Here in the United States there seems to be a common misconception that seems to pop up when I talk to my friends about the draft. Namely, that if you're the only living male in the family with your last name or even if you're an only son you're immune from getting drafted. This urban legend seems to have roots in a law passed in 1948 and popularized by movies like Saving Private Ryan and The Fighting Sullivans. The law, in its 1948 form, was designed to protect families during World War II from losing all of their offspring. Simply, if one or more of your sons or daughters had died in the war and you only had one son left, he could not be drafted. In the start of the Vietnam War in 1964 congress amended the law so that if your father had died in combat and you were the sole surviving son you could be exempted. However, congress also changed the law to be peacetime only, thus giving all the wannabe draft dodgers of the Vietnam War one less option. In 1971 Congress changed the law yet again and removed the sole remaining son clause so that any person in the military could leave if an immediate family member died in combat. Effectively, this law is only useful now if you've joined the military and need an excuse to quit before your peacetime service is up.
Laws like this are in effect in other countries and have been historically implemented. The governments of Lebanon, Egypt, and Syria require service for all 18 year old males, but all three will exempt anyone who is an only son. The government of Greek Cyprus gives an exemption if "the oldest or the only son of the family whose father or brother was killed or went missing while serving in the National Guard." Germany also has compulsory service for all 18 year old males, but with the odd twist that if you're the third brother in your family you have the option of not attending. I also remember from my history class that Nazi Germany would give the option to sole surviving sons not to fight on the front lines, but I can't find verification for this now.
However, these laws seem to be the exception and not the rule. Here in the United States there is significant push with the Universal National Service Act of 2003 (bills S 83 and HR 163, the Senate bill is as of now being considered by the Committee on Armed Services) to a system that is more equal in the way people are drafted. In its current form, the bill offers no recourse for people that are in college, allows both males and females to get drafted and helps prevent those that are well off from avoiding military duty.