Xga Blood Group System

Inheritance

The Xga (or Xg) system is one of the few BGS whose genes are sex-linked. Because the system has little significance other than its mode of inheritance, it is included in this chapter. As the name implies, the gene locus is on the X chromosome. Inheritance is controlled by two alleles (Xga and Xg) with Xga being dominant and Xg being recessive (or an amorph).

Because the system is sex-linked, the frequency of the Xg(a+) phenotype varies in males and females. The female frequency is 88% and the male frequency is 66%.

Males can be positive (XgaY) or negative (XgY). If positive, they are said to be hemizygous.

Females, if positive, can be homozygous (XgaXga) or heterozygous (XgaXg). XgXg females are Xg(a-).

Examples of Inheritance

Inheritance of the Xga antigen follows the expected pattern for sex-linked dominant traits. Figure 1-9 demonstrates a pedigree for an Xga-negative male mating with a homozygous-positive female.

When an Xga-negative male mates with a heterozygous-positive female , results are shown in Figure 1-10.

Lastly, the mating of an Xga-positive male with a negative female gives the results shown in Figure 1-11.

Significance

The Xga system is not very significant in transfusion medicine because the sole antibody in the system (anti-Xga) is extremely rare. This suggests that the Xga antigen must be poorly immunogenic.

The system is of interest to blood bankers because it is one of the few blood group systems whose genes are sex-linked. (It seems to interest blood bank instructors because it give them a chance to ask questions about terms like hemizygous.) It can sometimes be of use to geneticists in explaining genetic accidents (e.g., X-chromosome aneuploidy) or in helping to map the X chromosome (Figure 1-12).

Terms to look up

Enrichment Activity 5

Visit Klinefelter's and try the problem. Also review the tutorial at this site.

Learn more about another X-chromosome aneuploidy at Turner syndrome. Also visit the Turner syndrome FAQ.

  1. Post a comment or question to the class discussion list about any social-psychological issue related to females with Turner syndrome.

  2. Post a comment or reply to the class discussion list in response to a comment or question made by another participant about issues related to Turner syndrome.

Inheritance of Blood Groups Xga Blood Group System ABO BGS