Variation and Inheritance - Sickle-Cell Anaemia
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.
Source: BBC GCSE Bitesize
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.
Source: BBC GCSE Bitesize