While almost all genetic diseases are rare diseases, not all rare diseases are genetic. About 80% of rare diseases have a genetic origin and 90% of genetic diseases are rare.
What is a genetic disease?
Genetic diseases are pathologies due to abnormalities of genes or chromosomes. Some genetic abnormalities can be passed on to offspring; these genetic diseases are then family diseases. These are rare diseases that can appear at birth (or even in utero), or sometimes later in life. It is estimated that there are 8,000 to 10,000 genetic diseases.
In humans, it is estimated that there are between 20,000 and 30,000 genes. When, by chance, a genetic abnormality appears in one of these genes, its protein will be modified or will not be manufactured. However, it is essential for the functioning of our body.
What is the difference between DNA, genes and chromosomes?
The human body consists of 70,000 billion cells. Cells are the basic unit of living things. They have different functions and shapes. There are also gametes (sperm in men and eggs in women) which are the cells responsible for sexual reproduction.
The control center of the cell is the nucleus. Each nucleus of our cells contains the genetic heritage of each human being formed by 23 pairs of chromosomes (i.e. 46 chromosomes per person). Twenty-two of these pairs, the autosomes, are common to both sexes. The two remaining chromosomes are the sex chromosomes (XX for females and XY for males).
At the time of fertilization, the future child receives half of the genetic heritage of the father (23 chromosomes) and half of the genetic heritage of the mother (23 chromosomes). It is chance that determines this mixture of chromosomes.
The 46 chromosomes are formed by a filament: DNA (deoxyribunocleic acid). This filament looks like a spiral staircase called the double helix. This scale is made up of a sequence of interlocking elements called nucleotides. There are four types of nucleotides: adenine (A), cytosine (C), guanine (G) and thymine (T). The combination of these four letters out of 3 billion characters constitutes the genome, the encyclopedia where all the information necessary for the functioning of an individual is archived.
A gene is a piece of DNA with a specific function. It is the recipe allowing the production of a particular protein which will be responsible for a function of our body. Humans have around 20,000 genes.
None of us have exactly the same DNA. Each individual therefore has variations in their DNA. These variations make us different. However, when one of these genetic variations leads to a change in a protein that alters its function and can result in a disease, we speak of a pathogenic mutation.
Read also: Medical Genetics | The Key to Understand Chromosomes, DNA, Genes
How do you get a genetic diseases?
When a genetic disease results from an anomaly of a single gene, it is said to be monofactorial, or monogenic. In the same family, the mode of transmission of a monofactorial or monogenic genetic disease follows Mendel’s laws, which explains the use of the term “Mendelian disease”. 4 modes of transmission are defined for Mendelian diseases: autosomal or X-linked, depending on whether the gene involved is located on an autosomal chromosome or on the X chromosome; dominant or recessive, depending on whether the disease is dominant (a mutation in one copy of the gene is sufficient to cause the disease) or recessive (a mutation in each of the two copies of the gene is required to cause the disease).
1. What is autosomal dominant inheritance?
Some diseases are transmitted in a family according to the autosomal dominant mode. This means that a person inherits a normal copy of a gene and an abnormal copy of the same gene. However, the abnormal gene is dominant compared to the normal gene, in other words it is he who expresses himself. This explains that this person will be affected by a genetic disease. The particular characteristics of this genetic disease depend on the instructions that the abnormal gene will give to the organism.
Some dominant diseases occur from birth. Others appear only in adulthood. They are called “late”. Examples of dominant diseases are polycystic kidney disease and Huntington’s disease.
2. What is autosomal recessive inheritance?
Some diseases are transmitted in a family according to the autosomal recessive mode. This means that a person must inherit two defective copies of the same gene (one copy from each parent) to develop the genetic disease. If the person inherits only one defective copy and the other is normal, then in most cases that individual will be a healthy carrier of the gene change (mutation) because the normal copy of the gene compensates for the defective the abnormal copy. Being a healthy carrier means that you do not develop the disease, but carry an abnormal copy of the gene in the corresponding gene pair.
Examples of autosomal recessive diseases are cystic fibrosis or sickle cell disease.
3. What is X-linked inheritance?
The X chromosome contains several genes important for growth and development. The Y chromosome is much smaller and contains few genes. Women have two X chromosomes (XX) and if one of the genes on one of the X chromosomes is changed (mutated), the normal gene in the other X chromosome can compensate for its defect. In this case, the woman is a healthy carrier of the X-linked disease. Being a carrier means that you do not develop the disease, but that you carry a defective copy of the gene. Rarely, in some of these X-linked diseases, women may also show signs of the disease.
In men, if one gene on their single X chromosome is abnormal, the disease is expressed.
Examples of X-linked diseases are hemophilia, Duchenne muscular dystrophy, and fragile X syndrome. These diseases therefore often affect male subjects.
Am I suffering from a genetic disease?
Thanks to progress in genetics, it is possible to detect a genetic anomaly for a large number of these diseases, to know the mode of transmission of a genetic disease and to evaluate the risk of occurrence of this disease among the members of a family. These steps are carried out during a genetic counseling consultation in a genetics centre.
Zoom on examples of a disease located on each chromosome
Diagram of the human chromosome set, showing the location of some genes whose mutant forms cause hereditary diseases. Conditions that can be diagnosed using DNA analysis are indicated by a red dot. Ігор Пєтков, CC0, via Wikimedia Commons
Examples of genetic diseases and their origins
There are approximately 6,000 genetic diseases in the world. Their causes are as diverse as the resulting symptoms. This article provides an overview of how six genetic diseases work. It shows that there is no genetic disease BUT a genetic disease with each its own particularities.
MUCOVISCIDOSE (Cystic fibrosis): AN AUTOSOMAL RECESSIVE DISEASE
It is the most frequent genetic disease in Western populations (1 newborn in approximately 4000). Cystic fibrosis (mucoviscidose) is an inherited genetic disease. It affects the cellular functioning of almost all organs such as the lungs, the ENT system, the digestive system, the pancreas, the liver and the bile ducts, as well as the reproductive organs.
Cystic fibrosis is a disease that affects the respiratory tract and the digestive system. The main symptoms are bronchial congestion, respiratory infections, digestive problems and male sterility. A deregulation of the transport of chlorine in the cells leads to an increase in the viscosity of the mucus: this accumulates in the respiratory and digestive tracts, which causes the symptoms.
Cystic fibrosis is an autosomal recessive disease, therefore transmitted by both parents at the same time. It is caused by a mutation in both copies of the CFTR gene, located on chromosome 7. The parents of an affected child are carriers of a single mutation, with no consequences for their own health. But each of their children has a 1 in 4 chance of inheriting the two parental mutations and therefore of being affected by the disease.
Cystic fibrosis manifestations. Maen K Abu Househ, Public domain, via Wikimedia Commons
NEUROFIBROMATOSIS TYPE 1: A DOMINANT DISEASE
There are different types of neurofibromatosis. Type 1 is the most common and affects around 1 in 4,000 people. Neurofibromas are soft, flesh-like lumps that originate from nerve tissue.
The clinical manifestations vary greatly from one person to another, even within the same family. Skin manifestations of the “café-au-lait” type (dark spots on the skin) are the most frequent. Other possible symptoms include benign tumors of varying sizes (called neurofibromas), more or less disabling, located along the nerves or on the skin.
A small proportion of patients are affected by serious complications, mainly malignant tumours.
Neurofibromatosis is an autosomal dominant disease, which means that even if only one of the two alleles of the gene is mutated, the person is sick. The risk for a person of transmitting the disease to their child is 1 in 2.
There is a high rate of spontaneous mutations (de novo mutation): thus, in half of the cases, neurofibromatosis type 1 appears without any family history (neither the father nor the mother are ill).
The causative gene is the NF1 gene located on chromosome 17.
Back of an elderly woman with neurofibromatosis type 1. Almazi, Public domain, via Wikimedia Commons
TRISOMY 21: A CHROMOSOME DISEASE
Also called Down syndrome, trisomy 21 results from a chromosomal abnormality.
The usual consequences of trisomy 21 are a variable intellectual deficiency, a characteristic facial appearance (which does not prevent the child from having features resembling his parents); a decrease in muscle tone, malformations of variable importance, most often of the heart or the digestive system.
Trisomy 21 results from a chromosomal abnormality caused by the presence of a third copy (complete or partial) of chromosome 21. It occurs without any family history in the vast majority of cases, often in connection with a high maternal age.
Down syndrome (Trisomy 21), Trisomy 18 (Edwards syndrome), Trisomy 13 (Patau syndrome), Trisomy 9, Trisomy 8 (Warkany syndrome 2)
HEMOPHILIA: A DISEASE LINKED TO CHROMOSOME X
Hemophilia mainly affects boys (about 1/5000). It is a hereditary hemorrhagic disease due to the absence or deficiency of a blood coagulation factor.
Hemophilia is the cause of spontaneous bleeding or bleeding following trauma, even minor ones. Joint bleeding (haemarthrosis) is responsible for pain and can lead to joint destruction.
The hemophilia gene is located on the X chromosome, which explains the mode of transmission: only boys are affected because they have only one X chromosome, women carrying the mutation being only carriers. A female carrier can transmit her genetic anomaly with a probability of 1 in 2 to each of her children: one boy in two will be affected, one girl in two will be a carrier.
MYOPATHIES: MUSCLE DISEASES
Myopathies are a group of neuromuscular diseases, the best known of which is Duchenne muscular dystrophy (1 in 3300 male newborns). But there are many others, of variable expression and transmission. Myopathy is a general term that designates diseases whose point is generally associated with impaired muscle function.
Symptoms vary in their manifestations and intensity depending on the type of myopathy. Generally, these diseases result in generalized muscle weakness. This often causes heart and respiratory problems.
Each type of myopathy corresponds to a different deficient protein, playing a role in the production and functioning of muscles or in the junction between muscles and nerves. There are about a hundred different genes involved in myopathies: some are linked to the X chromosome, others are autosomal recessive or dominant.
Nervous and mental diseases. Method of rising from the ground in myopathy. Internet Archive Book Images, No restrictions, via Wikimedia Commons
SICKLE CELL DISEASE – GENETIK BLOOD DISORDERS
Sickle cell disease is a genetic blood disease that particularly affects populations of African origin, approximately 50 million people worldwide.
It is characterized by an anomaly of the hemoglobin, contained by the red blood cells, which ensures the transport of oxygen to the tissues. The abnormal hemoglobin will cause deformation and abnormal rigidity of these blood cells.
Abnormal red blood cells are destroyed in excess, which is a source of chronic anemia. In addition, they tend to aggregate and clog blood vessels. These “vaso-occlusive crises” are painful and sources of infection.
Sickle cell disease is an autosomal recessive disease: the person is sick only if they carry both mutated copies of the HBB (Hemoglobin Subunit Beta) gene. It is located on chromosome pair 11 and is responsible for coding one of the constituents of hemoglobin. If the parents of an affected child are carriers of a single mutation, each of their children has a probability of 1 in 4 of inheriting both parental mutations and therefore of being affected by the disease.
Figure (A) shows normal red blood cells flowing freely through a blood vessel. The inset shows a cross-section of a normal red blood cell with normal haemoglobin. Figure (B) shows abnormal, sickled red blood cells sticking at the branching point in a blood vessel. The inset image shows a cross-section of a sickle cell with long polymerized sickle haemoglobin (HbS) strands stretching and distorting the cell shape to look like a crescent moon. The National Heart, Lung, and Blood Institute (NHLBI), Public domain, via Wikimedia Commons
Medical genetics | The Key to Understand Chromosomes, DNA, Genes
Sources: PinterPandai, MedlinePlus, eMedicineHealth.com (Owned and Operated by WebMD), National Human Genome Research Institute, Johns Hopkins Health System
Photo credit (main picture): Ігор Пєтков, CC0, via Wikimedia Commons
Photo description: diagram featuring examples of a disease located on each chromosome.