The human body is made up of millions of cells. Most cells contain a complete set of genes.

Genes are the “recipe of life” acting as a set of instructions controlling our growth and the functioning of our body. They are responsible for many of our characteristics, such as eye color or body height. 

When a gene mutation occurs, the protein product may be defective, ineffective, or absent. Depending on the function of the particular protein, this may affect a single or several organ systems.

Genes are made up of a chemical called DNA and are located inside filamentous structures called chromosomes.

Each person has 46 chromosomes in most cells. These are 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes, i.e. X and Y.  Chromosomes are inherited from parents, 23 from the mother and 23 from the father, so each person has 2 full sets of 23 chromosomes or 23 "pairs". Because chromosomes are made of genes, everyone inherits 2 copies of most genes, one copy from each parent. The situation is slightly different in the case of sex chromosomes, where in case of male gender there is one X and Y chromosome and in case of female gender two X chromosomes, respectively.

Autosomal inheritance:

This type of inheritance concerns genes located on autosomal chromosomes, i.e., not related to sex.
In the case of dominant inheritance, one copy of the defective gene is enough for the symptoms of the disease to occur.
In the case of recessive inheritance, having one correct version of the gene prevents the disease from manifesting, i.e., two defective genes are necessary for manifestation.

Dominant autosomal inheritance:

If a child inherits one normal gene and one abnormal gene from the parents, and the abnormal gene is found to be stronger, it is autosomal dominant inheritance. The abnormal gene will dominate the normal gene, thus one abnormal gene will be enough to develop the disease, as it happens for Marfan syndrome, neurofibromatosis type 1 or also HNF1B nephropathy.

Recessive autosomal inheritance:

If a child inherits one abnormal gene and one normal gene from the parents, but the abnormal gene is weaker, it is autosomal recessive inheritance.  The abnormal gene will be dominated by the normal gene. Two abnormal genes are necessary to develop the disease, thus the disease will only come to light when two abnormal genes are passed on to the offspring, such in the case of Bardet Biedl syndrome, primary hyperoxaluria type 1 or autosomal recessive polycystic kidney disease (ARPKD).

The so-called "de novo" mutation means that the mutation has not been passed on by any parent (both parents have normal gene), but the gene change/mutation in the child occurs spontaneously during the development process.

X-linked inheritance

We talk about X-linked inheritance when the genes whose mutations cause a given disease entity are located on the X-chromosome. These diseases differ from autosomal diseases in affecting women and men in a different way. This is because both sexes have a different set of sex chromosomes:  While women have two X-chromosomes, and thus two versions of each gene on the X-chromosome, men have only a single X- chromosome, i.e., only one version of a given gene. Therefore, a defective gene on the X-chromosome will lead to disease in a male, while in a female the second X-chromosome can compensate (X-linked recessive inheritance)