The defective gene may be passed from parent to child at conception. If a parent has the gene, each son or daughter has a one in two (50/50) chance of inheriting HD.
A 50/50 risk factor does not mean that exactly half the children in a family will get the disease. One child in a family of four children could develop HD or two may get it or three or perhaps all four or none. Each person faces his or her own 50/50 risk regardless of whether any brothers or sisters are affected or not.
If a person does not inherit the defective gene from the affected parent they can't pass it on to their own children.
Huntington's Disease does not appear in one generation, skip the next, then reappear in a third or subsequent generation. However it may appear to skip a generation. Consider, for example, a theoretical case where a grandfather had the disease, his son apparently escaped but the son's daughter eventually developed the symptoms of HD. Rather than having skipped a generation we know that the defective gene was also present in the son but he died, perhaps in an accident or from a heart attack, before symptoms appeared.
[This page is based on the Association's publication Huntington's Disease. Originally written by Dennis H. Phillips, Ph.D. and first published in 1981, it has been frequently revised and republished since then. The current edition was published in 2001. (Australian Huntington's Disease Association (NSW) Inc.Huntington's Disease West Ryde, 2001.)]