Question

what is difference between gene and allele

Answer 1

Ok we'll start from gene Gene is the basic unit of heredity and is that part of DNA which codes for functional RNA and protein (which further forms enzyme). So, not every part of DNA is considered as the gene ( chromosome is condensed form of DNA, single chromosomes can contain several types of Gene)

Alleles: I'll explain you allele with an example because sometimes bookish language is hard to understand 
Eg- T is a gene, which controls the height of a pea plant, but this does not explain to us why some plants are tall, meanwhile others are dwarf what is the reason behind this?
Here comes the answer, " gene which codes for a pair of contrasting traits are known as alleles that is, they are slightly different forms of the same gene, that's why some plants are tall while others are dwarf "
'T' (allele) is used for a tall trait on the other hand 't' (allele) is used for the dwarf trait. 
An allele is always present in pairs ( well the reason is so obvious because chromosomes are always paired) 
Tall plant - TT 
Tall plant -Tt
Dwarf plant - tt 
Now, don't get all confused why Tt is tall and tt is dwarf because there are some laws which we have to follow, one of them is " law of dominance " according to which 
" In a dissimilar pair of factors (Genes) one member of the pair dominates ( dominant) another hand (recessive) " 
 when identical alleles are present (TT)in a pair then we call this condition as homozygous, while heterozygous (Tt) when contrasting pair is present( here am not going to explain you about crossing, because your question is all about the difference between a gene and an allele)

So, basically, An allele is a variant form of a gene. Some genes have a variety of different forms, which are located at the same position, or genetic locus, on a chromosome. Humans are called diploid organisms because they have two alleles, one at each genetic locus, with one allele inherited from each parent.