How Are Diseases Inherited?

Of genes and DNA 

Genes are how our information is saved in every cell that is part of our body. This information is very important because it has the code that tells our cells how to function and the time at which they have to start performing those functions. For example, specific genes control the behaviour of bone cells, which proliferate during childhood and early adolescence, but stop doing it after a certain age. Without this genetic control program, bone cells would not know when to stop multiplying and we would continue growing for as long as our cells could.

When genes get damaged: Mutations

Changes in the genetic code would mean changes in the information that they keep and therefore, changes in the instructions that cells might receive. To avoid this, genetic information is well preserved in the DNA chain, which is at the same time restricted to the nucleus of each cell, protected by several mechanisms that avoid any changes that could be done to it.

Even when the genes are very well protected, they do get damaged once in a while. This is a natural process that even helps in the evolution of species. However, when changes to the DNA content occur all of a sudden, they can severely affect the functions of a certain type or types of cells, giving as a result the development of medical conditions known as genetic diseases.

Genetic diseases and their cause

Not al genetic diseases are inherited, because mutations can be caused by external factors. For example, skin cancer is caused by DNA mutations after prolonged exposure to UV rays; another example is the exposure to toxic substances, which can also affect genes in white blood cells and cause certain forms of leukemia.

How does this happen? When the egg and the sperm merge in the uterus of a woman, they combine their genetic information to give rise to a baby. Each one of them has 23 chromosomes, which are structures formed by coiled DNA, giving them an appearance of balls of wool. Every individual has a total of 46 chromosomes in each and every cell of his or her body, which come from the 23 chromosomes provided from mom and the 23 chromosomes provided from dad.

And because half of our information comes from each of our parents, it is possible that we inherit our mom’s eye color or our dad’s face shape. Unfortunately, mutations that cause diseases are inherited too, and they can come from one or from both parents.

See Also: The 16 Most Common Genetic Diseases

Monogenic diseases are the easier ones to detect, since they occur as consequence of a mutation in a single gene. However, polygenic diseases are very difficult to study and to treat, because they can be caused by mutations in several genes, making it hard for researchers and doctors to determine their cause and to come up with effective treatments for them.

Some Examples Of Inherited Disorders 

For you to fully understand inherited diseases, here are a few examples of some of the most common monogenic diseases.  

Types of monogenic diseases

Monogenic disorders can either be dominant, recessive or X-linked. This has to do with the number of copies we have for each gene. As already said, we have two copies for each one of our genes, one that comes from our mom and one from our dad.

An example of this is Huntington’s disease, which is caused by a defect in chromosome 4. Normally, we all have a piece of DNA sequence, known as CAG repeat, inserted in chromosome 4 which repeats from 10 to 28 times. Patients with Huntington disease show an increase in the number of repeats that goes from 36 to 120.

The consequences of this are mainly seen in the nervous system of the patients. This illness affects brain cells, causing degeneration, the gradual lose of movement and alteration in other cognitive abilities, such as the ability to learn, think and reason.

The disease is inherited from one affected parent to daughters and sons. If one of the parents has the disease, there is a 50% chance that all his or her sons and daughters develop it too; if both parents have it, then it is certain that their childs will develop Huntington’s at some point of their lives. Every time the mutation is passed, the CAG repeats increase in number, which intensifies the disease.

If only one copy is affected, then this person won’t show any symptoms of the disease, but will certainly be able to inherit it to his or her descendants.

An example of a recessive genetic disorder is Tay Sachs disease. It is caused by a mutation in a gene present in chromosome 15 and it is a mortal disorder. The mutation, which is inherited from both parents in order for it the affect the child, affects the functionality of a protein that helps in the degradation of a fatty chemical brain substance known as gangliosides. Because the protein no longer exerts its normal function, gangliosides accumulate in the brain cells, causing damage that leads to deafness, blindness, paralysis, dementia, psychosis and finally death.

X-linked monogenic diseases

X-linked monogenic diseases, as its name indicates, are caused by mutations in genes present in the X-chromosome, which is a sex chromosome.

An example of this type of disorders is hemophilia. Hemophilia is a recessive X-linked disorder related to a lack of specific factors that contribute to the formation of blood clots. Since these factors are reduced in quantity or not present at all, the blood clotting process is defective and people suffering from this illness are prone to constant intense bleedings.

See Also: How do Genetics Impact Behavior and Personality

Since it is recessive, only one defective copy of the gene is needed for the disease to fully develop. Because men only have one copy of the gene (as they only have one X chromosome), they develop the disease, whereas women, who have two copies, can use the good copy instead of the bad one, and not have any symptoms or show a milder form of the disease. 

Inherited disorders are severe most of the time and are not easy to understand. Currently, there are more than 10,000 known monogenic disorders and scientists work everyday to fully understand them in order to find a treatment or cure.