For persons born between December 24, 1952 and November 14, 1986,
a person is a U.S. citizen if all of the following are true...
..For persons born out-of-wedlock (mother) if all the following apply:
1. the mother was a U.S. citizen at the time of the person's birth and
2. the mother was physically present in the United States or one of its outlying possessions for a continuous period of one year prior to the person's birth.
If you only passed down citizenship via one person, that would mean there are tons of US citizens now in countries were we have had US tourists and US servicemen. Tons little citizens running around from hookers.
This does not mean the mother must be in the US for the year immediately prior to the birth - it just means she must have lived in the US for at least one full year at some point prior to the birth. Since Ted's mother was raised in the US, she obviously met that requirement.