Autosomal dominant disorders
In this case the abnormality is on one of the 22 autosomes (body-defining chromosomes) and it tends to take precedence over the healthy allele. If the affected parent passes on the affected allele, it will also take precedence over the healthy allele that the embryo may have received from the other parent. In short, the risk of inheriting a dominant disorder is fifty percent in every pregnancy.
Whenever the abnormality is present, the disorder will occur. Known autosomal dominant disorders include Huntington's disease, Steinert's myotonic dystrophy and Marfan syndrome.