What is a combination of substances that are combined physically but not chemically

How does secondary succession differ from primary succession?

Plz ! what does it mean for an allele to be dominant?

Choose the best definition of allele. a. one of several possible versions of a gene, which each produce a different phenotype b. an organism that's purebred for a given trait c. the process of fertilizing a plant with pollen from another plant d. a set of genes that are located on the same chromosome and so are tightly linked