M

Blood Banking Genetics Vocabulary

Genetic Principles in Blood Banking

Blood Group Systems

  • Blood group systems are groups of antigens on the RBC membrane that share related serologic properties and genetic patterns of inheritance.

Blood Group Genetics

  • Genetic material is found in DNA.

  • DNA is contained in chromosomes in the nucleus of every cell.

  • Genetic material is replicated by mitosis (somatic cells) or meiosis (gametes).

Phenotype vs. Genotype

Phenotype

  • Physical (observed) expression of traits.

  • Determined by hemagglutination of RBC antigens using antisera.

  • Example: No agglutination with anti-A or anti-B antisera indicates type O blood.

Genotype

  • Actual genetic makeup.

  • Determined by molecular techniques or family studies.

  • Example: A person with phenotype A could have genotype A/A or A/O; family studies are needed to confirm.

Punnett Square

  • Used to predict the probability of an offspring’s genotype.

  • Summarizes every possible combination of maternal and paternal alleles of a particular gene.

Genes

  • Basic units of inheritance on a chromosome.

  • A locus is the site at which a gene is located on a chromosome.

  • Alleles are alternative forms of a gene, found at each locus.

    • Antigens produced by opposite alleles are antithetical (e.g., Kpa and Kpb antigens).

    • Multiple alleles at a single locus are considered polymorphic.

Inheritance

  • Recessive: Gene is expressed only when inherited by both parents.

  • Codominant: Equal expression of two different alleles. Blood group antigens are codominant.

  • Dominant: Gene that is expressed over another gene.

  • Genes that do not express a detectable product are considered amorphic (e.g., O gene).

Mendelian Principles

  • Mendelian principles can be applied to blood group antigen inheritance.

    • Independent segregation occurs when one gene from each parent is passed to the offspring.

    • Independent assortment is demonstrated when blood group antigens from different chromosomes are expressed separately, resulting in a mixture of genetic material.

  • Two exceptions to this law are linkage and crossing over.

Linkage

  • Occurs when 2 genes that are close to each other are inherited together.

  • Each set of linked genes is called a haplotype.

  • Haplotypes tend to occur at a higher frequency than for unlinked genes, a phenomenon called linkage disequilibrium.

Crossing Over

  • Occurs when 2 genes on the same chromosome combine and produce 2 new chromosomes.

Chromosomal Assignment

  • Most blood group system genes are on autosomes, except for those of the Xg system.

  • Xg genes are found on the X chromosome.

    • If the father carries the Xg allele, he will pass it to all of his daughters but not to any of his sons.

    • If the mother carries the Xg allele (not the father), all of their children will express Xg.

Heterozygosity and Homozygosity

  • A person who inherits identical alleles is called homozygous.

    • Examples: AA, BB, MM (M+ N–)

  • A person who inherits different alleles is called heterozygous.

    • Examples: AO, AB, MN (M+ N+)

Dosage

  • In some blood group systems, persons homozygous for an allele have a “double dose” of an antigen on their RBCs compared with those who are heterozygous for an allele.

  • Dosage is a variation in antigen expression due to the number of alleles present.

  • Homozygous expression of some antigens will show stronger agglutination compared with antigens that are heterozygous.

Genetic Interaction

  • The location of inherited genes in cis or trans positions can affect the expression of the antigen.

    • Alleles on the same chromosome are cis to one another.

    • Alleles on opposite chromosomes are in the trans position.

Population Genetics

  • To determine genotype or phenotype occurrence, two formulas are used:

    • A phenotype calculation enables finding a unit of RBCs with certain antigen characteristics (i.e., antigen negative).

    • The Hardy-Weinberg formula calculates a determination of the gene frequencies that produced a trait.

Phenotype Calculations

Example 1

  • A patient with multiple antibodies (anti-C, anti-E, anti-S) needs blood.

    • 70% are C positive, 30% negative.

    • 30% are E positive, 70% negative.

    • 55% are S positive, 45% negative.

  • Calculation: 0.30 \times 0.70 \times 0.45 = 0.0945 or 10%.

  • Conclusion: About 10% of the population will be negative for all three antigens; about 1 in 10 units will be compatible.

Example 2

  • A patient with multiple antibodies needs 2 units.

    • 66% are Fya positive, 34% negative.

    • 72% are Jkb positive, 28% negative.

    • 9% are K positive, 91% negative.

  • Calculation: 0.34 \times 0.28 \times 0.91 = 0.087 or 9% negative (9 out of 100).

  • 2 units needed / 0.09 (antigen negative frequency) = 22.

  • Conclusion: Antigen typing of 22 units may be required to find 2 compatible units.

Hardy-Weinberg Formula

  • Basic Formula:

    • p + q = 1

    • (p + q)^2 = 1.0 \quad \text{or} \quad p^2 + 2pq + q^2 = 1.0

  • Where:

    • p is the frequency of allele A.

    • q is the frequency of allele a.

  • Example:

    • What is the frequency of q if p is 0.3? 1 – 0.3 = 0.7

    • What are the genotype proportions?

      • AA = p^2 = 0.09 (homozygous for A)

      • Aa = 2pq = 0.42 (heterozygous for Aa)

      • aa = q^2 = 0.49 (homozygous for a)

Molecular Genetics Applications

  • Transplantation:

    • HLA antigen-level and allele-level typing for HPC and organ transplants

    • Engraftment studies for HPC transplants

  • Transfusion:

    • Red cell typing in multiply transfused patients

    • Determine blood type when the DAT is positive

    • Complex Rh genotypes, weak D expression

    • Screen for antigen-negative donor units when antisera are unavailable

    • Donor antigen screening for prevention of alloimmunization

  • HDFN:

    • Determine parental RhD zygosity

    • Type fetal blood

  • Donor testing:

    • Detect virus in donors that may be below detectable levels by anti-body detection methods

  • Relationship testing:

    • Establish paternity and legal relationships for immigration

Polymerase Chain Reaction (PCR)

  • PCR rapidly and precisely multiplies specific DNA sequences.

    • DNA is denatured.

    • A primer is added to attach specific areas of DNA.

    • DNA is amplified and replicated.

PCR-Based HLA Typing Procedures

Sequence-Specific Primers (SSPs)

  • Primers are available in PCR trays.

    • Low resolution—identifies antigen level.

    • High resolution—defines specific alleles for the antigen.

  • Amplified DNA (amplicons) are assessed using gel electrophoresis.

Sequence-Specific Oligonucleotides (SSOs)

  • A primer for each locus is used.

    • A, B, C, DR, DQ, DP (in separate wells)

  • A DNA probe allows the hybridized solution to be read and analyzed by a flow cytometer.

Sequence-Based Typing (SBT)

  • Provides high-resolution, allele-level typing.

  • Primers are similar to SSOs.

  • An instrument analyzes the nucleotide and amino acid sequences that correspond to the allele.

Short Tandem Repeats (STRs)

  • Donors and recipients have closely matched HLA alleles but may have slight variations in the DNA sequence called polymorphisms.

  • Chimerism evaluation uses these differences to determine the percentage of DNA from the donor in a stem cell recipient.

  • STRs are short sequences of DNA that are amplified to determine the percentage of engraftment in chimerism evaluation.

Molecular Applications of RBC Typing

  • (See table in Molecular Genetics Applications section)

BeadChip Technology

  • Uses oligonucleotide primers attached to colored silica beads on a substrate (slide).

  • Amplified and digested DNA in question binds to the primers.

  • Computer analysis determines which primers have attached.