In article <3g2gaa$qqk at agate.berkeley.edu>,
Ken Frauwirth (BioKen) <frauwirt at notmendel.Berkeley.EDU> wrote:
>In article <1995Jan21.202003.18443 at lafn.org>,
>Mark Baldwin <ag944 at lafn.org> wrote:
[snip snip]
>>========================= Mark Baldwin ag944 at lafn.org>>My memory says that you are (at least somewhat) correct. Sickle cell
>anemia is a recessive trait: you need both copies of the gene to get the
>disease (the mutation causes a single amino acid change in hemoglobin - as
>long as you have one "good" copy, you can make functional protein).
>However, I believe that heterozygotes (people with one "good" and one "bad"
>copy) *do* have an increased resistance to malaria - perhaps the presence of
>the defective protein interferes with the parasite's ability to live in
>the red blood cell. Since the mutant gene does offer a selective advantage
>(especially in tropical Africa, where malaria is still a problem), it is
>maintained, even though having two "bad" copies is lethal.
>>>BioKen
>--
There are a family of sickle cell traits however in general the problem is
that if you are homozygous for sickle cell then your red cells are very
fragile and as a result the individual becomes anaemic and has poor oxygen
transport along with other complications. If you are heterozygous for the
trait then to a large extent the individual has a normal phenotype. However
the red cells are more fragile and when stressed such as when for example
they are infected with malarial parasite they show the sickle cell phenotype
and are destroyed in the spleen and the liver thus helping to remove the
parasite at the same time. There are anumber of other mutations which are
protective for malaria and which are present in the african and mediteranean
populations and the high frequencies are thought to be maintained by
selection through the disease.
o/ \\ // || ,_ o Mike Clark, mrc7 at cus.cam.ac.uk
<\__,\\ // __o || / /\, Cambridge University, Dept. Pathology
"> || _`\<,_ // \\ \> | "to pay for my hobbies I have to work
` || (_)/ (_) // \\ \_ as an antibody engineer"
