Genes can be said to be the bedrock of Genetics. They are the basic unit of heredity that can be passed from parents to offspring. Coined from the name of the biologist that proposed the rules of inheritance, Gregor Mendel; Mendelian inheritance is used to describe the biological inheritance principles of how certain traits are inherited by offspring from their parents. There are different types of Mendelian inheritance like autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive and mitochrondial. These modes of inheritance are expressed in single-gene diseases. Let us discuss them:
In autosomal dominant, the offspring requires only a single copy of a gene variant to develop a disease or condition. In this inheritance mode, one of the parents of the child needs to possess the gene. It is important to note that this inheritance pattern occurs in every generation. Examples of diseases that can be expressed through autosomal dominant inheritance are neurofibromatosis, Huntington's disease and familial hypercholesterolemia among others.
On the other hand, in autosomal recessive, each parent contributes a gene variant each for the offspring to have two copies of the gene variant to develop a disease or condition. Here, the pattern may not be seen in every generation. Examples of diseases that are expressed through this inheritance pattern are sickle-cell anemia, Tay-sachs disease, phenylketonuria and so on.
The X-linked dominant is a sex-linked mode of inheritance. Here, it is common to see that both males and females carry the trait in each generation. The only thing is that fathers cannot pass their X chromosomes to their sons, instead they pass it to their daughters and pass their Y chromosomes to their sons. Consequently, females are mostly affected here. Examples of diseases that are expressed in this inheritance patterns are ornithine transcarbamylase deficiency and hypophatemic rickets.
X-linked recessive like their dominant counterpart are sex linked. However, here males that have an affected allele on their X chromosomes are unable to pass the traits to their sons because they are hemizygous. Their daughters nonetheless, will carry those traits. Mothers that are heterozygous and healthy can pass the affected traits to their sons. So here, males are mostly affected and these affected males are seen in every generation. Hemophilia A and Duchenne muscular dystrophy are some diseases expressed by the X-linked recesssive.
In mitochondrial, female parents are the only ones that can pass the affected traits even though both males and females can be affected. It is possible for this inheritance pattern to appear in each generation and Kearns-Sayre syndrome is a disease that can be expressed by this patterns.
National Human Genome Research Institute "Gene" October 16 2023
National Library of Medicine "Classic Mendelian Genetics (Patterns of Inheritance)"