INHERITANCE PATTERN OF DISEASE

Hi everyoneee!!! Welcome back to my blog ☺️

How're you guys doing? I know you must be very happy to be surrounded with your beloved family right.. hmmm I wish I was with them too :( But I cant,, because there is so many things need to be done. When I get up from sleep I immediately do my works for the entire day until they stuck in my mind and interrupt my sweet dreams. Ahh I need to get it done before going back to my hometown next week no matter what. I cannot let them lingering in my mind and keep following me everywhere, it is just so suffocating. 

Anyways,
Have you read my previous post? What do you think of it? 
It is good to know it right??  Because sometimes we just know the name of the disease but we never know what it's like. And sometimes, we see the abnormal signs which indicates the person is having a disease, but we don't know what it is.

Imam Syafi'e has once said, "the more knowledge that I gain, the more I know my ignorance"

So now, I want you guys to concentrate more this time, as I am going to tell you about this topic much deeper. 

Tonight, I want to share to you about how the disease develop, or how it is inherited.

Before we go into detail, let's read basic info about genes and chromosomes:

In all of the cells of our body, our genes are found on chromosomes (long strings of genes). We have many thousands of genes that provide information for our body to grow, develop and remain healthy. The gene sends messages to the cell to make important chemical products such as proteins. There are usually 46 chromosomes in each cell that are arranged into 23 pairs. One of each pair is passed on to us from our mother and the other from our father. 22 of these chromosome pairs are numbered. These numbered pairs are known as the autosomal chromosomes. 

The 23rd pair is made up of the sex chromosomes called X and Y. Since the chromosomes come in pairs, there are also two copies of each of the genes. The exception to this rule applies to the genes carried on the sex chromosomes called X and Y. A variation in a gene that creates a fault is called a pathogenic variant or mutation. Genes are sections of DNA that code for the proteins our body needs to function. A mutation in a gene will affect the body differently depending on how much it changes the resulting protein, how critical that protein is to the body and how much of that protein is needed in the body. 

Ok now, move on to the disease. 

Huntington's disease is an autosomal dominant disorder, whereby one mutated copy of the gene in each cell is sufficient for a person to be affected. This means that a person needs only one copy of the defective gene to develop the disorder. This defective gene is inherited from an affected parent either the mother or the father. In rare occasions, the condition may also result from a new (de novo) mutation in the gene and occur in people with no history of the disorder in their family. With the exception of genes on the sex chromosomes, a person inherits two copies of every gene (one copy from each parent). A parent with a defective gene could pass along the defective copy of the gene or the healthy copy. Therefore, each child in the family has a 50 percent chance of inheriting the gene that causes the genetic disorder. This disease usually begins between ages 30-45. It can also develop in childhood or adolescence, which is the juvenile form of Huntington's disease.

Let us look at the definition of autosomal dominant disorder one by one:-

Autosomal: a gene located on a numbered chromosome and usually affects males and females in the same way. 
(Dominant gene mutation is located on an autosome, one of the chromosomes numbered 1-22).

Dominant: having power and influence over others. 
(Individuals who have a dominant mutation on one gene, and a working copy of that gene on the other partner chromosome, will be affected by that condition despite the working copy. Therefore although one of the gene copies is correctly sending the instructions to make the gene product, the other copy with the dominant mutation is not sending the correct message and overrides the action of the working gene).

Disorder: disease or illness

So, autosomal dominant disorder means, a disease where it develops when a person is having one copy of affected gene on numbered chromosomes. Dominant is different with recessive because, autosomal recessive need both copies of affected gene to develop the disease.

If a person has an autosomal dominant gene mutation, they do not have the ability to make enough of the correct gene product and will have symptoms of the genetic condition from birth or be predisposed to developing the condition later in life. In this case, person with Huntington's disease has mutated HTTgene.

It is hard to just imagine right. Therefore, here I provide you some illustration that can help you understand :). In this illustration, D represents mutated HTT gene while d represents normal copy of HTT gene. 

Dd		Affected by or predisposed to Huntington's disease. This person has the gene pair Dd. This means that one copy of the HTT gene is working and producing the correct gene product however the other copy (D) is not working. They develop symptoms of Huntington's disease. We need both copies of these gene to be working to not have the disorder. When this person has a child, there is a 50% chance they will pass on a working d gene to each of their children and a 50% chance they will pass on the mutated HTT gene which is not working (D)
dd		Unaffected person.          This person has the gene pair dd. This means that both copies of HTT gene are working and they are able to produce the gene product. They will not develop symptoms of Huntington's disease.          When this person has a child, they will only pass on the working gene copy (d)
DD		Affected by or predisposed to Huntington's disease.          This person has the gene pair DD. This means that both copies of HTT gene are not working and none of the correct gene product is being made.          This person has a double copy of a dominant mutation (DD) and is often more severely affected by the condition than someone with just one dominant mutation (Dd)

Where an autosomal dominant gene mutation is represented by ‘D’ and the working gene copy by ‘d’, There are three possible combinations a person could have. This is regardless of whether the person is a male or female.

Autosomal dominant inheritance when one parent carries
the autosomal dominant faulty gene copy (mutated HTT gene)

If both parents carry one copy of mutant gene, each of their children has 25% chance to be unaffected, 50% affected or 25% severely affected

Autosomal dominant inheritance when both parents carry
the autosomal dominant faulty gene copy (mutated HTT gene)

The child who inherits both copies of mutated HTT gene from his/her parents may be more severely affected than their parents, or may not even survive, and/ or have a younger age of onset for conditions that develop later in life, depending on the condition. 

That's all from me! I hope you guys have a better picture of how Huntington's disease inherited from one generation to another. Goodnight and have a sweet dream all!!

REFERENCE : https://www.genetics.edu.au/publications-and-resources/facts-sheets/fact-sheet-8-autosomal-dominant-inheritance