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As discussed, Sickle Cell Disease is one of thousands of disorders caused
by a single gene. Sickle-cell anemia is an autosomal,
recessive human disease. It is caused by a flawed allele for a
polypeptide in hemoglobin. As a result, this oxygen carrying
protein in the red blood cell causes it to have a ‘sickled’
shape. Sickle-shaped cells are more rigid and tend to get
stuck in the capillaries. This lack of blood flow causes
damage to cells tissues and organs.
Scientists used SNPs (single nucleotide polymorphisms) to
locate the sickle cell allele in genes that code for hemoglobin
1. How many bases are involved in this mutation?
2. What kind of mutation causes the sickle cell mutation?
3. Now that you have an understanding of the rules of heredity, how
would you anticipate sickle cell disease to be passed down in a
population? Label the pedigree with genotypes
and phenotypes showing the probable
inheritance if both parents are heterozygous.
has disease
has disease
There is a connection between sickle cell disease and malaria.
Individuals who are heterozygous for sickle cell are at an advantage
because they have a greater chance of surviving malaria.
4. What is the evolutionary advantage to this? (hint: Is there a
genotype that is more advantageous to have in this case? What are the
problems with the other genoypes? See#3 for your possible genotypes.)
There are some mutations that give the recipient an advantage
over other people. Sometimes the advantage improves the ability to
survive a potentially deadly illness. The affected individual can then pass
his/her genes to the next generation more efficiently than other people
because they are more likely to reach reproductive age. This increases
the chance that the modified gene will survive into the first generation
(that of the children) and from there move into the following generation
(that of the grandchildren). This is a "positive" mutation.
Natural selection has to be considered in the context of "pre-modern"
societies. Modern medicine has altered the balance of nature and often
allows us to rescue people who otherwise would die of their condition. A
case in point is juvenile diabetes. Untreated, the disorder often is fatal in
childhood. Modern medicine allows most people with juvenile diabetes to
live essentially normal lifes. Natural selection consequently is no longer an
issue for most people with juvenile diabetes. The same is true for many
other hereditary disorders. When we examine traits in the human
population that rose to high frequency through natural selection, we are
in effect peering through the looking glass at our distant past.
5. If we had the opportunity to eradicated genes for disease from our
genome, how would that change the human population? For
example, if we eradicated the gene for sickle cell disease, what are
some possible results we would see in the population?