TLDR;
This lecture explains the concept of multiple alleles, where a gene has more than two variants within a population, using the ABO blood group system as a primary example. It covers the origin of multiple alleles through environmental factors and mutations, the genetic basis of ABO blood groups, the relationship between genotype and phenotype, the nature of antigens and antibodies, and the applications of this knowledge in blood transfusions and resolving parentage disputes.
- Multiple alleles arise from mutations and environmental pressures on genes.
- The ABO blood group system is determined by three alleles (A, B, O) of a single gene.
- Blood transfusions require matching blood types to avoid antigen-antibody reactions.
Multiple Alleles and origin [0:00]
Typically, a gene has only two forms (alleles), such as tall (T) and dwarf (t) for height. However, multiple alleles occur when a gene has more than two variants within a population. This means that instead of just two options, there are several different versions of the gene that can be inherited. The origin of multiple alleles lies in the fact that genes are subject to various environmental stresses like radiation, chemicals, and natural selection pressures, including crossing over during meiosis. These factors can induce mutations in a gene, leading to new variants. For example, a gene "A" might mutate into variants "B," "C," and "D" over time due to these influences. While a population can have many alleles for a gene, an individual can only express two at a time, one inherited from each parent.
Genetic Basis [10:29]
The ABO blood group system is a prime example of multiple alleles. Discovered initially by Landsteiner in 1905, who identified four blood types (A, B, AB, O), the genetic basis was later explained by Bernstein. Only one gene determines blood type, but it exists in multiple forms, making it a polymorphic gene. This gene resides on chromosome number 9 and has three alleles: IA, IB, and i (or IO). The IA and IB alleles are codominant, while the i allele is recessive.
Phenotype [14:32]
Phenotype, or external appearance, is closely linked to genotype, the genetic makeup. The ABO blood group system exhibits four phenotypes: A, B, AB, and O. Each phenotype corresponds to the presence or absence of specific antigens on the surface of red blood cells. Individuals with blood type A have A antigens, those with blood type B have B antigens, those with blood type AB have both A and B antigens, and those with blood type O have neither A nor B antigens. The genetic makeup for blood type A can be either homozygous (IAIA) or heterozygous (IAi), for blood type B can be homozygous (IBIB) or heterozygous (IBi). Blood type AB individuals have the genotype IAIB, demonstrating codominance, and blood type O individuals have the genotype ii, being recessive for both A and B alleles.
Nature of Antigens [25:02]
Antigens are molecules, often conjugated with proteins or lipids, found on the surface of cells, including red blood cells. They play a crucial role in cell recognition and communication. Chemically, antigens can be carbohydrates, proteins, lipids, or nucleic acids, or blends thereof. In the ABO blood group system, the antigens are carbohydrates. These antigens consist of a base structure of five monosaccharides: N-acetylgalactosamine, galactose, N-acetylglucosamine, galactose, and fucose. The O blood group has this base structure, while the A antigen has an additional N-acetylgalactosamine, and the B antigen has an additional galactose attached to the base structure. These antigens not only determine blood type but also play roles in biofilm formation (N-acetylglucosamine) and intercellular communication (N-acetylgalactosamine).
Nature of antibody [29:27]
Antibodies are proteins produced by the immune system to recognize and bind to foreign antigens. Unlike antigens, antibodies are exclusively proteins. In the context of ABO blood groups, antibodies defend the body's blood group by targeting foreign antigens. Individuals with blood type A have anti-B antibodies, those with blood type B have anti-A antibodies, those with blood type AB have no antibodies, and those with blood type O have both anti-A and anti-B antibodies. These antibodies are innate, meaning they are present from birth without prior stimulation. This knowledge is critical for blood transfusions, where matching blood types is essential to avoid antigen-antibody reactions that can cause agglutination and potentially death. Blood type A can donate to A and AB, blood type B can donate to B and AB, blood type AB can donate only to AB (universal recipient), and blood type O can donate to all blood types (universal donor). Blood type A can receive from A and O, blood type B can receive from B and O, blood type AB can receive from A, B, AB, and O, and blood type O can receive only from O. This understanding also helps resolve parentage disputes by applying genetic principles. For example, a child with blood type O cannot have parents if one of them has blood type AB.
