Biology Question #1551

jacqui, a 42 year old female from Stockleigh Pomeroy asks on August 21, 2003,

Explain recessive gene disease and what happens to the DNA to cause parents to be carriers.

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The answer

Barry Shell answered on August 25, 2003

Because every person's genetic makeup is a 50/50 combination of the genes that come from two parents, each of us have two copies of every gene. This goes for the genes that code for eye-colour as well as the genes that result in a disease. With many kinds of gene pairs, there is often one kind that is dominant, and one that is recessive. That means that, of the two copies, it will always be the dominant one that is expressed, not the recessive one. The recessive copy is there, inside you, but it is not used by the body to make any proteins. In a genetic disease, this means that it lies dormant and can come out in the next generation. For eye colour it could mean that a mother with brown eyes can produce a blue-eyed child.

This happens because during reproduction, only one set of the two copies is passed onto the child from each parent. For any given gene, a mother or father can have two "recessive type" copies, or two "dominant type" copies, or one of each. They pass only one of these on to their offspring. You can take a few minutes to draw a little chart to see how the probabilities work out. If one parent is pure dominant, and the other is pure recessive, the child will get one dominant gene copy and one recessive gene copy, and the result will be that the recessive copy will not be expressed, but will be "hidden" in his or her genetic code and may come out in the next generation. Remember, when one of the two copies is recessive, only the dominant one gets expressed, or "used" to create either the eye colour proteins or the proteins that result in the genetic disease. Blue eyes are caused by a pair of recessive genes so they are more rare than brown eyes which are dominant.

If both parents have one copy each of dominant and recessive types, then there's a 25% chance the child will get two dominant, or two recessive copies of the gene. If the child gets two recessives, then the blue eyes, or genetic disease will be expressed.

If both parents have double recessive copies, the child will have a 100% chance of getting the blue eyes or the genetic disease. As you can see from these probabilities, most of the time the dominant gene is expressed, and for this reason traits such as genetic disease can lie dormant within the gene pool, only to reappear by shear chance after several generations.

One might ask: why are these traits for genetic disease preserved at all? The answer is that sometimes they are linked to other very beneficial genes. For instance, the gene for cystic fibrosis, a genetic disease that causes the lungs to fill with mucous, is thought to be linked to a gene that confers immunity to certain diarrhoea-causing or other common widespread disease. It is not uncommon in nature to find the "good" linked together with the "bad".

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