Who though that aging was attributable to a loss of irritability in nervous and muscular tissue?

The answer is Erasmus Darwin

Explanation:

In the 19th century, Erasmus Darwin, Charles Darwin's grandfather proposed the theory that loss of irritability in the nervous system and a decreased response to sensation or stimuli was associated with ageing.

This is one of the first theories of biological ageing; the others being Hippocrates' and Eli Metchnikoff's.

Hippocrates attributed ageing to the loss of body heat whereas, Eli Metchnikoff associated it with autointoxication. A state caused by the poisoning or intoxication of the body by toxins produced by the body itself.

this transfer is call conjunction

"do a testcross with a homozygous recessive plant."