Humans receive a copy of gene from both parents and each gene has an alternative form called allele. Does every person possess two genes and four alleles for a single trait?
To fully answer your question it would take an entire course of molecular genetics and quantitative genetics. I will be only very brief and will let you seek for more information on your own.
Basics of molecular genetics
You are confounding gene copy and allele. As we are diploid, we have two copies (I am ignoring here sexual chromosomes and mtDNA) of every locus. This is true for genes too. We have two copies of each genes. Now, we may or may not have the same allele at these two copies. If we have the same allele at both copes, we are homozygous, otherwise we are heterozygous at this locus.
- You might want to have a look at the post Do men and women have the same number of genes?
Now, each chromosome is made of two sister chromatids (although this number varies depending upon the stage of life cycle). The sequences at the two sister chromatids are exactly the same, so you can't have different alleles at two sister chromatids.
- You might want to have a look at the post What is the relationship between DNA molecules and the chromosomes?
Basics of quantitative genetics
Of course, a single phenotypic trait is not necessarily mapped out by a single locus. There are usually plenty of loci involved (see Quantitative Trait Loci, epistasis and eventually pleiotropy). There is also other source of phenotypic variance than genetic variance such as environmental variance for example.
- You might want to have a look at the post Why is a heritability coefficient not an index of how “genetic” something is?
Ok ,but I can't get your point .As there are two copies of one gene each from father and mother and we have an allele for every gene copy so, combining these we got two genes and four alleles .?Can we?
We receive one copy of every locus (or gene as you seem to get stuck with this term) from each parent. If these two copies differ, then we have two different alleles (heterozygous), otherwise we have only one type of allele (homozygous). In some life stage of the cell cycle, each chromosome is made of two identical chromatids. When this is the case, there are 4 copies of the gene but never more than two different alleles.
You really seem to use the term "allele" when "gene copy" should be used. Think of an allele as a pokemon card. When you ask someone "do you have Pikachu?", you're not asking whether the person has the exact same card that the one that is in your deck of cards. Alleles are the same! So even if you have 2 copies of a given locus (that is two pokemon cards), you might or might not have two different alleles.
You also use the term "gene" where it would be more correct (or more general) to say "locus". You should check out the definition of locus.
Genes are sequences of DNA that do specific things (typically, that get encoded into proteins).
Chromosomes are long DNA strands that contain, well, DNA, and some sequences of that DNA are genes.
Those genes occur in specific places along the chromosome, just like the word "genes" occurs in specific places in this sentence (second and fourteenth word, namely), and if you copy-pasted this sentence they would be in the same places in the copy. This means two ways of recognizing the same gene in two copies of a chromosome are its sequence (which might not be identical if there are copying errors, but will be very close), and its location in the chromosome.
There is a bit of an ambiguity there; sometimes we use the word "gene" to refer to a literal sequence of DNA that forms a gene, but sometimes it refers to the more abstract concept of the same sequences that occur at the same locus in different people and do the same thing.
It's a bit like people can talk about "a Harry Potter book" and refer to the literal paper-and-ink book on their shelf, or they could be talking about the abstract stories in the Harry Potter series.
If you see a DNA sequence with ATCG and it's labeled as a "gene", it would be the first one. If people talk about "we've found the gene for Parkinson's", "how do we recognize the same gene in two copies of the chromosome", it will typically be the second one. And because the sequences aren't always identical you can have similar sequences of DNA, in the same place, that do basically the same thing, so they're "the same gene" in the abstract sense, but they do it differently from each other. Those can be thought of as variants of one another, and those variants are called "alleles".
In the Harry Potter analogy, alleles are like which of the six Harry Potter books we're talking about. So if you have two Harry Potter books in your library, The Philosopher's Stone and The Prisoner of Azkaban, you wouldn't count it up and say you have four books: the two Harry Potter books, and The Philosopher's Stone, and The Prisoner of Azkaban. You would understand that "Harry Potter books" is a general term that refers to a certain kind of books, that have enough in common that they can be considered "the same" in the sense of all being one series, and that this specific kind of books comes in six variants. You can't have "a Harry Potter book" that isn't also one of the six specific Harry Potter books that exist (I bet there are more now but let's ignore that for the analogy).
Also, if a person has on their bookshelf two copies of The Prisoner of Azkaban, do they have one Harry Potter book, or two Harry Potter books? It depends on whether we are talking in an abstract or a concrete sense. They have two literal paper-and-ink books on their shelf, but they have only one Harry Potter story.
Similarly, every person has two copies of a gene, i.e. a certain DNA sequence at a specific place, or "locus", on the chromosome, and if we're talking about literal DNA sequences we'd say they have two genes, but if we're talking about the more abstract concept we wouldn't say that. Usually when people talk about genes they're talking in the abstract sense, and thus talking about different sequences at different places on one chromosome, not copies of the same sequences in different chromosomes. Kind of like if someone asked you how many book series you own, you wouldn't count your paperback and hardcover copies of the same Harry Potter books as two series.
So to answer your question "Does everybody possess two genes" -> yes, for one sense of the word "gene", but it's not necessarily the one you meant.
As for the alleles, just like your two HP books aren't four just because they also have a specific title, people don't have four alleles because they have two copies of a gene and those copies are each a specific kind of that gene. People have one or two alleles, depending on whether the two copies they have for that locus are the same allele or two different ones.
By the way, it is not true that every gene has one variant called an allele (or, put in a better way, that every gene comes in two variants). Some genes don't vary much at all between people, so there's only one allele for the human race and everyone has the same one, while others have tons of alleles. But every person will only have one or two of those alleles, unless they have more than two chromosomes.