Types of genetic inheritance
Diseases of genetic origin occur due to the presence of alterations or mutations in the DNA. In general, these mutations can appear spontaneously in an individual (de novo mutation) or appear in the germ line and can be transmitted to the offspring. Within the latter we can find:
- Autosomal dominant inheritance or AD: A disease is inherited in 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 itself. Furthermore, being autosomal implies that the alteration is in one of the 22 non-sexual chromosomes or autosomes and can affect sons and daughters equally. In these cases, affected individuals usually appear in all family generations and, generally, the probability that each child inherits 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 the symptoms to appear. As it is of autosomal origin, the altered gene is found on one of the 22 non-sexual or autosomal chromosomes and can be inherited equally by sons and daughters. This type of disease does not usually appear in all generations of a family and it is necessary for both parents to be carriers of the mutated gene. In these cases, and in general, the probability of inheriting the disease would be 25%, 50% of being an asymptomatic carrier and 25% of not carrying the mutated gene.
- X-linked dominant inheritance: this type of inheritance occurs when the altered gene is 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 speaking, due to females inherit a copy of this chromosome from both their father and mother (males inherit their copy of the X chromosome only from their mother), they are more likely to be affected by these types of diseases. If the mother has the disease, her children have the same chance of inheriting the condition, regardless of their sex. However, if it is the father who is affected, all his daughters will be sick while his sons will be healthy.
- X-linked recessive inheritance: this type of inheritance occurs when the altered gene is on the X chromosome but the normal gene predominates over the mutated one. This type of disease is much more common in men as they only have one copy of the X chromosome and if they inherit the mutated gene they will develop the disease, whereas women who inherit one mutated copy of the gene will be carriers but will not have the disease because they have two X chromosomes.
- Mitochondrial Inheritance: The most important part of the genetic material is found in the chromosomes, inside the nucleus of the cells, but there is a small amount of DNA found in the mitochondria, which are intracellular organelles involved in metabolism. This mitochondrial DNA is transmitted with a characteristic inheritance pattern because mitochondria are only inherited from the mother. Because of this, all the children of an affected mother will inherit the alteration, not being so in the case that the affected is the father.
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 that express the expected phenotype. Thus, we speak of complete penetrance when 100% of individuals present the expected phenotype according to their genotype, and of incomplete penetrance when this percentage is lower, so that not all individuals with a pathological genotype present the disease.