Chapter 8 - Balanced Polymorphism

Notes for Understanding Evolution - Chapter 8

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Under Construction

Notes for Chapter 8: Balanced Polymorphism

Introduction

I. Sickle-Cell Anemia

RQUE8.1: Why is the normal versus sickle-cell alleles case strictly a case of co-dominant rather than simple dominant/recessive inheritance pattern?

II. Sickle-Cell Hemoglobin

RQUE8.2: Explain the genetic basis for the sickle-cell trait at the level of the part of the affected gene.

RQUE8.3: Biologists use the protein electrophoresis method to detect polymorphism at a gene locus among individuals in a population, or even within a family. If the parents of eight children are both carriers for the sickle-cell trait, explain the expected banding pattern on a gel if a scientist electrophoreses one blood sample from each child side by side along a gel, and then stains for hemoglobin.

III. Theory of Balanced Polymorphism

RQUE8.4: Explain the notion of balanced polymorphism for two alleles at a gene locus by temporarily assuming that homozygotes for either allele always die before they reproduce, whereas heterozygotes do not.

IV. Experimental Verification of Balanced Polymorphism

V. Superiority of Heterozygotes

RQUE8.5: Why is there sometimes a heterozygous advantage with respect to the sickle cell versus normal hemoglobin alleles?

VI. Relaxed Selection

RQUE8.6: Given a potential heterozygote advantage associated with the sickle-cell trait, what is a reasonable hypothesis for explaining that the frequency of the responsible allele actually has decreased among African Americans living in the U.S. in recent decades?

VII. Tay-Sachs Disease

RQUE8.7: Why is the number of cases of people with Tay-Sachs disease lower than that expected from the incidence of carriers living in the U.S.?

VIII. Cystic Fibrosis

RQUE8.8: The treatment of the relatively common mostly Caucasian genetic disorder, cystic fibrosis, has improved substantially in recent decades, but no cure has been forthcoming. What potential ethical implications arise from our improved knowledge of the disease?

IX. Selection Against the Heterozygote

X. The Control of Rh Disease

RQUE8.9: Explain why Rh disease was effectively selecting against heterozygous individuals, and why the degree of selection has diminished in the U.S. since the 1960s.

XI. Implications of Balanced Polymorphism

RQUE8.10: With the discovery that most animals including us humans are heterozygous at roughly a third of our gene loci, is balanced polymorphism a likely explanation for how so many alternative alleles are present, or is there another more plausible cause?

Click link to return to Biology 409 Schedule
or back to Chapter 7 or ahead to Chapter 9