Diseases of genetic origin occur due to the presence of alterations or mutations in the DNA. Generally speaking, these mutations can appear in an individual spontaneously (de novo mutation), or appear in the germ line, making their transmission to offspring possible. Among the latter we can find:
- Autosomal dominant inheritance or AD: a disease is considered to be inherited with this pattern when the altered gene is dominant over the normal one and a single copy of the mutation is sufficient for the disease to manifest. Furthermore, being autosomal implies that the alteration is located on one of the 22 non-sex chromosomes or autosomes, potentially affecting sons and daughters equally. In these cases, affected individuals usually appear in all family generations and, generally, the probability of each child inheriting the pathology is 50%.
- Autosomal recessive inheritance or AR: this type of inheritance occurs when the normal gene predominates over the mutated one, so that two copies of the altered gene are needed for symptoms to appear. Being of autosomal origin, the altered gene is located on one of the 22 non-sex chromosomes or autosomes, and can be inherited equally by sons and daughters. This type of disease does not usually appear in all generations of a family and requires both parents to be carriers of the mutated gene. In these cases, and generally, the probability of inheriting the disease would be 25%, a 50% probability of being an asymptomatic carrier, and a 25% chance of not carrying the mutated allele.
- X-linked dominant inheritance: this type of inheritance occurs when the altered gene is located on the X sex chromosome and the mutated gene predominates over the normal one. Females carry two X chromosomes while males have only one. Generally, as females inherit a copy of this chromosome from both the father and the mother (males inherit their copy of the X chromosome only from their mother), they are usually more affected by these types of diseases. If the mother has the disease, her children have the same probability of inheriting the pathology, regardless of sex. However, if the father is the one affected, all his daughters will be ill while his sons will be healthy.
- X-linked recessive inheritance: this type of inheritance occurs when the altered gene is located on the X chromosome but the normal gene predominates over the mutated one. These types of diseases occur much more frequently in men because they only have one copy of the X chromosome and, if they inherit the mutated allele, they will develop the disease, while women who inherit one mutated copy of the gene will be carriers but will not suffer from the disease because they possess two X chromosomes.
- Mitochondrial inheritance: The most important part of the genetic material is found in the chromosomes, within the cell nucleus, but there is a small amount of DNA found in the mitochondria, intracellular organelles involved in metabolism. This mitochondrial DNA is transmitted with a characteristic inheritance pattern because mitochondria are only inherited from the mother. Due to this, all children of an affected mother will inherit the alteration, which is not the case if the father is the one affected.
In addition to the types of inheritance explained above, an important concept must be taken into account: penetrance. This is the percentage of individuals with a specific genotype (DNA) who express the expected phenotype (symptoms). Thus, we speak of complete penetrance when 100% of individuals show the expected phenotype according to their genotype, and incomplete penetrance when this percentage is lower, so that not all individuals with a pathological genotype present the disease.