Why are pedigrees in understanding genetic disorders?

Pedigrees help in understanding the genetic basis of human traits. This can be done by analysing the results of matings that have already occurred. Pedigree are useful in showing the inheritance pattern of particular traits in a family of related individuals. Pedigree can be used to assess the risk for various genetic conditions. This pedigree analysis had been used to find out the breast cancer gene (BRCA1) by looking across the generations of realated individuals.

answer and explanation;

volcanic rocks are igneous rocks which form from lava, molten rock which has been thrust out of a volcano onto the surface of a rocky body such as an asteroid, planet, or dwarf planet.  

plutonic rocks are igneous rocks which form from subterranean chambers of molten rock or magma. plutonic rocks make up the base of continental crust as well as oceanic crust. their slow rate of formation causes them to be coarse grained since there is plenty of time for large crystals to form before the magma solidifies into rock.

the textural difference is that plutonic rocks are more coarse grained than volcanic rocks. plutonic rocks are coarse grained while volcanic are fine grained ; glassey or fragmental. since fine and coarse are adjectives the decision as to whether a rock is fine or coarse grained is generally made on the bases of whether or not the individual crystals can be seen in most if the rock.