Mendelian Inheritance: Genes and Types

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.

References

National Human Genome Research Institute "Gene" October 16 2023

https://www.genome.gov/genetics-glossary/Gene#:~:text=Genes%20are%20passed%20from%20parents,approximately%2020%2C000%20protein%2Dcoding%20genes.

 National Library of Medicine "Classic Mendelian Genetics (Patterns of Inheritance)"

https://www.ncbi.nlm.nih.gov/books/NBK132145/#:~:text=There%20are%20five%20basic%20modes,and%20complex%20multi%2Dfactorial%20diseases.

New Gene Therapy for X-Linked SCID

A new gene therapy is showing promise in the treatment of X-Linked Severe Combined Immunodeficiency (X-Linked SCID). Because this is an X-linked disease, males are almost always the ones affected by the condition. People affected by this having recurring infections due to bacteria, viruses, and fungi that naturally occur in the body. In healthy individuals, these do not usually cause any problems, but due to a weakened immune system people with X-Linked SCID can't protect themselves from it.

X-Linked SCID is linked to a mutation in the IL2RG gene. This mutation prevents immune cells from developing and functioning correctly. The new gene therapy involves removing bone marrow from the affected patient and inserting a normal IL2RG gene. Then the bone marrow is inserted back into the patient where it hopefully makes new blood cells and improves immune function.
Three years later, one patient has had continuous improvements in immune function. Another patient died from damage to the lungs before treatment began. This stresses the importance of starting early for the treatment to work. Another 3 patients received treatment 3 to 6 months ago and are now beginning to see improvement.

There have been too few of studies in this to know for sure, but this gene therapy seems to be working so far as long as it is started early. There would need to be more studies done to find out when the optimal time for treatment would be. Hopefully if done early enough, the people affected by this disease could live much longer than usual.