Genetic diseases are caused by abnormalities in genes or chromosomes. They are caused due to mutations in certain genes and are inherited when the genetic material is passed down from one generation to another. Cystic fibrosis, sickle-cell anemia and hereditary multiple exostoses are some of the inherited genetic diseases. Diseases are inherited when defective genes in either parent are duplicated in their offspring. Here is an overview of the various ways in which genetic diseases may be inherited.
Genetic Inheritance Patterns
Before going through the various inheritance patterns, let us see how dominant and recessive genetic traits differ from each other. As their names suggest recessive genetic traits remain latent in organisms, while dominant traits express themselves.
X-linked Genetic Diseases: The mutations in genes on X chromosomes can lead to X-linked genetic disorders. If a male is affected by an X-linked dominant disorder, there are hardly any chances that his sons will suffer from the same disease. However, all his daughters will inherit the disorder. Aicardi syndrome and Rett syndrome are examples X-linked dominant disorders.
In case of a male suffering from an X-linked recessive disorder, there are no chances that his sons will be affected while his daughters are sure to carry a single copy of the mutated gene. If a female suffers from an X-linked dominant or recessive disorder, there are 50% chances of her offspring having that disease. Hemophilia A (type of blood clotting disorder caused by mutation of the factor VIII gene) and color blindness are the most common examples of X-linked recessive disorders.
Y-linked Genetic Diseases: Genetic mutations on the Y chromosome lead to Y-linked disorders. Y chromosome is small and does not carry many genes. As less number of genetic traits are linked with the Y chromosome, Y-linked diseases are less common. As only males have a chance of suffering from Y-linked genetic diseases, they are inherited from father to son. Male infertility is a Y-linked genetic disease.
Autosomal Dominant: In this pattern of inheritance, only one mutated copy of a gene is necessary for an individual to suffer from the disorder. There is 50% chance that a child will inherit this type of genetic disorder. Hereditary nonpolyposis colorectal cancer and Huntington's disease are some examples of autosomal dominant genetic disorders.
Autosomal Recessive: In this inheritance pattern, two copies of the gene are mutated and each copy of the gene is known as a carrier. Cystic fibrosis and sickle-cell anemia are some examples of autosomal recessive genetic disorders.
Mitochondrial Inheritance: Mitochondria are the energy houses of a cell. They are responsible for the production of energy. They have their own DNA that is different from that of the cells. Mutations in the non-chromosomal DNA of mitochondria cause mitochondrial inheritance diseases. Mitochondrial inheritance is also referred to as maternal inheritance. Male sperms do not contain mitochondria; it is the egg-cells that contribute mitochondria to the developing embryo. So, only mothers can pass mitochondrial inheritance genetic diseases to their offspring. Leber's hereditary optic atrophy, an eye disease and mitochondrial encephalopathy, a form of dementia are some examples of mitochondrial inheritance diseases.
Multifactorial Inheritance: Also known as polygenetic inheritance, it is caused by mutations in multiple genes. Environmental factors can also lead to multifactorial inheritance disorders. They are often known as complex disorders as they do not have a defined inheritance pattern. Heart diseases, obesity, diabetes, cancer and arthritis are some of the common examples of multifactorial inheritance diseases.
Genetic engineering studies and genetic research have brought to light many of the previously unknown facets of human genetics. However, much is still unknown and research in genetics continues.