Genotypic Traits and Blood Groups

Genotypic Traits in Human Population Biology

Introduction to Genotypic Traits

  • Genotypic traits are characteristics influenced by the genetic makeup of an individual.

  • Definitions and characteristics:

    • Simple inheritance: Traits determined by a single gene or allele.

    • Monogenic: Typically involve a single locus.

    • Discontinuous traits: Present in distinct categories, often binary in nature (e.g., trait present or absent).

    • Common examples include:

    • Blood groups: A, B, AB, O types.

    • Hemoglobin variants.

    • Proteins and enzymes.

  • Contrast with polygenic traits:

    • Polygenic inheritance: Involves multiple loci contributing to a single trait.

    • Continuous traits: Exemplified by characteristics such as height, skin color, eye color, and hair color and texture.

    • Polygenic traits represent the majority of human traits.

Early Blood Transfusions

  • Historical context of early blood transfusions:

    • Ancient physicians in Greece experimented with blood transfusions.

    • Results were variable:

      • Some patients benefitted from the procedure.

      • Others experienced fatal illnesses due to transfusion reactions.

ABO Blood Groups

  • ABO blood groups are defined by the presence of specific antigens on red blood cells (RBCs).

  • Note on initial discovery:

    • Discovered by Karl Landsteiner in 1900, recognized as the first human polymorphism.

  • Characteristics of the ABO blood group system:

    • Polymorphism: A genetic trait existing in two or more forms (alleles).

    • Most extensively studied example of simple genetic traits in humans.

  • Genetic basis:

    • The ABO gene located on chromosome 9.

    • Contains three alleles: A, B, and O.

    • A and B alleles: Codominant.

    • O allele: Recessive to both A and B alleles.

  • Blood types corresponding to genotypes (combination of alleles):

    • Type A: Genotype can be AA or AO.

    • Type B: Genotype can be BB or BO.

    • Type AB: Genotype must be AB.

    • Type O: Genotype must be OO.

Antigen-Antibody Reactions

  • Understanding self vs non-self in immunology:

    • Antigen: A protein found on the surface of foreign substances causing an immune response, specifically the production of antibodies.

    • Antibody: A protein that recognizes and binds to antigens to initiate an immune response.

Agglutination and Blood Typing

  • Agglutination: A phenomenon where antigens on whole cells (e.g., foreign RBCs) cause clumping of cells.

    • Agglutinogens: Specific antigens present on RBCs, vital for blood typing.

    • Examples include Antigen A and Antigen B, both glyolipids found on the RBC surface.

  • Agglutinins: These are antibodies present in the plasma that can lead to transfusion incompatibility.

    • Types include Anti-A, Anti-B, and additionally Anti-H, the implications of which will be explored further later in the material.

Chemical Basis of ABO Types

  • The type of antigen on an individual's blood is determined by the glycolipid composition and order on the RBC surface membranes:

    • Glycolipids consist of membrane phospholipids with attached short carbohydrate sequences, forming a chain of sugars.

    • Key components include:

    • Galactose

    • N-acetylgalactosamine (GalNac)

    • N-acetylglucosamine (GlcNac)

    • Fucose (Notably, every chain terminates with a fucose unit!).

    • The H antigen: Serves as a precursor for A and B antigens and is the original building block for other antigen formations.

    • The addition of galactose to the H antigen leads to the formation of the B antigen.

    • The addition of N-acetylgalactosamine leads to the formation of the A antigen.

ABO Blood Group Determinations

  • Detailed mechanisms of ABO type determination and the role of antigens:

    • Blood type is established by assessing the presence/absence of specific agglutinogens (antigens on RBCs).

    • Breakdown of blood types:

    • Type A: Presence of A antigens.

    • Type B: Presence of B antigens.

    • Type AB: Presence of both A and B antigens.

    • Type O: Absence of both A and B antigens.

    • The frequency of blood types in populations with type O being the most common and type AB being the rarest.

Association of Agglutinins and Blood Type

  • Based on the agglutinogens on RBCs, the types of antibodies present in blood plasma can be predicted:

    • Blood type A: Presence of A antigens and Anti-B agglutinins.

    • Blood type B: Presence of B antigens and Anti-A agglutinins.

    • Blood type AB: Presence of both A and B antigens, no antibodies present.

    • Blood type O: Absence of both antigens but presence of both Anti-A and Anti-B agglutinins.

Blood Components and Transfusion Implications

  • Components of blood:

    • Plasma: Liquid portion containing proteins, nutrients, gases, and antibodies.

    • Formed elements: Comprising RBCs, white blood cells (WBCs), and platelets.

  • Types of transfusions:

    • Either RBC only (packed)

    • Or whole blood (including plasma).

Genotypes and ABO Blood System

  • Mapping ABO genotypes indicates the following relationships:

  • Blood Types and Corresponding Genotypes:

    • Type A: Alleles AA or AO (A agglutinogens only)

    • Type B: Alleles BB or BO (B agglutinogens only)

    • Type AB: Alleles AB (both A and B agglutinogens)

    • Type O: Alleles OO (no agglutinogens)

H Antigen Variation and Bombay Phenotype

  • H antigen variations result from the FUT1 gene on chromosome 19:

    • Various genotypes for H antigen: HH, Hh, or hh.

    • Approximately 99.9% of RBCs across all human populations express H antigen.

  • Individuals who are homozygous recessive (hh) are referred to as having the Bombay Phenotype:

    • Extremely rare: Estimated at 1 in 10,000 in India and 1 in 1 million in Europe.

    • Health implications: No significant effects except during blood transfusions.

Bombay Phenotype Details

  • Individuals with the hh genotype have an inactivated FUT1 gene, leading to:

    • A lack of H antigen and thus an absence of fucose.

    • Inability to produce A or B antigens consequently.

    • Such individuals develop antibodies against H, A, and B antigens (Anti-H, Anti-A, Anti-B).

  • Genetic implications of Bombay phenotype described as representing a “little” h due to inactivation.

Distribution Patterns of Blood Groups

  • Geographical distributions of alleles A, B, and O across early populations:

    • Allele B, representing the rarest in populations, is highest in Central Asia and Africa and low in the Americas and Australia.

    • Allele A shows high frequencies in small, isolated populations like Blackfoot Indians and in Northern Europe, especially Scandinavia.

    • Allele O, the most common, holds high frequencies in the Americas and Africa, contrasted with low frequencies in Eastern Europe and Central Asia.

Evolutionary Mechanisms and Disease Susceptibility

  • Mechanisms of evolution influencing distribution and frequency of ABO alleles:

    • Blood groups impact the severity, susceptibility, and mortality rates related to various infectious diseases.

Natural Selection Influencing Disease Severity

  • Natural selection factors for Type O individuals concerning specific pathogens:

    • Helicobacter pylori: Associated with increased gastric and duodenal ulcer risks and enhanced bindings of the bacteria to epithelial cells in the gastrointestinal tract.

    • Historical instance (1996): Outbreak of E. coli O157 resulted in 87.5% of deaths occurring among individuals with type O blood.

    • Other conditions associated with Type O blood, such as traveler’s diarrhea.

Cholera and Blood Group Interactions

  • Cholera (caused by Vibrio cholerae) shows differential impact on blood types:

    • Type O blood displays higher disease severity during cholera outbreaks, such as in Peru (1991).

    • Noted increased hospitalization rates linked with Type O blood during endemic events.

    • Ganges River Delta had high cholera instance but low frequency of the O allele suggesting selective pressures involved in the variations.

Rh Blood Group System

  • The Rh system is highly polymorphic with over 45 antigens, including C, D, and E:

    • Notably, the D antigen is the most reactive among them.

    • Determination of Rh status is linked to the absence or presence of a functional RHD gene located on chromosome 1.

    • Genetic characteristics:

    • Rh+ individuals have genotypes DD or Dd.

    • Rh- individuals carry the genotype dd.

Risk of Anti-D Antibodies and Hemolytic Disease

  • When an Rh- person is exposed to Rh+ blood, they can produce Anti-D antibodies (anti-D agglutinins):

    • These antibodies are not usually found in Rh- individuals unless exposed.

    • Risk for Hemolytic Disease of the Newborn (HDN) exists if an Rh- mother produces Anti-D antibodies, posing risks for subsequent pregnancies.

    • In HDN, the baby's RBCs agglutinate due to maternal antibodies leading to hemolysis and anemia at birth.

Notable Case: James Harrison

  • James Harrison, known as the man with the golden arm, donated blood/plasma over 1,173 times, helping save 2.4 million babies from HDN:

    • Harrison had exceptionally high anti-D antibodies; likely due to prior exposure to Rh+ blood following surgeries in his teens.

  • Instances of Rh- men opting for exposure to Rh+ blood to become donors are noted.

Rh- Allele Prevalence and Distribution

  • The Rh- allele is extremely rare in many populations:

    • High frequency noted in Europe; complex dynamics suggest historic gene flow between Rh- and Rh+ populations.

    • Evidence supported by ancient DNA linking Rh- hunter-gatherers to farming populations with Rh+ traits.

    • A high frequency of Rh- found in Basque populations potentially suggests historic admixture events.

Duffy Blood Group System

  • The Duffy blood group system involves the DARC gene on chromosome 1, which encodes a membrane protein/receptor:

    • Found in RBCs, spleen, liver, and kidneys.

  • There are two main alleles in this system:

    • FYA and FYB, leading to four major phenotypes:

    • Fy(a+b−), Fy(a−b+), Fy(a+b+), Fy(a−b−).

Duffy Negative Distribution Trends

  • Examination of the frequency of Duffy negative (null) phenotype across various geographical regions:

    • Duffy negative frequencies vary as depicted across global demographics.

Historical Context: Duffy Negative and the African Slave Trade

  • The impacts of Duffy negative frequencies in the context of historical movements, notably the African slave trade:

    • A detailed map illustrates the transportation volumes and destinations over the period from 1701-1810, highlighting significant population movements.