IB Genetic disorders © Oxford University Press 2011 Genetic disorders

IB Genetic disorders © Oxford University Press 2011  There are about 5000 genetic (inherited) disorders caused by a defect in just one gene.  Cystic fibrosis and sickle cell anaemia are caused by a defective recessive gene; polydactyly and Huntington’s chorea are caused by a defective dominant gene.  There are other genetic disorders, such as Down’s syndrome, caused by the presence of an extra chromosome.  It is possible to carry a genetic disorder without suffering from it.

IB Genetic disorders © Oxford University Press 2011 Polydactyly  This genetic disorder results in more fingers or toes than usual.  About 1 in every 500 babies is born with an extra digit – usually on the little finger side of the hand or little toe side of the foot, but sometimes on the thumb side.  The extra digit causes no harm and is usually removed surgically at birth.  Sometimes seven or nine digits are formed on a hand or foot.  Polydactyly is more common amongst black African and African American children than among white children.

IB Genetic disorders © Oxford University Press 2011 Cystic fibrosis (CF)  This is a genetic disorder of the cell membranes lining the airways and pancreas which have faulty chloride ion channels.  It is one of the most common genetic disorders amongst white people – about 1 in 2000 births each year in the UK has CF.  CF results in thick, sticky mucus building up in the airways and pancreas.  In addition, there will be scarring (fibrosis) and cysts (cystic) in the pancreas.

IB Genetic disorders © Oxford University Press 2011 Cystic fibrosis (CF) Other symptoms of CF are:  frequent lung infections and difficulty breathing  failure to thrive as children cannot properly digest food because enzymes from the pancreas do not pass into the gut  shorter life - although antibiotics, physiotherapy and lung transplants have extended life expectation beyond 30 years  Infertility - men can make sperm but do not have a sperm duct; women have thick mucus that blocks their cervix.

IB Genetic disorders © Oxford University Press 2011 Sickle cell anaemia This genetic disorder causes the red blood cells to be shaped like sickles instead of the normal round shape. This makes them more prone to damage, so many are destroyed in the circulation, depriving the body of oxygen and leading to anaemia. Sickle cell anaemia is rare in Britain but not in some African countries. Huntington’s chorea The symptoms of this genetic disorder (shaking, erratic body movements, severe mental deterioration) do not usually appear until after the age of 40, by which time the defective dominant allele has been passed on to children and even grandchildren.