In this example, a father who is Xga-positive has mated with an Xga-negative female to produce a positive son. This would seem to be impossible. (It looks like non-maternity.) However, it can be explained by nondisjunction in the father in which he transmitted Xg a to the son. The mother would transmit an X chromosome also, making the son XXY (Klinefelter syndrome). The Xga system was useful in this case because the father was positive and the mother was negative. If the mother had also been Xg(a+), we would not have known where the son's extra X came from.
 Xga Blood Group System
| Figure 1-12 |