Their red blood cells contain both kinds of haemoglobin. However, if you live in an area of the world where malaria is a common disease there is an advantage in having the sickle cell gene. It is malaria which kills the most people in the world every year. Providing that you do not get overstressed and build up an Oxygen debt, you will live happily ever after. The number of chromosomes varies between different animals and plants.
Most mutations result in recessive genes.
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