Genetic Disorders

A dominant gene means that a single allele can control whether the disease develops. If only one parents (usually affected) passes on an autosomal, defective gene which results in the child having a genetic disorder, then the disorder is called autosomal dominant. A recessive gene means that there is enough normal protein product to function properly from the normal gene and, therefore, two copies of the defective gene are necessary for the disease to develop. If both parents are unaffected and they each pass on a defective gene causing their child to be affected, then the genetic disorder is autosomal recessive. The parents are called carriers. For example, sickle-cell anemia is a recessive disorder characterized by abnormal hemoglobin production. The genetic defect involves a gene that produces hemoglobin. Although sickle-cell carriers produce, in part, abnormal hemoglobin, although they usually do not experience clinical manifestations since the normal hemoglobin produced from the normal gene is enough to function normally.

However, Many other genetic disorders are caused by defects related to the sex chromosomes, or the X and Y chromosomes. If a defective gene on the X-chromosome are inherited, it is called X-linked. Like autosomal disorders, X-linked genetic diseases also can be inherited by dominant and recessive mechanisms. X-linked dominant means that if the father passes on the defective gene on his only X chromosome, all his offspring (which will be females) will be affected. If he passes on his Y chromosome, none of these males will be affected. Therefore, there is no male-to-male transmission. If it is X-linked recessive, all daughters will be carriers. If the mother passes on a recessive X-linked gene, then all her sons will be affected and all her daughters will be carriers. Understanding the mechanisms by which genetic disorders are inherited are very important for interpreting recurrence risks.

A variation of Mendelian patterns of inheritance is called incomplete dominance. Incomplete dominance occurs when both alleles are expressed. An example of this is observed in Four-O-Clock flower color. A white and a red phenotype are neither dominant nor recessive. If a flower is heterozygous and carries genes that produce both the white color and the red color, the flower color results in pink. Another variation is codominance. In incomplete dominance, the phenotype is a blending of the two different gene effects. In codominance, both gene variants are expressed at the same time, representing a third phenotype. For example, a flower with alleles that can produce either white or red in the homozygote form will produce both colors in the heterozygote form expressed as spotted flowers.