Hereditary diseases: what are they, types, characteristics and examples

With the passage of time, sciences such as medicine have progressively developed, something that has allowed that life expectancy, quality of life and well-being have been increasing greatly measure.

Thanks to this, many of the diseases that were once deadly today can be treated successfully, and in some cases the disease itself has even been eradicated. However, there are still different types of diseases that continue to pose a great challenge for medicine, such as AIDS, cancer or diabetes.

In addition to these, there is a large group of diseases that has to do with the genes transmitted by our ancestors and for which there is mostly no cure (although Sometimes treatments can be found to reduce or slow down the symptoms, or correct, reduce or eliminate the affectation they cause in the subject and his daily life). We are talking about the set of hereditary diseases, a concept on which we will reflect throughout this article.

• Related article: "Differences between DNA and RNA"

Hereditary diseases: what are they?

They receive the name of hereditary diseases the set of diseases and disorders that have the particularity of being able to be transmitted to offspring, that is, from parents to children, through the transmission of genes that cause them.

Thus, these are diseases that arise at the chromosomal, mitochondrial or Mendelian level and that are due to the existence of genetic mutations that come from our ancestors. It is not always necessary for one of the parents to manifest the disorder or disease, depending on the type of inheritance that is: it may be carrier of a recessive gene that does not trigger the onset of the disease in him or her, but it can develop in the decendents.

It's important to put attention on genetic diseases and inherited diseases are not necessarily synonymous. And it is that although all hereditary disease is genetic, the truth is that the inverse relationship does not always have to occur: there are genetic diseases that arise from spontaneous de novo mutations that appear without any antecedents relatives.

For a disease to be heritable, it is necessary that the genes and mutations linked to its appearance must be present in the germ cells, that is, the sperm and / or eggs that will be part of the new to be. Otherwise we would be facing a genetic but not hereditary disease.

Types of gene transmission

To be able to talk and know where hereditary diseases arise from it is necessary to take into account the multiple methods of genetic transmission from which a mutated gene can be transmitted. In this sense, some of the main modes of genetic transmission are as follows.

1. Autosomal dominant inheritance

One of the main and best-known types of inheritance is autosomal dominant inheritance, in which there is a mutation in one of the non-sexual or autosomal chromosomes. The dominant gene will be the one that is always expressed, so in the event that there is a mutation in it linked to the appearance of a disease, it will express itself and develop.

In this case, there will be a 50% chance that each child the person in question has will manifest the disease (depending on who inherits the dominant gene). It can have complete penetrance (one allele dominates over the other) or incomplete (two dominant genes are inherited, the inherited traits being a mixture of those that come from the parents).

2. Autosomal recessive inheritance

Autosomal recessive inheritance is one that occurs when there is a mutation or alteration in a recessive gene and this is transmitted to the new generation. Now, the fact that the alteration is in a recessive gene implies that the disease will not develop unless it exists in more than one allele of a chromosome, in such a way that having a copy of this gene does not mean that the disorder has to appear.

For this to occur, it will be necessary for both alleles of a gene to present the mutation, that is, how much father and mother must transmit an altered copy of the gene to the child for the development of disease.

3. Sex-linked inheritance

Although they must integrate into sex cells to be transmitted, many inherited diseases are autosomal, that is to say that the alteration is present in one of the non-sexual chromosomes that are going to to transmit. However other disorders are transmitted through copies of the sex chromosomes, X or Y. Since only males at the genetic level carry Y chromosomes, if there is an alteration in this chromosome it could only be transmitted from parents to male children.

In the event that the alteration occurs in the X chromosome, they can be transmitted from both parents to their children regardless of their sex.

4. Polygenic inheritance

The two previous types of genetic inheritance are monogenic, that is, they depend on a single gene. However, there are often multiple genes linked to the onset of a disease. In this case we will be talking about a polygenic inheritance.

5. Mitochondrial inheritance

Although they are not as well known or common as the previous ones, there are various inherited diseases and disorders that do not arise from the DNA present on the chromosomes, but its origin is in the organelles known as mitochondria. In these structures we can also find DNA, although in this case it comes exclusively from the mother.

Examples of inherited diseases

There are many hereditary diseases that exist, being possible to find thousands of them. However, in order to put a face and name to some hereditary diseases, below we leave you with a total of a dozen examples (some of them well known).

1. Huntington's disease

Huntington's disease, formerly known as Huntington's chorea, is a hereditary disease with full penetrance autosomal dominant transmission.

This disease is characterized by a progressive neurodegeneration that causes, among other symptoms, movement disorders (highlighting the choreic movement they perform due to the involuntary contraction of the muscles when moving), as well as a profound alteration of cognitive functions and especially of executive functions, which worsen with the pass of the time.

• You may be interested: "Huntington's chorea: causes, symptoms, phases and treatment"

2. Hemophilia

This dangerous disease, which is characterized by difficulty in blood clotting and which causes profuse and uninterrupted bleeding that can be life-threatening if not stopped, it is also a disease hereditary. Specifically its most common form, type A hemophilia, is a disease linked to the sex chromosomes (specifically linked to the X chromosome) and is transmitted recessively. That is why hemophilia is a disease that is suffered almost exclusively by men, since women have two copies of the X chromosome in such a way that its appearance is difficult.

3. Achondroplasia

Achondroplasia is a genetic disorder that it is characterized by causing alterations in the formation of cartilage and bone, being the main cause of dwarfism.

Although in most cases (around 80%) we are faced with spontaneous mutations, in 20% of them the presence of family antecedents from which the mutation has been inherited is observed. In these cases, an autosomal dominant pattern is observed, in which a single cup of the mutated gene can lead to cause the disease (if one of the parents suffers from it, her children have a 50% chance of manifesting achondroplasia). The main associated genes are G1138A and G1138C.

• You may be interested: "Achondroplasia: symptoms, causes and treatments"

4. Marfan disease

A disease of genetic origin that characterized by affecting connective tissue. It is an autosomal dominant disease in which the bones grow uncontrollably, in addition to other possible symptoms such as at the level cardiovascular (highlighting murmurs and affectations in the aorta that can become life-threatening) or at the ocular level (there may be retinal detachments, myopia and cataracts).

5. Cystic fibrosis

Cystic fibrosis is one of the inherited diseases that are produced by an autosomal inheritance recessive, and is characterized by the accumulation of mucus in the lungs in a way that makes it difficult to breathing. Mucus can also appear in organs such as the pancreas, in which cysts can also appear. It is a life-threatening disease, usually due to severe infections, which is more common in children and young people.

6. Leigh syndrome

In this case we are facing a hereditary disease of the mitochondrial type (although it can also be caused by autosomal recessive genetic transmission), which characterized by rapid neurodegeneration that occurs early (usually before the first year of life) and in which the presence of damage to the brainstem and basal ganglia stands out.

Problems such as hypotonia, movement and gait problems, breathing problems, neuropathy, and impaired heart, kidney, and lung function are some of the common symptoms.

7. Sickle cell anemia

This disorder is characterized by the presence of changes in the shape of red blood cells (instead of round, they acquire an irregular shape and become rigid) in such a way that they make possible the blockage of blood flow, in addition to due to a reduction in the life of these globules (something that can mean a reduction in the levels of this essential component of the blood). It is another hereditary disease, through autosomal recessive inheritance.

8. Thalassemia

Another blood-related disorder that is inherited through recessive autosomal inheritance is thalassemia. This disease causes difficulty in synthesizing specific parts of hemoglobin (especially alpha globin), something that can cause fewer red blood cells to be made and even generate anemias of different consideration and severity (although with treatment they can lead a life normal).

9. Duchenne muscular dystrophy

Characterized by progressive muscle weakness (both at the level of voluntary and involuntary muscles), the presence of frequent falls, constant fatigue and sometimes intellectual disability, this serious degenerative disease is fundamentally hereditary, with a recessive inheritance pattern linked to the X chromosome.

10. Phenylketonuria

Phenylketonuria is a hereditary disease that is acquired through autosomal recessive inheritance, and is characterized by absence or deficiency of phenylalanine hydroxylase, something that causes the inability to break down phenylalanine in such a way that it accumulates in the body. It can lead to brain damage, and usually occurs with delayed maturation, intellectual disability, uncontrolled movements and even seizures, as well as a peculiar smell of urine and sweat.

11. Leber Congenital Amaurosis

A rare disease characterized by abnormalities or progressive degeneration of the photoreceptors in the retina. It can generate a great affectation at the visual level, deteriorating the sense of sight and it is usual that the sufferer has a very limited vision capacity. It is an autosomal recessive inherited disorder.

12. Autosomal dominant polycystic kidney disease

One of the most common inherited kidney diseases, autosomal dominant polycystic kidney disease is characterized by the presence of cysts in both kidneys as well as secondarily in other organs such as the liver. Kidney stones, pain, high blood pressure, strokes or cardiovascular problems (including mitral valve prolapse as one of the most frequent). It can even lead to end-stage kidney failure. It is an autosomal dominant, complete penetrance disease with mutations in the PKD1 and PKD2 genes.

Bibliographic references:

• Hib, J. & De Robertis, E. D. P. 1998. Fundamentals of cellular and molecular biology. Buenos Aires: The Athenaeum.
• Krakow, D. (2018). FGFR3 disorders: thanatophoric dysplasia, achondroplasia, and hypochondroplasia. In: Copel JA, D'Alton ME, Feltovich H, et al, eds. Obstetric Imaging: Fetal Diagnosis and Care.