How is an allele described?
form of a gene
one haploid cell
part of a chromosome
short segment of dna

The correct answer is : Form of a gene

Explanation:

Alleles can be defined as the alternate forms of the same gene. A gene may have two or multiple alleles.The alternate forms express contrasting characters that can be  observed in the phenotypes of the individuals.The phenotypes of individual alleles is best observed in individuals carrying homozygous combination of either alleles.According to Mendel's law of dominance, among the alleles, one remain dominant over the other alleles, that is, if the dominant allele is present it can mask the effect of any other allele present along with it. The other allele  called the recessive allele can show its effect only when it is in homozygous form, that is when the dominant allele is absent.An allele is a form of the gene and hence a part of the chromosome.