No, there are many more than just one. There's also the thallasemias and CF alleles, just to name a few.
Yes, evolutionists have one beneficial mutation that they can cite as proof that positive, helpful mutations do occasionally occur.
Actually, scientists don't like to use the word "proof" as it is not the objective of science to prove anything, but instead, science is about the accumulation of evidence that leads to logical and rational conclusions that explain and make predictions about the natural world. However, science does have great utility for disproving things, such as creation science.
It is sickle-cell anemia, which is a mutation which occurred in someone in Africa centuries ago. Was that mutation beneficial? Far from it; it damaged the red blood vessels so they became quarter-moon shaped instead of round.
No, it's red blood cells, not vessels.
This produced a special type of anemia. The person with sickle-cell anemia cannot properly absorb food and oxygen.
How then can anyone call that mutation beneficial?
Well, the evolutionists do it on the basis of the fact that people with sickle-cell anemia are less likely to contract malaria from mosquitoes!
Really now, that is begging the question! If I had bulbar polio, I would be less likely to be killed in an auto accident because I would be paralyzed on a bed and less likely to be riding in a car. But one would not say that polio was, for that reason, beneficial!
First of all, polio is not a genetic disease, it's an infectious disease (caused by a virus).
Secondly, you have a failed understanding of genetics. Sickle cell anemia arises when an individual is homozygous mutant at the hemaglobin locus (HbS/HbS). Sickle cell trait is when one is heterozygous (HbA/HbS), while normal individuals are homozygous wildtype (HbA/HbA). Those who are S/S will, in all likelyhood, die from complications of sickle cell anemia before they reach reproductive age. Those who are wildtype A/A don't get sickle cell anemia, but they have a higher risk of contracting cerebral malaria, which is almost always fatal. Those, however, who are heterozytous A/S do not get sickle cell anemia AND when they do get malaria it almost never progresses to cerebral malaria (because the oxygen tension in the red cells prevent the malarial parasite from efficiently completing its life cycle and also reduces infectivity) and they seldom die from malaria. If you do the math you find that the fitness in the malarial belt is:
sickle-cell trait (A/S) = 1.00
normal hemaglobin (A/A) = 0.90
sickle cell anemia (S/S) = 0.14
(where 1 is best chance of survival, 0 would be worst chance of survival). So, you see, this is a beneficial mutation because those who are heterozygous (50%) are the most likely to be reproductively successful than those who are homozygous (25% each A/A and S/S). We know from genetic studies that the HbS allele occurs at about 20-30% in the malarial belt of Africa.
In return for the advantage of being 25 percent less likely to contract malaria, 25 percent of the children of people, in Africa, with sickle-cell anemia will die! What advantage is that? pp. 21, 23.
This statement is in error as I have demonstrated.
Also, the theory of evolution would predict that in the absence of malaria, the sickle cell allele (HbS) would decline and possibly disappear from a population (because the allele itself is poorer at oxygen carrying than is the normal allele, HbA). Is this testable? Sure! Two experiments could be conducted to examine this prediction.
The first experiment would be to simply eliminate the malarial parasite from a population carrying the sickle-cell allele. Although this appears simple, it is much more challenging than it sounds. If you eliminated the agent that exerts a positive pressure on the allele, the allele should become less frequent during subsequent reproductive cycles.
The second experiment would be to remove a population of Africans from an environment where malaria is endemic. Since the pressure to maintain the sickle-cell allele would disappear, its frequency should also become less.
Has either of these experiments been done? Well, yes - sort of. When the slave trade was occurring 400 years ago, Africans were brought to the west, and they were indeed removed from the malarial environment (as cerebral malaria hasn't been a problem in the US). What's the frequency of the sickle-cell allele in those Americans of African descent? Why, it's down to 8% and continuing to decline.
This is evolution.
This is a fact.
The Spanish Survery is self explanitory. Please have another read of it if you are having troubles understanding it.