Title: Human genetic disease - Autosomal dominant inheritance | Britannica
Headings: 
Autosomal dominant inheritance
Human disorders attributable to a single dominant gene
Autosomal recessive inheritance
Human disorders attributable to a single pair of recessive genes

Content: An individual who carries one copy of a dominant mutation ( Aa) will produce two kinds of germ cells—eggs or sperm—typically in equal proportions; one half will bear the mutant gene ( A ), and the other will bear the normal gene ( a ). As a result, an affected heterozygote has a 50 percent chance of passing on the disease gene to each of his or her children. If an individual were to carry two copies of the dominant mutant gene (inherited from both parents), he or she would be homozygous ( AA ). The homozygote for a dominantly inherited abnormal gene may be equally affected with the heterozygote. Alternatively, he or she may be much more seriously affected; indeed, the homozygous condition may be lethal, sometimes even in utero or shortly after birth. Such is the case with achondroplasia, so that a couple with one affected partner and one unaffected partner will typically see half of their children affected, whereas a couple with both partners affected will see two-thirds of their surviving children affected and one-third unaffected, because 1 out of 4 conceptions will produce a homozygous fetus who will die before or shortly after birth. Although autosomal dominant traits are typically evident in multiple generations of a family, they can also arise from new mutations, so that two unaffected parents, neither of whom carries the mutant gene in their somatic cells, can conceive an affected child. Indeed, for some disorders the new mutation rate is quite high;