How Are Recessive Traits Inherited?

Dominant and recessive are terms used in Mendelian genetics. Dominance means that the condition or effect of that gene appears as a trait, such as eye or hair color; or a condition, such as night blindness. If an individual carries two dominant genes, the trait will appear in that individual and invariably pass on to offspring. If an individual inherits both a dominant and a recessive gene for the same characteristic, the dominant will prevail in the individual, but the recessive remains and can be transmitted to offspring. When two recessives are passed on -- one from each parent -- the trait or condition appears in that child. Other modes of transmission, such as mitochondrial and multifactorial, can also pass inheritable traits from parent to child.

Autosomal

• Autosomal genes are not sex-determining. If one parent has two dominant genes for brown eyes and the other has a both dominant gene for brown eyes and a recessive gene for blue eyes, their child will have brown eyes. However, they will have one in four -- or 25 percent -- chance of inheriting a recessive trait for blue eyes. Two brown-eyed parents can have a blue-eyed child if each of them carries a recessive gene -- and that gene passes from each of them to the child.

X-linked

• X-linked genes determine gender. Every child inherits one X-linked chromosome from each parent: an X from the mother and either an X or a Y from the father. Two Xs make the child female and an X and a Y make it male. Since an X-linked gene is carried only on the X chromosome, an X-linked trait cannot pass from a father to his son, because the trait is not carried by the Y chromosome that creates a male. It is, however, passed from a father to all his daughters, because he has only one X to give them and the gene goes with the X chromosome. The most notorious example of such a trait is hemophilia -- also called bleeder's disease -- in which the blood does not clot properly; it causes uncontrollable bleeding and eventual death. This condition became politically significant when it passed through the children of Queen Victoria of England into several royal families of Europe and affected the Romanovs, the ruling house of Czarist Russia.

Mitochondrial

• Alteration (mutation) of mitochondrial DNA causes rare medical conditions, such as Kearns-Sayre Syndrome. Mitochondrial DNA is inherited only from the mother. All children of a mother with a mitochondrial-transmitted condition may either show the condition, or carry the gene for it. How severely a child is affected depends on how much of the mitochondrial genetic material is normal and how much is altered.

Multifactorial

• Multifactorial genetic problems can be transmitted genetically, but environmental factors are also involved. As many as one newborn in 10 will show a multifactorial disease at some time in its life. A good example of this is diabetes. A predisposition to diabetes is genetic, but that's not enough to cause the disease to appear. In identical twins, if one has diabetes, about 50 percent of the time the other does not. The trigger that starts the disease process in motion may be lifestyle habits; such as overeating or lack of exercise. It also appears to occur more often in Caucasians and in cold climates. It could also be triggered by a virus. Research into environmental causes is ongoing.