Combine the given two facts to write a concluding fact. Note that there should be some parts of the first and second facts that are not mentioned in this conclusion fact. Your combined fact should be the result of a chain between the two facts. Chains form when two facts connect together to produce a concluding fact. An example of a chain is: "pesticides cause pollution" (fact1) + "pollution can harm animals" (fact2) → "pesticides can harm animals" (conclusion fact). Parts of the concluding fact overlap with the first and the second fact. To construct such a  concluding fact, a useful way is to borrow the subject from one fact and the ending conclusions from another fact.

Fact 1: plasma is formed by electrons separating from atoms in stars. 
Fact 2: Plasma is more common in stars, because making plasma requires high temperatures.
Plasma is formed in high temperatures of the sun.

Fact 1: skin is used for breathing air by frogs. 
Fact 2: Air contains oxygen.
skin is used for breathing oxygen by frogs.

Fact 1: Skin color is another polygenic trait. 
Fact 2: Most dental variations are polygenic, that is controlled by several genes.

Skin color is a trait controlled by several genes.