Question

What do u mean by pure line??

Answer 1

PURE LINE: A breed or strain of animals or plants in which certain characters appear in successive generations as a result of inbreeding or self-fertilization.

Answer 2

pure line simply means homozygosity.
when an organism or plant is continously selfed for four or five generations it becomes homzygous (TT)
That is the reccesive trait is completely excluded from that specific lines gene.

in cattle breeding it may cause breed deppression