One copy of the gene or one allele is sufficient to create all the damage and cause the disorder in
Autosomal dominant disorders
If a person has an autosomal dominant gene mutation, he/she would not have the ability to make enough of the correct gene product and would have symptoms of the genetic condition from birth or be predisposed to developing the condition later in life (depending on the gene involved). Humans need two copies of each gene in order to have the correct balance of DNA. One copy of each gene is passed to a child from the mother and the other, from the father. If the gene is autosomal dominant, even if you inherited the abnormal gene from only one parent, you would get the disease. Affected children would have at least one affected parent. There would be no question of carriers in case of dominant disorders; individuals would be either affected or normal.