasis, are caused by the action of several genes, i.e., are multigenetic traits. In contrast, members of the FIRST. family of disorders are all single gene (“Mendelian”) traits.
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.
This information is provided as a service to patients and parents of patients who have ichthyosis. It is not intended to supplement appropriate medical care, but instead to complement that care with guidance in practical issues facing patients and parents. Neither FIRST, its Board of Directors, Medical & Scientific Advisory Board, Board of Medical Editors, nor Foundation staff and officials endorse any treatments or products reported here. All issues pertaining to the care of patients with ichthyosis should be discussed with a dermatologist experienced in the treatment of their skin disorder.