Genetic jinxes | Bangkok Post: learning

THE CHINGCHOK Hunter

Imagine that you are living in the countryside, away from pollution. You drink the finest spring water and eat only organic meals. You never smoke or drink alcohol. You are fit and healthy and exercise daily. You would imagine that you are destined for a healthy, happy, long life, and you probably are, unless you have a killer lurking in your genes.

If that is the case, there is nothing you can do to stop the onset of a genetic disease and its affects, and if it is one of the lethal genetic diseases, you will die from its effects. It is the ugly truth of the matter, but hopefully, in the not too distant future, there may be a remedy.

What are genes?

Genes are lengths of DNA (deoxyribonucleic acid) that exist in the nucleus of your cells. They tell your body how to build proteins. You have roughly 20,000 to 25,000 genes that exist on structures called chromosomes. Each human has 23 pairs of chromosomes. Your total genes make up your genome.

There are two variations of each gene (one on each chromosome pair). These two gene variations are called alleles. You get one allele from your mother and one from your father, which means that you have two forms of each gene. Whether these alleles are dominant or recessive can determine whether you have that genetic trait as a part of your physical make up: your phenotype.

Genetic diseases can come in various forms. You can have a single-gene disorder which is the result of a mutation (change) in one gene. Relatively common examples include sickle-cell anaemia, Huntington's disease and cystic fibrosis, although there are thousands more.

There are other, more complicated diseases (although not necessarily more lethal) that are the result of the mutations of many genes. Examples include Alzheimer's disease and diabetes, but many more conditions are polygenic.

When whole chromosomes (which are huge lengths of DNA and contain many genes, remember) are duplicated or deleted, or when parts of chromosomes are joined or lacking, severe disabilities can occur, with the most famous being Down's syndrome, which is the result of having a triplet of chromosome 21 as opposed to just a pair.

Yet more variations of genetic disorders exist, with some being specific to sex chromosomes (the XX for females or XY for males as chromosome pair number 23) and others being only inherited along the female line in mitochondrial DNA, the separate DNA not in the nuclei but in the organelles that allow respiration to take place.

A genetic assassin

Huntington's disease (HD) is an inherited progressive form of dementia in which personality, memory and moods change as the disease advances. It is also a dominant single gene disorder. What that means is that if you only have one form of the mutated gene, the result is that you will get HD.

All humans have a gene called the huntingtin gene that codes for the huntingtin protein, the protein that causes HD, which was first described by Dr George Huntington in 1872.

If only one huntingtin gene mutates and becomes longer, it leads to severe consequences and results in that person suffering from HD. Even though every cell in the body has the gene, only the cells in the brain are affected. People with HD usually show signs of the disease at around the age of 40, although this can vary widely between individuals.

What happens is that the mutated gene codes for a protein with very different properties than the usual huntingtin protein, with one of the characteristics being that it increases the decay of your nerve cells or neurons. This leads to brain degeneration a few years after the initial symptoms, and this eventually results in death.

Despite HD being a gene-dominant disease, it remains in society only because the symptoms, and the inevitable death, happen after the reproductive phase in life. Should people know it is in their family, however, testing can be done to see whether the children have the disease. As it is dominant, there is a 50-percent chance at best that the children will inherit the disease.

Should a child have a mutated allele, depending on the gene length, a realistic lifespan can be estimated. If the onset is later in life, a couple can undergo in vitro fertilisation to attempt to have children while ensuring that the disease is not inherited.

More killers

Cystic fibrosis is a recessive condition that is lethal to most sufferers before they reach the age of 30. A friend of mine at university had cystic fibrosis and died in her early 20s. By recessive, I mean that you have to have both forms of the gene (both alleles) to be a sufferer. If you only have one of the alleles, you are a carrier and will not develop cystic fibrosis.

This complicates matters as the gene could be in you and you will probably never know and therefore never get tested for it. If you happen to have a partner who is also a carrier, and a surprisingly large number of people are, there is roughly a 25-percent chance that your child will have the disease.

The disease leads to a thick mucus building up in the lungs, which leads to respiratory infections. Difficulties in breathing and the absorption of nutrients are a result of the disease, although it has many other symptoms and effects. Although there is no cure, the life expectancies of sufferers have increased substantially.

Sickle-cell disease is a recessive disease that affects people from sub-Saharan Africa or who are of such origin. The reason it affects this specific race of people is that if a person carries one allele of the mutated gene (which is a mutation of the haemoglobin gene) in his or her genome, he or she shows significant resistance to malaria.

This has led to about a third of people from sub-Saharan African origin carrying the allele, although should both partners carry it, there is a chance of full-blown sickle-cell disease that ultimately leads to death, usually between the ages of 40 and 50 years.

Canavan's disease?

Believe it or not, there is a disease with my last name! Canavan's disease is a genetic disease that, like so many others, is fatal. It is a mutation of an enzyme-producing gene, and it leads to mental retardation and brain damage.

It is a recessive condition, meaning that if two parents are unaffected carriers, the child has a 25-percent chance of developing the disorder through inheriting both recessive alleles. The result, tragically, is an early death for the child, and not a great life beforehand.

What can be done?

There is some hope that a technique called gene therapy may be the answer to some genetic diseases, especially the single-gene diseases. The concept is to replace the mutated gene in your genome with a correct copy of the gene that will then replicate in your cells and allow normal functioning and development. There are also treatments that can expand one's life and improve the life quality of victims of certain genetic diseases.

Whether gene therapy is a realistic procedure in the near future is unknown, but it certainly gives hope to sufferers or soon-to-be sufferers of genetic diseases. I have named but a few among thousands. It really is a scary world where your fate is already predetermined at your birth by your DNA.

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Dave Canavan, who has an MSc in Behavioural Ecology, is the principal of Garden International School. Dave is fascinated by science and loves animals, especially the dangerous kind! You may contact him at

davidc@gardenbangkok.com .