Dominant genetic disorders are diseases that need only one defective gene copy to be passed down from parent to child. This means that it only takes one parent with the defective gene for the disorder to be inherited. Disorders like these have 50% chances of being passed down from parent to child. For a clearer example, two out of four offspring of an affected parent will most likely get the disorder.
The usual cause for dominant genetic disorders is the mutation of a gene or a chromosome, the latter being made up of many genes. This mutation often happens at the early stages of pregnancy, when the 13 chromosomes of the mother bind together with the father’s 13 other chromosomes, creating a cell that contains 26 chromosomes. During this phase when the cells are being copied, an error happens that results in either a lacking or an extra chromosome, resulting in a genetic disorder.
Many of the dominant genetic disorders are caused by mutated autosomes, which are chromosomes that do not determine the sex of an offspring. Disorders like these are specifically called autosomal dominant disorders. One example would be Huntington’s disease, in which the mutation appears on “chromosome 4.” Other examples include Marfan’s disease that causes a person to be abnormally tall, and achondroplasia, the opposite of Marfan’s disease, which results in dwarfism.
Another specific type of the dominant disorders are the “X-linked dominant genetic disorders,” wherein only the X chromosomes experience a mutation. In these cases, men are more prone to pass down and inherit these disorders and manifest worse symptoms compared to women. The reason behind this is that women have two X chromosomes, in which the defected copy is somehow “protected” by the non-defect copy of the X chromosome. Men, on the other hand, have of X and Y chromosomes, so the one X chromosome does not have any protection.
Passing down the X-linked dominant disorders may depend on the affected parent. Typically, a male parent bearing the mutated X chromosome will not pass the disorder down to male offspring as the male transfers the Y chromosome to the next generation of males. The female offspring, instead, will all obtain the disease. With affected women, there will always be a 50% chance of passing down the disorder to both male and female offspring. In some cases, an offspring will inherit and become a “carrier” for the disease, but will not necessarily exhibit the symptoms.
Examples of the X-linked dominant disorders are Rett syndrome, which features shorter limbs and a smaller head, and incontinentia pigmenti, which involves skin and hair discoloration. Generally, all dominant genetic disorders tend to be rare, primarily because affected patients usually die at an early age and are, thus, unable to produce children. In occasions when affected patients give birth, the offspring will also most likely die young.