Genetic Principles in Blood Banking

Definition of Blood Group Systems: These are categorized groups of antigens located on the Red Blood Cell (RBC) membrane. They are classified together because they share related serologic properties and follow specific genetic patterns of inheritance.

Genetic Material: All hereditary information is contained within DNA (Deoxyribonucleic Acid).
Location and Storage: DNA is organized into structures called chromosomes, which are housed within the nucleus of every cell in the body.
Replication Processes:
- Mitosis: The process by which genetic material is replicated in somatic (body) cells.
- Meiosis: The process by which genetic material is replicated in gametes (reproductive cells).

Phenotype: The physical expression of inherited traits. In blood banking, this often refers to the observable results of serologic testing (e.g., a person testing as Type A).
Genotype: The actual genetic makeup of an individual. This represents the specific alleles inherited from the parents (e.g., an individual with a Type A phenotype may have an $AA$ or $AO$ genotype).
Punnett Square: A diagnostic tool used to predict the probability of an offspring’s genotype. It summarizes every possible combination of maternal and paternal alleles for a specific gene.

Gene: The basic unit of inheritance located on a chromosome.
Locus: The specific physical site or location on a chromosome where a gene is situated.
Alleles: Alternative forms of a gene found at a specific locus.
- Example (ABO): At the ABO gene locus, the alleles can be $A$ , $B$ , or $O$ .
Antithetical Antigens: Antigens produced by opposite alleles at the same locus (e.g., $Kp^a$ and $Kp^b$ antigens).
Polymorphic: A term used to describe a locus that has multiple possible alleles (e.g., the ABO locus).
Amorphic: A gene that does not produce a detectable product (often called a "silent gene").
Syntenic: Two or more genetic loci located on the same chromosome, regardless of whether they are linked.

Dominant: A gene that is expressed even if only one copy is inherited, effectively masking the expression of another gene.
Recessive: A gene that is expressed only when it is inherited from both parents (homozygous state).
Codominant: A pattern where there is equal expression of two different alleles when present together. Most blood group antigens follow this pattern.

Independent Segregation: This occurs when one gene from each parent is passed to the offspring, ensuring genetic diversity.
Independent Assortment: This is demonstrated when blood group antigens from different chromosomes are expressed separately, resulting in a mixture of genetic material in the offspring.
Exceptions to Mendelian Laws:
- Linkage: Occurs when two genes are located very close to each other on the same chromosome and are inherited together as a unit.
- Crossing Over: Occurs when two genes on the same chromosome undergo a physical exchange of material, producing two new recombinant chromosomes.
Haplotype: A set of linked genes that are inherited together from a single parent.
Linkage Disequilibrium: A phenomenon where haplotypes occur at a higher frequency in a population than would be expected if the genes were unlinked.

Autosomal Inheritance: Most blood group system genes are located on autosomes (non-sex chromosomes).
Sex-Linked Inheritance:
- Xg and Kx Systems: These genes are located on the X chromosome.
- Paternal Transmission: If the father carries the $Xg$ allele, he will pass it to all of his daughters (who receive his X) but to none of his sons (who receive his Y).
- Maternal Transmission: If the mother carries the $Xg$ allele (and the father does not), all children have a chance to express the trait depending on which X chromosome they inherit.

Homozygous: An individual who inherits identical alleles for a given trait (e.g., $AA$ , $BB$ , or $MM$ / $M+ N-$ ).
Heterozygous: An individual who inherits different alleles for a given trait (e.g., $AO$ , $AB$ , or $MN$ / $M+ N+$ ).
Dosage Effect: A variation in antigen expression due to the number of alleles present.
- Double Dose: Homozygous expression (e.g., $Jk^a Jk^a$ ) often results in a higher density of antigens on the RBC surface.
- Single Dose: Heterozygous expression (e.g., $Jk^a Jk^b$ ) results in a lower density of each specific antigen.
Clinical Significance: Double dose expression may show stronger serological agglutination compared to single dose. Systems demonstrating dosage include:
- Rh
- Duffy ( $Fy$ )
- MNS
- Kidd ( $Jk$ )
- Example: RBCs with a genotype of $Jk(a+b-)$ (double dose of $Jk^a$ ) might react with Anti- $Jk^a$ at a $3+$ strength, whereas $Jk(a+b+)$ cells (single dose) might only react at a $1+$ strength.

Cis-Interaction: Occurs when genes are located on the same chromosome of a homologous pair.
Trans-Interaction: Occurs when genes are located on opposite chromosomes of a homologous pair.
These orientations can sometimes affect the expression of the antigens produced by the genes.

Purpose: To calculate gene frequencies and genotype proportions within a population.
Basic Formula for Allele Frequency:
- $p + q = 1$
- $p$ represents the frequency of the first allele (e.g., allele $A$ ).
- $q$ represents the frequency of the second allele (e.g., allele $a$ ).
Genotype Proportions Formula:
- $(p + q)^2 = 1.0$
- $p^2 + 2pq + q^2 = 1.0$
- $p^2$ = Frequency of individuals homozygous for allele $A$ ( $AA$ ).
- $2pq$ = Frequency of individuals heterozygous ( $Aa$ ).
- $q^2$ = Frequency of individuals homozygous for allele $a$ ( $aa$ ).
Calculation Example:
- If $p = 0.3$ , then $q = 1 - 0.3 = 0.7$ .
- $AA = (0.3)^2 = 0.09$
- $Aa = 2 \times (0.3) \times (0.7) = 0.42$
- $aa = (0.7)^2 = 0.49$

Clinical Application: Used to find the number of RBC units that are likely to be antigen-negative for patients with specific antibodies.
Method: Convert the known percentage of the population that is negative for the antigen into a decimal, then multiply the decimals for each required antigen.
Example 1: Finding a unit negative for $C$ , $E$ , and $S$ .
- $70\%$ $C$ positive $\rightarrow$ $30\%$ negative ( $0.30$ )
- $30\%$ $E$ positive $\rightarrow$ $70\%$ negative ( $0.70$ )
- $55\%$ $S$ positive $\rightarrow$ $45\%$ negative ( $0.45$ )
- Calculation: $0.30 \times 0.70 \times 0.45 = 0.0945$ (approximately $10\%$ or 10 out of 100 units).
Example 2: Finding a unit negative for $Fy^a$ , $Jk^b$ , and $K$ .
- $66\%$ $Fy^a$ positive $\rightarrow$ $34\%$ negative ( $0.34$ )
- $74\%$ $Jk^b$ positive $\rightarrow$ $26\%$ negative ( $0.26$ )
- $9\%$ $K$ positive $\rightarrow$ $91\%$ negative ( $0.91$ )
- Calculation: $0.34 \times 0.26 \times 0.91 = 0.080$ (approximately $8\%$ or 8 out of 100 units).

Applications:
- Used to identify RBC antigens when serologic hemagglutination is difficult or impossible.
- Determining the zygosity of an individual.
- Resolving complex cases involving weak or variant antigens.
Advantages over Hemagglutination:
- Does not require potent or rare antisera.
- Can be performed even if the patient has a positive Direct Antiglobulin Test (DAT) and recently transfused cells, which can interfere with standard serology.