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Tipsabout the Sickle-Cell Allele

 First, don’t give up because this is a harder question. The people who don’t succeed in the class
are the ones who don’t keep trying. We’re all here to learn together, and being challenged is part
of that learning process.

 When you read Chapter 3 as assigned this week, give extra attention to the section called,
“Microevolution and Patterns of Human Variation” on pages 70-72. You might need to read
it several times with the tips below in mind.

 Don’t let the vocabulary completely block you. Try substitutions. Allele = a form of a gene.
Population = group of people.

 Here are some of the basics about the sickling allele. HbA = a gene form that functions
normally. HbS = a gene form that can cause sickle-cell anemia; this is the sickling allele. Each
person inherits two gene forms from their parents. That means, when you are born, you might
have one HbS from one parent and one HbA from another parent (this is called the HbS/HbA
genotype). There are other possibilities. You could get HbS from both parents (this is called the
HbS/HbS genotype) or you could get HbA from both parents (this is the HbA/HbA genotype).

 People with the HbA/HbA genotype have normal health.

 People with the HbS/HbS genotype have sickle-cell anemia – a serious condition that usually
prevents someone from reproducing and passing on their genes.

 People with the HbA/HbS genotype are carriers of the gene form that causes sickle-cell anemia,
but they don’t get sick because they only have one of them, not both.

 We would expect the HbS gene form to disappear because most people carrying it can’t
pass it on through reproduction.

 However, in some populations, there is a lot of the HbS gene form (allele) either in the HbS/HbS
genotype and in the HbA/HbS genotype.

 As you reread the relevant section in the textbook, ask yourself: What populations have a lot
of the HbS allele (gene form)?

 Then you need to answer the second question in the discussion prompt: Why is there a high
proportion of the sickling allele (HbS) in those populations and not others? Be sure to
use one or more of the following vocabulary to answer the question: gene flow, genetic drift,
natural selection, and mutation.

 Remember, you don’t have to have the completely correct answer in your discussions!
You just need to read the materials and do your best to answer the question, and then try to
come to a more complete understanding as you read your classmates’ posts and my comments.