Anatomy and Physiology: Blood Types and Genetics

Blood Types and Genetics

Before I talk about blood type, I should probably back up and talk about genetics. I talked about DNA and protein synthesis, as well as the chromosomes involved in mitosis. Don't forget that, unless you have a chromosomal defect such as Down's syndrome (Trisomy 21), each of your chromosomes comes as a homologous pair with one chromosome from each parent. These homologous chromosomes have the same type of gene on them, but the alleles are different. A gene is section of DNA that codes for a particular protein, but an allele is a variation of a gene. Since genes are on chromosomes, and chromosomes come in pairs, our genes usually come in pairs of alleles (those on the sex chromosomes—the twenty-third pair, that determine gender—might not, because females are XX and males are XY, or one X chromosome and one Y chromosome).

Alleles can be either dominant or recessive, so the combination of alleles in the pair determine the trait. A dominant allele, indicated by a capital letter, is one that is expressed as a trait (such as a widow's peak hairline) with either one or two copies of the allele: WW or Ww. A recessive trait, indicated by a lowercase letter, is one that is only expressed (such as a straight hairline) with two copies of the allele: ww. The combination of alleles—WW, Ww, and ww—is called a genotype: WW is homozygous dominant, Ww is heterozygous, and ww is homozygous recessive. The physical trait that is expressed is the phenotype, such as the type of hairline, and it is determined by the genotype: WW and Ww are both dominant (widow's peak), while ww is recessive (straight hairline).

That's a lot to get out of the way, but it will be useful when I talk about reproduction. Now, on to blood type. Blood type refers to the antigens (in this case glycoproteins) on the membrane of the RBC, and it is determined genetically by two genes: the ABO gene and the Rh gene. Let's start off with the Rh gene, which gets its weird name from its initial discovery in the blood of the Rhesus monkey.

If the allele for the glycoprotein (indicated as Rh +) is inherited, the glycoprotein will appear on the cell membrane; there is another allele, which means that no antigen is displayed (indicated as Rh -). The only way to have a cell with no Rh antigen is if both chromosomes in the homologous pair are Rh negative (--); Rh negative is therefore recessive. You will be Rh positive if you inherit at least one Rh + allele (that is, ++ or +-).

The ABO gene, which gets its odd name from the three alleles, determines the other blood type: A, B, and O, of course. This gene illustrates another interesting thing that sometimes happens in inheritance: some genes are codominant. Codominant genes are ones in which two alleles, such as A (IA) and B (IB), are both dominant, and a heterozygous genotype (AB) produces a phenotype where both traits are expressed (the A and B glycoproteins).

Similar to the Rh--allele, the O (I^O) allele can only produce the O phenotype if both inherited alleles are O (OO), which makes the O allele recessive. All of this is important because emergency procedures often involve loss of blood, and the only way to keep a patient alive is to replace some of the volume lost; without enough blood, tissues will not be able to receive the materials, especially O₂, needed to survive (the brain, in particular, dies very quickly in the absence of O₂). A simple idea is to take a little of the blood from one person and put it into another, a process called transfusion.

Compatibility of blood types is based on the fact that incompatible blood sample will agglutinate, or clump, when mixed. This is due to the presence of antibodies that attack foreign surface antigens (A, B or Rh +, but not O or Rh -). For this reason, these surface antigens are sometimes called agglutinogens, and the antibodies are called agglutinins. The three antibodies are called, and rightfully so, anti-A, anti-B, and anti-Rh. Anti-A and anti-B are always found in the blood of specific blood types (see the following table of blood types), even without prior exposure to the foreign antigen, but anti-Rh is only produced after exposure.

A quick look at the table of blood types illustrates some interesting points. Any incompatible blood type must have an antigen that is foreign to the recipient. AB+ blood contains all three antigens (A, B, and Rh +), and none of the antibodies; as such, there is no blood type that is incompatible to AB+, the universal recipient.

Most people learned somewhere that type O is the universal donor, meaning anyone is able to receive it. That is only partly true. Do they mean O+ or O-? In order to be compatible to everyone, there can be no foreign antigens. Look at the table again. O+ has an Rh + antigen that is foreign to anyone with an Rh--(A-, B-, AB-, O-). O-, however, has no foreign antigens, making it the universal donor.

Excerpted from The Complete Idiot's Guide to Anatomy and Physiology © 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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