How are recessive traits inherited when multiple alleles or sex-linked genes are involved?

The first allele is dominant and the second allele is recessive. For genes on an autosome (any chromosome other than a sex chromosome), the alleles and their associated traits are autosomal dominant or autosomal recessive. Dominance is a key concept in Mendelian inheritance and classical genetics.

"do a testcross with a homozygous recessive plant."

the areas around the equator