Posts Tagged ‘sickle’

Sickle Cell Anemia and Genetic Fence-Sitting

// June 22nd, 2009 // 5 Comments » // How Things Work

Following on from the theme of biological errors and the noise of life, I thought I’d talk about sickle cell anemia, quite a common ailment inherited Mendelian fashion through recessive genes.

If you’re new (very new) to genetics, then check out a picture for what I mean by recessive, and click through to Wikipedia to explain it all (man I love Wikipedia, is it natural to love an online encyclopedia of dubious accuracy? It feels so RIGHT!) Anyways, in the picture white is the recessive gene, and the flower will only have a white phenotype (as in, looks white) if both versions of the genes are the white recessive one.

mendelian_inheritance_3_1

Substitute red with normal haemoglobin (Hb), and white with mutated haemoglobin (HbS – S for Sickle or Screwed) and that’s what’s going on with sickle cell. If both your copies of haemoglobin have the sickle cell mutation, then you get the door prize – haemoglobin that tends to polymerise when it releases oxygen to form long rods (hehe, long rods) in the red blood cells, making them look all deformed and shabby like thus:

Sickle Cell Red Blood Cell
Shaped like a moon, or a sickle – think Grim Reaper!

Okay, I’m being a bit jokey about this, but it’s actually an awful disease – the cells are the wrong shape, and get caught in the blood stream causing blockages, which means progressive organ damage and pain, and they also don’t last as long (10-20 days rather than 120 days) causing anemia. Most people die early.

The weird thing is – this is terrible disease has been around for a long time with a clear genetic link – why hasn’t Darwin fished out out the mutation from the gene pool yet?

Researchers think that the sickle cell mutation was actually selected FOR, because there is evidence that having one copy of HbS and one normal Hb gene protects people against malaria.

Malaria is a highly lethal disease caused by protozoa carried by mosquitos, and even today kills about one million people a year. People who have African heritage often have a higher incidence of sickle cell, because malaria is endemic there. So the picture looks more like this –

scd-malaria

You’re damned if you do and damned if you don’t, but all good if you’re a genetic fence-sitter and take one of each. Indecision for the win!

(Wish I could take credit for the pictures, but still setting up the computer… it arrived yesterday – yay! It’s very sexy)






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