Different forms of a given gene are known as

Allele

A gene is a basic unit of inheritance. A gene is located on a fixed position in a chromosome. They consists of genetic information in the form of nucleotides.  They provide instructions to the cell machinery or organelles to produce specialized molecules called as proteins. Proteins are produced in order to perform a specialized function in the organism. An allele is a form of gene. The genes have two alternate forms of alleles, either of the allele when expressed in the organism produces a particular phenotype.  

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.