Genetic diseases are those that are due to a mutation or alteration of the genome. When this alteration is obtained through genetic inheritance, then we speak of inherited genetic diseases.
On the contrary, if the genetic alteration occurs throughout life, then we speak of acquired genetic disease.
Inherited genetic diseases can also be congenital if they manifest from birth or the neonatal period. There are also hereditary genetic diseases not congenital; in these, the genetic alteration is inherited and therefore is present at birth, but its effects do not develop until later in any stage of life, from childhood to old age.
But how are these disease-causing genes inherited?
General aspects of genetic inheritance
The transmission of inherited genetic diseases follows the same inheritance patterns as any other gene, so it is important to know the basic inheritance patterns to understand how these diseases are inherited.
Each person has two gene copies distributed in 23 pairs of chromosomes. Having two copies of each gene, the human being is said to be an organism. diploid. One copy is received from the father and the other copy from the mother. Each copy is called allele.
There may be variations in genes that do not cause disease. These variations are called polymorphisms and they are not only normal but also desirable, since they are the variations that contribute biodiversity and genetic variability within the same species.
In contrast, variations that affect the function of a gene and can cause disease are called mutations.
Genetic diseases due to mutations in a single gene are generally transmitted in simple patterns known as mendelian patternsby the name of Gregor Mendel, the first to describe this type of inheritance patterns.
Other genetic diseases are due to mutations in several genes, and others also depend on certain environmental conditions. These genetic diseases are inherited in much more complex non-Mendelian patterns.
In Mendelian inheritance patterns it is important to understand the concept of dominant and recessive alleles. As mentioned before, the human being is diploid and has two copies of each gene, two alleles. If the two alleles of the same gene are different, the one that is manifested is the dominant allele and the other is the recessive allele.
For the characteristics of the recessive allele to manifest, the two alleles have to be the same, otherwise the characteristics of the dominant allele will always be displayed.
The same thing happens with disease-causing mutations. If the mutation occurs in a dominant allelethe disease will appear even if the mutation is in a single allele. On the contrary, if the mutation is recessive, the disease will only occur if both alleles have the mutation, for which the mutation has had to be inherited from both the father and the mother. If there is only one recessive copy, the person will be a carrier but will not develop the disease or develop symptoms.
Another large group of Mendelian inheritance is the sex chromosome linked inheritance (X and Y chromosomes, chromosome pair 23). Genes located on sex chromosomes follow different patterns than genes located on autosomal chromosomes (all other non-sex chromosomes, 1 through 22).
For example, a gene located on the Y chromosome is only passed from father to son. Rather, fathers only pass on the genes on the X chromosomes to female daughters. Women do not have a Y chromosome, the two sex chromosomes are X type, and they can transmit the genes of the X chromosome in a dominant or recessive way as it happened with the genes of autosomal chromosomes.
Types of genetic diseases according to the hereditary pattern
autosomal dominant
A single copy of the mutated gene is enough for a person to develop the disease. The parent who passed on the mutation also has the disease. Autosomal dominant diseases usually appear in all generations of an affected family.
examples: Huntington’s disease, Marfan syndrome, familial hypercholesterolemia, achondroplasia, neurofibromatosis.
autosomal recessive
In autosomal recessive inheritance, both alleles or copies of a gene have to have the mutation. Since each copy is inherited from one parent, both the father and the mother must have the mutation, although they may be carriers and not develop the disease if they themselves do not have both mutated copies. Autosomal recessive diseases do not usually occur in all generations.
examples: cystic fibrosis, sickle cell anemia, phenylketonuria, Tay-Sachs disease.
X-linked dominant
X-linked dominant hereditary diseases are due to dominant genes located on the X chromosome. They follow a pattern similar to autosomal dominant, with a special compartment in male individuals.
In women, who have two X chromosomes, the mutation in one allele is enough to cause the disease, so the disease occurs in all cases in which the gene is inherited. Women can transmit these diseases to both sons and daughters.
In men, who have an X chromosome and a Y chromosome, they also always develop the disease if they inherit the gene, since the only copy of the gene they have is the mutated copy, but unlike women, they will transmit the disease 100% of their female offspring and 0% of their male offspring.
Men who suffer from an inherited disease linked to the X chromosome they do not pass it on to their sons.
examples: Vitamin D resistant hypophosphatemia, Fragile X syndrome, Rett syndrome.
X-linked recessive
X-linked recessive diseases are also due to mutations of genes on the X chromosome, but in this case to genes that follow a recessive pattern of inheritance.
Men only have one X chromosome and, therefore, they only have the copy of the gene with the mutation, so as long as they inherit the gene, they will develop the disease even if it is a recessive gene.
In women, however, the mutation must affect both copies of the gene or they will not develop the disease.
However, X-linked recessive diseases are much more common in men than in womensince inheriting two copies of a recessive mutation on the X chromosome is relatively rare.
examples: hemophilia A, hemophilia B, Fabry disease.
Y-linked
A disease is considered Y-linked if it is due to a mutation in a gene whose locus is on the Y sex chromosome. Since the Y sex chromosome is only present in males, only they suffer from these diseases and only they transmit it. They are only inherited from fathers to sons.
examples: Y chromosome infertility, certain cases of Swyer syndrome.
co-dominant
In codominant inheritance both alleles are expressed of the same gene, there is not one dominant and one recessive. Each gives rise to slightly different proteins, and both influence a certain genetic trait, characteristic, condition, or disease.
The ABO blood type It is inherited by codominance. There are not many diseases that follow this pattern, an example is alpha-1 antitrypsin deficiency.
mitochondrial
Mitochondria are a type of cellular organelle that has your own genetic material. The replication of this genetic material, as well as its inheritance, is independent of the genetic material in the nucleus.
Mitochondrial DNA follows its own replication cycle and is only inherited through ovules. Namely, only mothers pass it on.
Inherited diseases due to mitochondrial mutations can appear in all generations and are transmitted to the 100% offspring.
Examples: Leber hereditary optic neuropathy.
Other forms of inheritance
Many diseases and health conditions are caused by the combined effect of several genes. These diseases or conditions are said to be polygenic. There are also diseases that, even though they require a certain genetic load, depend on the interaction with environmental factors to manifest or develop.
Genetic transmission patterns in these cases become extremely complex and do not follow the Mendelian patterns described above.
Examples of polygenic diseases or diseases that depend on environmental factors are some heart diseases, type 2 diabetes or many kinds of cancer.
The diseases caused by changes in the structure or number of chromosomes, and not only by genetic mutation, they also do not follow Mendelian inheritance patterns. For example, trisomy X, a disease characterized by the presence of three X chromosomes that only affects women.