One would think that deleterious genes would be eliminated by natural selection, yet we have two human genetic
disorders with a high carrier frequency, sickle-cell anemia and cystic fibrosis. Why has this high frequency for these
conditions remained in the population? What will be an ideal response?
ANSWER: The distribution of the sickle cell allele matches the geographic distribution of malaria that is
transmitted to humans by infected mosquitoes. Those who carry one or two copies of the
sickle cell allele are resistant to malaria, and experiments on human volunteers have
confirmed this. In heterozygotes and recessive homozygotes, the plasma membrane of red
blood cells is resistant to infection by the malarial parasite. This resistance gives
heterozygotes a better chance at survival and the opportunity to leave more offspring than
those with a homozygous dominant genotype. Those with a homozygous recessive genotype
are resistant to malaria but also have sickle cell anemia and have a much lower survival rate.
In this case, natural selection favors the survival and differential reproduction of
heterozygotes at the expense of both the recessive homozygous genotype and homozygous
dominant genotype. Because of their resistance to malaria and the ability of heterozygotes to
leave more offspring, the sickle cell allele is spread through the population and maintained at
a high frequency.
You might also like to view...
Jacob and Monod were intellectually primed to draw the conclusions they did concerning regulation of the lac operon. In part, this was due to their fascination with mechanisms of enzyme regulation. They knew that the activity of some enzymes is regulated
when their reaction product binds to the enzyme, changing its shape and, therefore, its activity. This knowledge allowed them to easily make the intellectual leap to propose _____. A) the existence of the CAP binding site B) allosteric regulation of the repressor C) positive regulation of the lac operon D) the existence of the operator E) co-transcribed and co-regulated genes of bacterial operons
Reverse transcriptase is an enzyme that can synthesize DNA from an RNA template. What DNA sequence is produced from an RNA molecule with the sequence AUUGACGGU?
A. UAACUGCCA B. TTTCTGCCT C. TAACTGCCA D. ATTCAGCCT E. TUUCTGCCU
Evaporation exerts tension on the narrow columns of water that fill xylem tubes. The tension occurs because liquid water has ____
a. cohesion due to hydrogen bonds b. cohesion due to ionic bonds c. translocation of sugars d. pressure due to compression within xylem e. high oxygen content due to soil conditions
Identify the labeled structures on the human skeleton in the picture. 
A: ________B: ________C: ________D: ________E: ________F: ________G: ________H: ________I: ________J: ________K: ________L: ________M: ________N: ________O: ________P: ________
What will be an ideal response?