Which characteristic is used to classify galaxies?

The galaxies were classified based on the shape and structure exhibited by them.

In the 1920's, the galaxies were classified based upon the shape and structure by famous scientist Edwin Hubble. He did it by a telescope with which he captured various galaxies existing. He ordered (or "arranged") their shapes as winding, banned winding, curved, sporadic, and impossible to miss. This framework was known as the Hubble morphological succession of cosmic system types.  

Hubble noticed that a few universes, similar to the M31-Andromeda Galaxy, showed up as circles and had arms of stars and residue which showed up in a winding example. Like M31, these worlds showed up almost uniform in splendor. Likewise, Hubble saw that in a portion of these sorts of worlds the arms were all the more firmly twisted around the cosmic system. He called these winding systems. Our Galaxy, the Milky Way, is a case of a winding universe.  

your answer is "the 1950's"

c) 50% there are four chromosomes. after crossing over, 2 of the four have been changed. that means any gamete has a 2 in 4 chance or 50% chance of receiving the mutated chromosome.