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Blood Bank

Blood Group Inheritance

Let’s tie together the basic genetics and molecular concepts to see how blood groups are actually passed down through families. Understanding inheritance patterns is fundamental to predicting phenotypes, resolving discrepancies, and understanding population frequencies

Inheritance of Blood Groups: Passing the Torch

Inheritance in blood group systems follows the fundamental principles established by Gregor Mendel. We inherit one set of chromosomes (and thus, one allele for each gene locus) from our mother via the egg, and one set from our father via the sperm

Mendel’s First Law: Law of Segregation

  • Concept: During gamete formation (sperm and egg production), the two alleles for a trait separate (segregate) from each other, so each gamete carries only one allele for that trait
  • Blood Bank Relevance: If a parent has the genotype AO, they don’t pass on AO to their child. They will pass on either the A allele or the O allele in their sperm or egg. The other parent does the same. The combination determines the child’s genotype

Mendel’s Second Law: Law of Independent Assortment

  • Concept: Alleles for different traits (genes located on different chromosomes or very far apart on the same chromosome) are inherited independently of one another
  • Blood Bank Relevance: The inheritance of your ABO type (gene on chromosome 9) is independent of your RhD type (gene on chromosome 1). Getting an A allele doesn’t make you more or less likely to get a D allele
  • Important Exception - Linkage: This law doesn’t strictly apply to genes located close together on the same chromosome. These are called linked genes

Linked Genes and Haplotypes

  • Concept: Genes physically close together on the same chromosome tend to be inherited together as a block because the chance of a crossover (recombination) event happening between them during meiosis is low
  • Haplotype: A set of linked alleles inherited together on the same chromosome
  • Blood Bank Relevance
    • Rh System: The RHD and RHCE genes are very tightly linked on chromosome 1. We inherit a block of Rh alleles from each parent. For example, a common haplotype in Caucasians is DCe. Someone might inherit DCe from one parent and dce from the other. Their genotype is often written as DCe/dce. They pass on either the DCe combination or the dce combination to their children
    • MNS System: The GYPA (coding for M/N antigens) and GYPB (coding for S/s antigens) genes are closely linked on chromosome 4. Common haplotypes include MS, Ms, NS, and Ns. Someone with genotype MS/Ns inherited MS from one parent and Ns from the other

Predicting Inheritance: The Punnett Square

  • A Punnett square is a simple grid used to predict the possible genotypes of offspring based on the genotypes of the parents
  • How it works
    1. Determine the possible alleles each parent can contribute in their gametes (based on their genotype and the Law of Segregation)
    2. List one parent’s possible gametes along the top of the square and the other parent’s along the side
    3. Fill in the boxes by combining the alleles from the corresponding row and column. Each box represents a possible genotype for an offspring, and typically, each box has an equal probability (e.g., 25% if there are 4 boxes)
  • Example 1: ABO Inheritance
    • Parent 1: Genotype AO (Phenotype A) -> Possible gametes: A or O
    • Parent 2: Genotype BO (Phenotype B) -> Possible gametes: B or O
    B O
    A AB AO
    O BO OO
    • Predicted Offspring Genotypes: 25% AB, 25% AO, 25% BO, 25% OO
    • Predicted Offspring Phenotypes: 25% AB, 25% A, 25% B, 25% O
  • Example 2: RhD Inheritance (Simplified)
    • Parent 1: Genotype Dd (Phenotype RhD Positive) -> Possible gametes: D or d
    • Parent 2: Genotype Dd (Phenotype RhD Positive) -> Possible gametes: D or d
    D d
    D DD Dd
    d Dd dd
    • Predicted Offspring Genotypes: 25% DD, 50% Dd, 25% dd
    • Predicted Offspring Phenotypes: 75% RhD Positive (DD and Dd), 25% RhD Negative (dd)
  • Example 3: MNS Inheritance (M/N only for simplicity)
    • Parent 1: Genotype MM (Phenotype M) -> Possible gametes: M only
    • Parent 2: Genotype MN (Phenotype MN) -> Possible gametes: M or N
    M N
    M MM MN
    M MM MN
    • Predicted Offspring Genotypes: 50% MM, 50% MN
    • Predicted Offspring Phenotypes: 50% M, 50% MN

Dominance, Recessiveness, and Codominance in Inheritance

  • How alleles interact (as discussed in Basic Genetics) determines the resulting phenotype from the inherited genotype
  • Codominance (e.g., A and B, M and N): If alleles like A and B are inherited (AB genotype), both are expressed (AB phenotype)
  • Dominance/Recessiveness (e.g., RhD, ABO O): If a dominant allele (D) is inherited with a recessive one (d), the dominant phenotype (RhD Pos) is expressed (Dd genotype). The recessive phenotype (RhD Neg) only appears when two recessive alleles are inherited (dd genotype). The O allele is recessive to A and B

Why Understanding Inheritance Matters in Blood Bank

  • Family Studies: Helps resolve complex antibody problems or identify compatible family members for transfusion, especially for patients with multiple antibodies or antibodies to high-prevalence antigens
  • Predicting HDFN Risk: Knowing the parental genotypes (especially for RhD, Kell, etc.) helps predict the likelihood of the fetus inheriting an antigen the mother lacks, potentially leading to HDFN. Fetal genotyping directly assesses this
  • Understanding Antibody Likelihood: A person will generally only make antibodies to antigens they lack. Inheritance patterns dictate which antigens are present or absent. For example, an O person inherited O from both parents and thus lacks A and B antigens, making them capable of forming anti-A and anti-B
  • Population Frequencies: Allele frequencies vary among different ethnic populations. This affects the probability of finding antigen-negative blood for specific patients and the likelihood of encountering certain antibodies within that population
  • Resolving Discrepancies: Apparent contradictions between typing results and family history (e.g., a Group O child from an AB parent) might indicate sample mix-ups, misidentification, or rare genetic events (like the Bombay phenotype or chimerism)

In essence, the inheritance patterns dictated by basic Mendelian genetics, combined with our understanding of dominance/codominance and linkage, provide the framework for understanding how blood group diversity arises and is maintained within families and populations

Key Terms

  • Law of Segregation: Mendel’s first law, stating that the two alleles for a trait separate during gamete formation, so each gamete receives only one allele
  • Law of Independent Assortment: Mendel’s second law, stating that alleles for different traits (on different chromosomes or far apart on the same chromosome) are inherited independently of each other
  • Linked Genes: Genes located close together on the same chromosome that tend to be inherited together because crossing over between them is infrequent
  • Haplotype: A set of specific alleles located close together on a single chromosome that are inherited as a unit (e.g., DCe in the Rh system, Ms in the MNS system)
  • Punnett Square: A diagram used to predict the probability of offspring inheriting particular genotypes based on the genotypes of the parents
  • Pedigree: A chart that shows the presence or absence of a trait within a family across generations, used to track inheritance patterns