Genetic Mutations & Gene Therapy

 What is a genetic disorder? 

A genetic disorder is a disease caused by an abnormality in an individuals DNA. The abnormalities in the DNA, however, differ from one person to the next. Abnormalities can range from a small mutation in a single gene to the addition or subtraction of an entire chromosome or set of chromosomes (Citation 1). Genes code for proteins; proteins are molecules essential to life and make up a majority of cellular structures. Proteins can no longer carry out their function properly when a gene is mutated, resulting in a disorder.(Citation 2).

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 What are the causes of genetic diseases?

Inherited Mutations

Many genetic diseases are caused by mutations passed down through families. There are 4 different types of inherited genetic disease; single gene inheritance, multifactorial inheritance, spontaneous chromosome abnormalities, and mitochondrial inheritance.

Single Gene Inheritance 

There are 2 types of single gene inheritance; autosomal dominant and autosomal recessive. When an autosomal genetic disease occurs, only one mutant gene is passed down to the child from the parent (father or mother) and only occurs if at least one parent has the genetic disease. Haemophilia is an example of autosomal dominant genetic disease. When an autosomal recessive genetic disease occurs, the mutation must be present in both inherited genes. Autosomal mutations are passed down to children whose parents do not have the disease, but are instead carriers of the disease; carriers being the people who only have one mutated gene. Generally, there is a 25 percent chance that a child of two carrier parents will develop the genetic disease. Cystic fibrosis is an autosomal recessive genetic disease (Citation 3).

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 Multifactorial Inheritance

Multifactorial disorders are also called complex or polygenic inheritance. They are conditions caused by the collaboration of genetic and non-genetic agencies of environmental factors. Asthma, congenital heart disease, dementia, coronary heart disease, venous thrombosis, and diabetes mellitus are multifactorial disorders. For some of these disorders, such as coronary heart disease, there is an idea of the genetic and environmental risks that contributes to the disorder. For others, such as asthma and diabetes mellitus, the information regarding the environmental risks tops the information known about the genetic risks. However, researchers predicated that over the coming ten years, knowledge and information on genetic disorders will greatly increase as well as the contributions causing the genetic disorders (Citation 9).

 Spontaneous Chromosomal Abnormalities

It is not rare for children to suddenly develop a genetic disease that is not inherited. Instead, the disease results because of spontaneous genetic abnormalities that result normally during the fertilization of the egg by the sperm. Egg and sperm cells usually develop during a process called meiosis. After the process of meiosis, the result is usually a set of cells that contain 23 chromosomes. Two sets of 23 chromosomes result after the egg and sperm fuse together. Unfortunately, this process doesn’t always happen as planned. Sometimes defects occur during meiosis which leads to the egg cell either lacking or carrying an extra copy of a chromosome. If the resulting cell obtains an extra chromosome, a cell with 47 chromosomes is created, such as those found in people with Down Syndrome. Although some studies have shown that Down Syndrome can be inherited, the majority of studies have shown that Down Syndrome instead is a result of spontaneous chromosomal abnormalities (Citation 3). However, during cell division can occur after fertilization instead of during the actual process, resulting in mosaicism. Mosaicism is a disorder in which the cells of a person have a different chromosomal composition. Turner syndrome is a result of mosaicism. The range of which mosaicism affects an individual differs and can range from mild to severe depending on the percentage of abnormal cells (Citation 8).

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Mitochondrial Inheritance 

Mitochondrial inheritance is a genetic disorder resulting from mutations in the nonchromosomal DNA of mitochondria. Mitochondria are small organelles found in the cytoplasm of plant and animal cells. They are part of the main process during cell division. About 5-10 pieces of DNA are found in mitochondria. In contrast to sperm cells, egg cells keep their mitochondria during fertilization therefore making mitochondrial DNA and hence the disorder is only and always inherited from the female parent (Citation 11). Normally the mother will have mitochondria containing a combination of healthy, normally working genes along with defected genes. The number of defected genes, however, would have to outnumber the amount of normal genes in order for a disorder to occur. Depending on the individual, the number of mitochondria with defected genes vary from one organ/ tissue to the next. Usually a mother will have a mixture  of mitochondria containing the working gene copy and others containing the faulty gene. While it is possible to test for the presence or absence of some defected mitochondrial genes during pregnancy, the result may not always be accurate and difficulties interpreting the result may occur. Symptoms of the disorder resulting from mitochondrial inheritance may not occur if the amount of genes defected is not substantially high and therefore interfering with energy production (Citation 13).

 Acquired Mutations

Acquired mutations can often result in genetic diseases. They result due to mutations in the cell while undergoing cell division. Thus, a mistake must occur in the sequence of the cell while it is copying the billions of nucleotide bases that make up DNA. The mutations that occur while copying the DNA get passed down to the daughter cells, which in return, multiply into more cells. As a result, of the many mutated cells that have been passed on because of the mistake that occurred in the sequence of nucleotide bases, the cells do not carry out their normal function and begin to behave abnormally. Cancer results from acquired mutations, and is the most common genetic disease according to The Human Genome (Citation 3).

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                            Overview of inherited birth defects.

           Overview of what happens when DNA mutations occur. 

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