Anthony Allison: The Relationship Between Sickle-Cell Anemia and Malaria
In the late 1940's, a 20-something Anthony Allison was finishing his coursework in basic sciences and was about to start his clinical medical training. Following a bout of malaria, Allison decided to help Kenyans (and other peoples) by seeking a cure for
Deme
When physical anthropologists and geneticists study the genetics of individuals, they focus on the reproductive population, or deme: members of a species that produce offspring.
Gene Pool
When physical anthropologists talk about populations, they often refer to the gene pool, which is all the genetic material within a population. When geneticists talk about the gene pool, they are even more specific, referring to all the variation within a
Species
The concept of the breeding population is also central to the definition of species. A species is comprised of all the populations (and their individual members) that are capable of breeding with each other and producing viable (fertile) offspring.
Reproductive Isolation
Species, therefore, are defined on the basis of reproductive isolation. In biological terms, if two populations are reproductively isolated, members of one population cannot interbreed with members of the other. Reproductive isolation is largely related t
Population Genetics
Population genetics is the study of changes in genetic material�specifically, the change in frequency of alleles.
Microevolution
Evolution over the course of a few generations.
Macroevolution
Evolution over the course of many generations.
Genetic Equilibrium
Just as interesting, however, are instances in which frequency does not change over time�that is, when the frequencies of a population's alleles for a particular trait are in a state of equilibrium
Who Are The Inventors of the Hardy-Weinberg Equilibrium Formula?
In 1908, Godfrey Hardy (1877-1947), an English mathematician, and Wilhelm Weinberg (1862-1937), a German obstetrician, independently recognized that some alleles are in a state of equilibrium.
What Are The Requirements for Hardy-Weinberg Equilibrium?
If no mutation or natural selection or gene flow occurs, if the population is large, if mating is random, and if all members of the population produce the same number of offspring, then genotype frequencies at a single gene locus will remain the same afte
Hardy-Weinberg Law of Equilibrium
Moreover, the equilibrium frequencies will be a function of the allele frequencies at the locus. This is called the Hardy-Weinberg law of equilibrium. In the simplest case, a single locus has A (dominant) and a (recessive) alleles, with respective frequen
Mutations (When Are They Serious or Harmless?)
During cell reproduction, DNA almost always replicates itself exactly. Sometimes, however, the replication process produces an error or a collection of errors in the DNA code. If the problem is not at once detected and correct by a set of enzymes that mon
What Is The Only Source of New Genetic Variation in Populations?
Mutations are the only source of new genetic variation in a population.
Point Mutations
Mutations involving incorrect base pairing are called point mutations.
Synonymous Point Mutations
A synonymous point mutation creates an altered triplet in the DNA, but the alteration carries with it the original amino acid. Because the amino acid is the same, the protein formed is the same.
Nonsynonymous Point Mutations
A nonsynonymous point mutation results in a matchup that brings along a different amino acid. Such a mutation can have dramatic results for the individual carrying it.
Frameshift Mutation
As a result of the shifting base pairs caused by base insertion, the reading frame of a gene is altered or stopped entirely. This frameshift mutation produces a protein having no function. Such a mutation usually involves a small part of the DNA sequence,
Transposable Elements
Other kinds of mutations can affect far more of the genome. Transposable elements are genes that can copy themselves to entirely different places along the DNA sequence. If such a gene inserts itself into another gene, it can fundamentally alter the other
Klinefelter's Syndrome
In the most extreme mutations, entire chromosomes can be duplicated (a trisomy) or lost altogether (a monosomy). Examples of trisomies are Down syndrome, and Klinefelter's syndrome, a common sex chromosome variant that results in XXY.
Spontaneous Mutations
Spontaneous mutations have no known cause.
Induced Mutations and Mutagens
Induced mutations are caused by specific environmental agents, usually associated with human activity. These agents, or mutagens, are increasingly becoming known.
Fitness
In particular, fitness is defined as some measure of the propensity to contribute offspring to future generations, usually by the next generation. Fitness can be defined in reference to individuals in a population or to specific genotypes. The implication
Directional Selection
Directional selection favors one extreme form of a trait.
Stabilizing Selection
Stabilizing selection favors the average version of a trait.
Disruptive Selection
In disruptive selection, the pattern of variation is discontinuous. Individuals at both extreme ends of the range produce more offspring than do the remainder of the population. Given enough time, this pattern can result in a speciation event.
The Case of the Peppered Moth and Industrial Melanism
Perhaps the best evidence ever documented of natural selection operating on a heritable trait concerns the peppered moth, a species common throughout Great Britain. Prior to the mid-1800's, all members of the species had a peppered appearance, their white
Positive Selection
The above case of industrial melanism is an example of positive selection, whereby an organism's biology is shaped by selection for beneficial traits.
Hemoglobin S Gene and Sickle-Cell Anemia
Natural selection for beneficial traits in humans is best understood by studying genes that control specific traits. Of the 90 or so different loci that are targets of natural selection, among the most compelling examples is the sickle-cell gene�the Hemog
Hemolytic Anemias/Abnormal Hemoglobin
Millions of people suffer from such hemolytic anemias, which involve the destruction of red blood cells. A low number of red blood cells can produce health problems because of the resultant lack of hemoglobin, the chemical in red blood cells that carries
Sickle-Cell Anemia
The SS genotype results in many red blood cells' having a sickle shape caused by the abnormal hemoglobin. The cells' abnormal shape prevents them from passing through the capillaries, the narrow blood vessels that form networks throughout tissues. When th
The Geography of Sickle-Cell Anemia and a Possible Association with Malaria
Beginning in the mid-twentieth century, the medical community observed that many people living in equatorial Africa had the S gene. Individuals heterozygous for normal hemoglobin and the sickle-cell mutation (AS) either did not contract malaria or suffere
Balanced Polymorphism
Situation in which selection maintains two or more phenotypes for a specific gene in a population. Example: the S gene for sickle-cell anemia and its resistance for malaria.
The Biology of Sickle-Cell Anemia and Malarial Infection
AS red blood cells do not sickle under most conditions (that is, except when severely deprived of oxygen). They are, however, somewhat smaller than normal cells, and their oxygen levels are somewhat lower. For reasons not yet understood, the AS red blood
The History of Sickle-Cell Anemia and Malaria
In the late 1950s, the American physical anthropologist Frank B. Livingstone (1928-2005) sought to strengthen the case for natural selection by historically linking sickle-cell anemia and malaria. He hypothesized that the Bantu carried the mutation with t
Hemoglobinopathies
Sickle-cell anemia turns out to be just one of a number of hemoglobinopathies and other genetic abnormalities in Africa, Asia, and Europe that provide a strong selective advantage in regions of endemic malaria. Heterozygous carriers of abnormal hemoglobin
Thalassemia
Thalassemia, a genetic anemia found in Europe, Asia, and the Pacific, reduces or eliminates hemoglobin synthesis.
G6PD
An association has long been recognized between deficiency of the enzyme glucose-6-pohsophate dehydrogenase (G6PD) and malaria. A recessive hereditary mutation leads more males than females to lack the gene that is coded to produce this enzyme. Without th
Conditions for Genetic Drift
Genetic drift is random change in allele frequency over time. Provided that no allele confers a selective advantage over another, a random change can lead to a change in gene frequency, such as one allele being lost and the other becoming fixated. This fo
Endogamous
A group that discourages reproduction outside of the group.
Exogamous
An exogamous society extends reproduction outside its community.
Founder Effect
Founder effect, one form of genetic drift, occurs when a small group (fewer than several hundred members) of a large parent population migrates to a new region and is reproductively isolated. The new region is either unoccupied or occupied by species with
Huntingdon's Chorea
Founder effect has also been documented in several genetic diseases that affect humans. Among the best known is Huntingdon's chorea, a genetic abnormality caused by an autosomal dominant gene. The gene is located on chromosome 4 at the locus that codes fo
Conditions for Gene Flow
Another force of evolution, gene flow is the transfer of genes across population boundaries. The key determinant for the amount of gene flow is accessibility to mates�the less the physical distance between populations, the greater the chance of gene flow.
Genetic Markets Document Gene Flow: Agriculture and Origins of Modern Europeans
Agriculture has existed in Europe since at least 7000 yBP. There, its profound implications for society included the beginnings of cities, of complex social organization, and of warfare. In 1925, the Australian archaeologist V. Gordon Childe (1892-1957) p
Demic Diffusion
A population's movement into an area previously uninhabited by that group.