Blood types (A, B, AB, O) are familiar, often discussed in medical contexts like transfusions. These classifications are determined by specific biological markers on red blood cells. An individual’s genetic makeup, inherited from their parents, dictates the presence or absence of these markers. Understanding these genetic principles reveals how blood types illustrate complex inheritance patterns.
Understanding Multiple Alleles
In genetics, an allele is a specific variant form of a gene. While many traits involve genes with only two alleles, some genes exist in more than two forms within a population. This condition, where a single gene has three or more alternative alleles, is known as multiple allelism. An individual organism can only carry two alleles for a given gene. However, the presence of multiple alleles at the population level increases genetic diversity. These variations often arise through mutations and contribute to the range of observable traits within a species.
The ABO Blood Group System: A Key Example
The human ABO blood group system clearly illustrates multiple alleles. This system is governed by a single gene on chromosome 9, which has three main allelic forms: IA, IB, and i. The IA allele produces A antigens on red blood cells, while the IB allele produces B antigens. The third allele, i, does not produce any functional antigen.
The combination of these three alleles determines an individual’s blood type. The IA and IB alleles demonstrate co-dominance; if both are present, both A and B antigens are expressed, resulting in AB blood type. Both IA and IB alleles are dominant over the i allele.
Therefore, an individual with genotypes IAIA or IAi will have Type A blood, and those with IBIB or IBi will have Type B blood. Only individuals who inherit two copies of the recessive i allele (ii) will have Type O blood, as no A or B antigens are produced.
How Blood Types Are Inherited
The inheritance of ABO blood types follows Mendelian patterns, with each parent contributing one allele to their offspring. A child’s blood type is determined by the specific combination of the two alleles they receive. For instance, if one parent has Type A blood (genotype IAi) and the other has Type B blood (genotype IBi), their children could potentially inherit Type A (IAi), Type B (IBi), Type AB (IAIB), or Type O (ii) blood. This demonstrates how two parents with Type A and Type B blood could have a child with Type O blood, if both carry and pass on the recessive ‘i’ allele. Blood typing can provide insights into family relationships.
Beyond ABO: Other Blood Group Systems
While the ABO system is widely recognized for its role in blood transfusions, it is only one of many blood group systems identified in humans. The International Society of Blood Transfusion (ISBT) recognizes 48 different blood group systems, each defined by specific antigens on red blood cell surfaces. The Rh factor, for example, is another important system, classifying blood as either positive or negative based on the presence or absence of the RhD antigen.
Other systems include the Duffy, Kell, Lutheran, and Kidd blood groups. While the ABO system is a classic example of multiple alleles influencing a trait, not all blood group systems exhibit the same genetic complexity or inheritance patterns. Some systems involve fewer alleles or different dominance relationships. The existence of these numerous blood group systems highlights the extensive genetic diversity present in human populations.