What is the difference between relatedness and descendancy?
This question looks like it came from your biology exam study guide. While answer #1 is an excellent explanation, I think what you are looking for is the following.
In evolution, relatedness (taxa, see link 2 below) refers to the degree of biological similarity between organisms or the degree of separation or distance between organisms. We call that the biological change between one organism and its common ancestor. Homo sapiens, (human beings) are said to be closely related to chimpanzees. We might say distant cousins. Relatedness does not mean members of a species are exactly alike, but they have many similar characteristics — homo sapiens walk upright, chimpanzees can also walk upright but that may not be their primary stature. The related organisms/units/species, homo sapiens and chimpanzees for example, have common characteristics that set them apart from other related organisms/units/species, such as mushrooms. You are more closely related to you sister than to your cousin because you are all descendants of your grandparents. Because descendancy refers to the common ancestry of a species, and all humans came from a common ancestor, they are all related. However, relatedness -- the degree of separation or the distance, between the human organisms -- increases over time. If we go far enough back we get to the common ancestor of homo sapiens and chimpanzees.
Descendancy deals with genetic linkage, while relatedness doesn't necessarily have anything to do with genetics.
Try it this way. All descendants are related to each other, but not all relatives are descendants of each other.
I am a descendant of my grandfather. He passed his genes on to my father. My father passed some of those genes to me. I passed those genes to my children, and my children will pass on that genetic information to my grandchildren. Descendancy deals with a genetic linkage.
Relatedness does NOT ALWAYS deal with genetic linkage. Yes, I am related to my father and descended from him. I am related to my in-laws, but not descended from them. Relatedness can be established through a genetic linkage, but it can also be established through marriage. Here's another example. I am related to my nephews on my wife's side of the family, but they are not my descendants. No part of my genetic information is in those children.