In line 19 inertness most nearly means

If you provided the aforementioned line 19, that would kinda be great.

Inertness refers to the non-reactive property of a substance. In chemistry, a substance is said to be inert if it does not react with another substance. For  example, noble gases like Helium and Neon are inert gases.

In physics, inertia refers to the property of a body by virtue of its mass where in it does not change its state of motion unless acted upon by an external force.

It most nearly refers to non-activity.

d. air pressure decreases as altitude increases.

multiple allele inheritance is traits controlled by more than 2 alleles (ex: pigeons having 3 alleles of one gene govern their feather color).