Variation and Inheritance is an interesting and important chapter. You may want to use BBC's GCSE Bitesize to help you conceptualize the chapter before you proceed with revision with the textbook. Note: Evolution is not tested, and for cloning you only need to know such things exist but not learn them.
Sickle-cell anaemia is caused by a recessive allele for the haemoglobin gene - Haemoglobin S, instead of the normal Haemoglobin A - which causes the production of abnormal haemoglobin. Individuals who are homozygous for this allele (ie, Haemoglobin SS) suffer sickle-cell anaemia.
Red cells that contain the abnormal haemoglobin change shape and become sickled when the oxygen levels are low. The haemoglobin forms crystals that distort the membrane of the cells and may even destroy them. The loss of red blood cells makes the sufferer anaemic and weak. The irregularly-shaped cells may block capillaries and cause clotting, which can affect the blood supply to the heart and brain.
Some people are heterozygous for the sickle cell condition - they are AS (one normal Haemoglobin A and one Haemoglobin S). Here the red cells are affected but only some become sickle-shaped, so the effect is not so great. Such heterozygous individuals who have the sickle cell trait but not the disease itself are called carriers.
Use the animation below to work out the chances of both the trait and the disease occuring in the children of AA/AS parents, and in the children of AS/AS parents.
One field in which genetic engineering has had a huge impact is the mass production of insulin to help diabetics. Scientists have isolated the gene responsible for making human proteins, including the insulin hormone. This gene is inserted into the bacterial DNA, and the microbes then clone themselves rapidly, making identical copies of themselves, all with the new gene and all capable of making human insulin. This is a cheap way of producing sufficient quantities of exactly the right hormone, for everyone who needs it.
Other applications of genetic engineering:
Producing interferon, a human protein which stops viruses multiplying inside the body
Producing human growth hormone to treat growth abnormalities
Blood clotting factor to treat haemophiliacs
Used in industry to produce enzymes for use in biological washing powder
Producing pest resistant crop varieties
Producing tomatoes that stay fresh much longer
Genetic engineering makes it possible to transfer genes across the species and has unlimited potential for influencing the genetic make-up of living organisms. It could potentially change not just what we do but who we are, which is why the topic of genetic engineering triggers heated moral and ethical debates.
