An Introduction to Molecular Anthropology

Mark Stoneking

• Publisher: Wiley-Blackwell
• Year: 2016
• Print ISBN: 9781118061626
• eBook ISBN: 9781119050872
• Edition: 1
• Topic: Biowissenschaften
• Subtopic: Entwicklung

Molecular anthropology uses molecular genetic methods to address questions and issues of anthropological interest.  More specifically, molecular anthropology is concerned with genetic evidence concerning human origins, migrations, and population relationships, including related topics such as the role of recent natural selection in human population differentiation, or the impact of particular social systems on patterns of human genetic variation.

Organized into three major sections, An Introduction to Molecular Anthropology first covers the basics of genetics – what genes are, what they do, and how they do it – as well as how genes behave in populations and how evolution influences them. The following section provides an overview of the different kinds of genetic variation in humans, and how this variation is analyzed and used to make evolutionary inferences. The third section concludes with a presentation of the current state of genetic evidence for human origins, the spread of humans around the world, the role of selection and adaptation in human evolution, and the impact of culture on human genetic variation.  A final, concluding chapter discusses various aspects of molecular anthropology in the genomics era, including personal ancestry testing and personal genomics.

An Introduction to Molecular Anthropology is an invaluable resource for students studying human evolution, biological anthropology, or molecular anthropology, as well as a reference for anthropologists and anyone else interested in the genetic history of humans.

CHAPTER 1
Genes: How They Are Inherited

Like begets like: dogs have puppies, cats have kittens, and humans have baby humans. Moreover, you tend to look more like your parents or other relatives than people you are not related to. The mechanics behind these simple statements—the laws of heredity—were first worked out by Gregor Mendel in the 1860s, who studied how variation in garden peas was transmitted from parents to offspring (Mendel 1865). But peas aren't so terribly interesting–-and after all, this is an anthropology textbook–-so we will use variation in humans to illustrate the mechanics of inheritance. The variation we will use is the ABO blood group system, but before explaining how the ABO blood groups are inherited, you first need to know something about blood.

Blood and ABO Blood Groups

Suppose you stick a needle with a syringe into a vein, withdraw a few ccs (cubic centimeters—a cc is about 20 drops or so) of blood, squirt the blood into a test tube, and let it sit. After 30 minutes or so, the blood will have spontaneously formed a clot—all it takes is exposure of the blood to air to initiate clotting. Remove the clot and what is left behind is a clear, yellowish fluid called serum. If you instead add a chemical to the test tube that inhibits clotting and spin the blood at high speed in a centrifuge, you will find that the blood has separated into different components (Figure 1.1). At the bottom are the red blood cells (RBCs, also known as erythrocytes), which transport oxygen around the body. Immediately on top of the RBCs is a ghostly white layer, sometimes referred to as the buffy coat, that consists of white blood cells (also known as lymphocytes), which are important for protecting the body from invading cells. And on top of the white blood cells is a clear, yellowish fluid called plasma. Plasma is like serum, except plasma also contains the various factors that are involved in blood clot formation.

Figure 1.1 The components of blood, after adding an anticoagulant, followed by centrifugation. RBC, red blood cells; WBC, white blood cells.

Suppose now we take serum from one person and mix it with RBCs from another person and do this for many different people. Sometimes nothing will happen, but sometimes the RBCs will clump together (agglutinate). Agglutination is entirely different from clotting (Figure 1.2). You may think that mixing blood components from different people is a strange thing to do, but in fact Karl Landsteiner won a Nobel Prize for doing just that. During the nineteenth century, physicians began giving blood transfusions to people who had lost life-threatening quantities of blood through injury or illness. Seems reasonable enough—someone needs more blood, so give them blood from somebody else—and indeed, sometimes the blood transfusion recipients recovered spectacularly. But sometimes they actually got much sicker from the transfusion, to the point of even dying, and nobody knew why this would happen. Landsteiner, an Austrian physician, took it upon himself to figure out why such adverse reactions to blood transfusions occurred. Through his mixing experiments, he discovered that people's blood could be classified into four groups (Landsteiner 1900), corresponding to what are now known as blood groups A, B, AB, and O. Mix together blood from people with the same blood group and nothing happens. But mix together blood from a group A person with blood from a group B person and you get agglutination—and if you do this in a blood transfusion, clumps of agglutinated cells will form in the veins, blocking small capillaries and leading to tissue death, which is bad news indeed.

Figure 1.2 Left, a version of red blood cells that have not agglutinated. Right, a version of red blood cells that have agglutinated.

So what causes agglutination? It turns out that RBCs carry on their surface substances called antigens, and these antigens cause the formation of substances in the serum called antibodies, which bind to antigens. Each antibody has two binding sites for its particular antigen, and there are many copies of each antigen on each RBC. So, mix together RBCs with serum containing antibodies against an antigen on those RBCs, and you get lots of antibodies binding to lots of RBCs, resulting in agglutination. But if the serum does not contain antibodies against the antigens on the RBCs, then there is no agglutination.

Table 1.1 lists the antigens present on the RBCs and the antibodies present in the serum of the A, B, AB, and O blood groups (for those of you who have seen blood groups with + or −, such as A+ or B−, don't worry, we'll get to that later in the chapter). The O blood group can be thought of as a “null” blood group, in that there are no O antigens or anti-O antibodies. Note that if you have a particular antigen on your RBCs, you don't have antibodies against that antigen—otherwise you would be agglutinating your own blood cells, which would be very bad news indeed (however, there are diseases known in which the body starts making antibodies against its own antigens; such diseases are known as autoimmune diseases and examples include lupus and some types of arthritis). Note that people with blood type O are known as “universal donors,” because their RBCs lack A or B antigens and hence can be safely transfused into people of any blood type–-that's why you often hear emergency room physicians on TV shows shouting for type O blood when a patient comes in who needs blood immediately. Conversely, people of blood type AB are known as “universal recipients,” because they can receive RBCs of any blood type in a transfusion, as they lack anti-A and anti-B antibodies.

Table 1.1 Antigens and antibodies for the ABO blood groups

Blood group	Antigens on RBCs	Antibodies
A	A	anti-B
B	B	anti-A
AB	A,B	none
O	None	anti-A, anti-B

RBCs, red blood cells.

Inheritance of ABO Blood Groups

Now that you know something about ABO blood groups, we can go into how they are inherited. First, some facts and terminology. Humans are diploid, meaning that each gene is present in two copies (for now, just think of a gene as the instructions for doing something, as in “the gene for the ABO blood groups”; in the next chapter, we'll see what genes actually are). One copy is inherited from the mother, through the egg, and one copy is inherited from the father, through the sperm. Any particular gene can come in different forms, or variants, and these are called alleles. For the ABO blood group gene, there are three alleles, namely, the A allele, the B allele, and the O allele. And since everyone has two alleles, there are six possible combinations of alleles; the pair of alleles that you have is your genotype. For three genotypes, the two alleles are the same (namely, AA, BB, and OO), and these are called homozygous genotypes or homozygotes. For the other three genotypes, the two alleles are different (namely, AB, AO, and BO), and these are called heterozygous genotypes or heterozygotes. The astute reader may wonder how it is that six different genotypes result in just four different blood groups. The actual blood group, or phenotype, associated with each genotype is shown in Table 1.2. Note that both the AA genotype and the AO genotype result in blood type A, and both the BB genotype and the BO genotype result in blood type B, thereby explaining how six different genotypes result in just four different blood groups.

Table 1.2 ABO blood group genotypes and corresponding phenotypes

Genotype	Phenotype (blood type)
AA	A
AO	A
BB	B
BO	B
AB	AB
OO	O

The ABO blood groups also nicely illustrate the concept of dominant versus recessive alleles. If the heterozygote for two alleles exhibits exactly the same phenotype as the homozygote for one of the alleles, then that allele is said to be dominant, and the allele that does not exhibit a phenotype in the heterozygote is said to be recessive. Thus, since the AO genotype results in exactly the same phenotype (blood group) as the AA genotype, the A allele is dominant with respect to the O allele, and the O allele is recessive with respect to the A allele. Similarly, the B allele is dominant with respect to the O allele, and the O allele is recessive with respect to the B allele, because the phenotype of the BO heterozygote is exactly the same as that of the BB homozygote. What about the A and B alleles—which is dominant and which is recessive with respect to each other? To figure this out, look at the phenotype (blood group) associated with AB heterozygotes. It turns out that AB heterozygotes have a different phenotype than either AA or BB homozygotes—they are type AB. We therefore say that the A and B alleles are codominant with respect to each other (other terms you may come across, such as partial dominance or incomplete dominance, mean basically the same thing as codominance: the heterozygote has a different phenotype than either homozygote).

Note that the dominance relationship is a property of a pair of alleles, not of a single allele, and, therefore, can vary depending on which pair of alleles are considered. For example, it would be incorrect to simply say that the A allele is dominant, because even though it is dominant with respect to the O allele, it is codominant with respect to the B allele. Determining the dominance relationships of a pair of alleles simply involves comparing the pheno...

Table of contents

• Cover
• Title Page
• Copyright
• Dedication
• Preface
• Chapter 1: Genes: How They Are Inherited
• Chapter 2: What Genes Are, What They Do, and How They Do It
• Chapter 3: Genes in Populations
• Chapter 4: A Simple Model: Hardy–Weinberg Equilibrium
• Chapter 5: Evolutionary Forces
• Chapter 6: Molecular Evolution
• Chapter 7: Genetic Markers
• Chapter 8: Sampling Populations and Individuals
• Chapter 9: Sampling DNA Regions
• Chapter 10: Analysis of Genetic Data from Populations
• Chapter 11: Analysis of Genetic Data from Individuals
• Chapter 12: Inferences About Demographic History
• Chapter 13: Our Closest Living Relatives
• Chapter 14: The Origins of Our Species
• Chapter 15: Ancient DNA
• Chapter 16: Dispersal and Migration
• Chapter 17: Species-Wide Selection
• Chapter 18: Local Selection
• Chapter 19: Genes and Culture
• Chapter 20: Ongoing and Future Developments in Molecular Anthropology
• References
• Suggestions for Additional Reading
• Index
• EULA