1) Single-gene/monogenic Genetic Diseases: In this type of genetic diseases the abnormality happens in one gene. To be more specific, the starting point is a mutation in a single gene. This can lead to production of a defective protein that cannot carry out its function. This is what causes the single gene disorder. There are 6000 known single gene diseases, including cystic fibrosis, Aicardi Syndrome etc.
2) Multifactorial/Polygonic Genetic Diseases: This category of genetic diseases is much more complicated. In this case mutations happen in many genes. The phenotype (the symptoms of the disease), in this case, is a result of mutations combined with a specific environment. These disorders are difficult to analyze, because scientist should take into consideration a number of factors in order to succeed a useful analysis. Diseases like diabetes, Alzheimer, arthritis are due to multi mutations.
3) Chromosomal Genetic Diseases: This type of genetic diseases is due to changes in the structure and number of chromosomes. Besides, chromosomal disorders can be caused by segments and joins of parts of chromosomes. We all know that chromosomes are big DNA molecules composed from genes. The human genome consists of 23 x 2 chromosomes. It is much easier to observer because, sometimes, a chromosomal genetic disease can be detected with microscopic examination. The most popular chromosomal disorder is the Down Syndrome, where there is a third copy of chromosome 21). In this disorder there is a third copy of chromosome 21.
4) Mitochondrial Genetic Diseases: These are the rarest genetic diseases. As we know, mitochondrial DNA is a DNA molecule found in mitochondria, which is an organelle necessary for cellular respiration. It seems that mutations in the mitochondrial DNA can also cause undesirable disorders.
