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Blood Bank

Basic

Our blood type is determined by genes, which are specific instructions in our DNA found at particular locations called loci. We inherit different versions of these genes, called alleles (like A, B, O or D, d), getting one from each parent. This specific combination of inherited alleles is our genotype (e.g., AO, Dd). What we actually detect on the red blood cells in the lab is the phenotype (e.g., Group A, Rh Pos). How these alleles interact matters: codominant alleles (like A and B) both show up, dominant alleles (like D) show up even if paired with a recessive one, and recessive alleles (like O or d) only show up if two copies are inherited. Understanding these fundamental genetic principles is crucial for predicting inheritance, explaining why people make certain blood group antibodies, and interpreting our lab tests accurately

The Blueprint: Genes, Alleles, and Loci

  • Gene: Imagine a gene as a specific instruction in our DNA blueprint. For blood groups, these instructions often tell the body how to make a specific protein or enzyme that results in a blood group antigen on our red blood cells
  • Locus (plural: Loci): This is the specific address or location of a gene on a chromosome. Think of it like the street address for that specific instruction. For example, the ABO gene has its own specific locus on chromosome 9
  • Allele: Genes can come in different versions, like different flavors of the same instruction. These different versions are called alleles. For example, at the ABO locus, you can have the A allele, the B allele, or the O allele. You inherit one allele from each parent for most genes

Your Genetic Makeup: Genotype vs. Phenotype

  • Genotype: This is the actual pair of alleles a person has inherited for a specific gene (one from each parent). It’s their genetic code. Examples: AA, AO, BO, AB, OO for the ABO system; DD, Dd, dd for the RhD system
  • Phenotype: This is the observable trait – what we actually detect in the lab! It’s the physical expression of the genotype. For blood groups, the phenotype is the collection of antigens we find on the red blood cells through serological testing
    • Example: A person with the genotype AO has the phenotype A, because the A antigen is expressed, and O is generally considered “silent” or amorphous in basic terms. A person with genotype AB has the phenotype AB because both A and B antigens are expressed

Inheritance Patterns: How Genes are Passed Down

Blood group genes follow basic Mendelian inheritance patterns:

  • Mendel’s First Law (Law of Segregation): When sperm and egg cells are formed, the two alleles at a locus separate (segregate) so that each gamete (sperm or egg) receives only one of the alleles. This is why you pass on only one of your ABO alleles to your child, not both
  • Mendel’s Second Law (Law of Independent Assortment): Alleles for different traits (genes located on different chromosomes or far apart on the same chromosome) are inherited independently of each other. For example, inheriting the A allele from the ABO system doesn’t influence whether you inherit the D allele from the Rh system (as they are on different chromosomes). Caveat: Genes located close together on the same chromosome (linked genes) tend to be inherited together more often – we see this in systems like MNS and Rh

Expressing Yourself: Dominant, Recessive, and Codominant Alleles

How alleles interact determines the phenotype:

  • Dominant Allele: This allele expresses its trait even if only one copy is present (heterozygous state). In the Rh system (simplified), the D allele is dominant. Genotypes DD (homozygous) and Dd (heterozygous) both result in the RhD-positive phenotype
  • Recessive Allele: This allele only expresses its trait if two copies are present (homozygous state). The d allele (representing the absence of D antigen) is recessive. Only the genotype dd results in the RhD-negative phenotype. The O allele in the ABO system is generally considered recessive to A and B
  • Codominant Alleles: Both alleles are expressed equally in the heterozygous state. The A and B alleles in the ABO system are classic examples. If you inherit A from one parent and B from the other, your genotype is AB, and your phenotype is AB – both A and B antigens are present on your red cells. The M and N alleles in the MNS system are another great example (MN genotype leads to MN phenotype)

Homozygous vs. Heterozygous

These terms describe the pair of alleles at a specific locus:

  • Homozygous: Having two identical alleles for a trait (e.g., AA, OO, dd, MM)
  • Heterozygous: Having two different alleles for a trait (e.g., AO, BO, AB, Dd, MN)
    • Why this matters: Sometimes, individuals who are homozygous for an antigen allele (like MM) might have a stronger expression of that antigen on their cells compared to those who are heterozygous (MN). This can occasionally affect reactivity strength in the lab, a concept known as “dosage.”

Why is this Basic Genetics Stuff Important in Blood Bank?

  • Predicting Offspring Phenotypes: Helps in understanding potential blood types of children based on parents’ types (useful in some clinical scenarios)
  • Understanding Antibody Formation: Why does a group O person make anti-A and anti-B? Because they don’t have the A or B genes/antigens!
  • Resolving Discrepancies: Sometimes lab results don’t make sense. Understanding inheritance patterns can help figure out why (e.g., presence of a rare allele, weak subgroup)
  • Paternity Testing (Historically): Blood groups were early tools used in parentage testing, though DNA methods are now standard
  • Population Studies: Frequencies of different alleles vary in different populations, impacting the availability of certain blood types and the likelihood of encountering specific antibodies

Key Terms

  • Gene: The fundamental unit of heredity; a segment of DNA that carries the instructions for producing a specific protein or enzyme, ultimately determining a blood group antigen characteristic
  • Allele: An alternative form or version of a gene found at a specific locus
  • Locus (plural: Loci): The specific physical location of a gene on a chromosome
  • Genotype: The specific combination of alleles an individual has inherited at a particular locus (or loci). It represents their genetic makeup
  • Phenotype: The observable physical or biochemical characteristics of an individual, determined by their genotype and environmental factors. In blood banking, it’s what we detect serologically (the antigens present on the red cells)
  • Homozygous: Having two identical alleles for a specific gene at a given locus
  • Heterozygous: Having two different alleles for a specific gene at a given locus
  • Dominant Allele: An allele whose trait is expressed in the phenotype even when only one copy is present (in the heterozygous state)
  • Recessive Allele: An allele whose trait is only expressed in the phenotype when two copies are present (in the homozygous state). It is masked by a dominant allele in the heterozygous state
  • Codominant Alleles: Two different alleles at a locus that are both fully expressed in the phenotype of a heterozygote