Why do genetic counselors need information about many generations of a family in order to draw conclusion about a heredity condition?

Some conditions are recessive while others are dominant. In addition, a condition might be X-linked or chromosomal, environmental (due to a teratogen) or sporadic. A comprehensive evaluation of the entire family history (3 or 4 generations) is highly useful in differentiating between these different disease causes.

Some conditions present very similarly, but can be inherited in different ways and through the family history, a trained genetic counselor can help distinguish between different types of inheritance and may help narrow down options for diagnostic testing.