All organisms possess characteristics of their own, which ensure their uniqueness. These characteristics are morphological (eye, hair, ear shape, etc) as well as physiological (blood type, etc). The latter require more complex analysis than the former.
All these hereditary characteristics constitute the phenotype.

    I) Phenotype depends on proteins :
        a) Phenotype diversity :
A species is defined as a group of individuals able to reproduce naturally with one another. Thus individuals from the same species have characteristics in common, which are specific to  their species (all dogs have a muzzle and can bark). However, each individual has its own characteristics which differentiate it from all others.
Some of these characteristics, such as face shape, skin color, eye color, etc, are easy to detect. Others, such as blood type, leukocyte antigen, etc, require complex analysis. These are just a few examples among a host of other characteristics. Their combination ensures  genotype diversity and thus the uniqueness of each individual.
Further analysis has enabled scientists to establish that many of these characteristics depend on proteins.
For instance, skin color depends on a pigment called melanin; Melanin is a protein which governs skin color, hair color, eye color… Albinism is a disease caused by the absence or lack of melanin. This protein is not synthesized by albinos because the enzyme which controls melanin synthesis, either functions badly or is simply absent. This enzyme usually ensures the conversion of the tyrosine amino acid into melanin. Thus, in this example, a protein directly controls a phenotypic characteristic : pigmentation.

        b) Protein diversity :
More generally, we can also say that phenotype diversity is due to protein diversity, which controls genetic expression. Studies carried out on these proteins have revealed that the same protein can occur in different forms for the same function. There is thus protein polymorphism.
Protein properties depend on the spatial structure and the amino acid sequence of this protein. Just as the amino acid sequence on a protein depends on a genetic program, protein diversity, and thus phenotypic diversity, also depend on a genetic program. However, we can observe that environmental living conditions may have an influence on an individual's phenotype.
The phenotype results from the expression of the genotype, that is to say of the gene pool. This explains why dramatic consequences might occur due to an error in the genetic program. Even a small error (on one nucleotide) might cause severe damage on the phenotype expression.

    II) An allele, a gene version: many allele for one gene :
        a) What is an allele ? :
An allele is a version of a gene. In fact, a gene often exists in many versions. This allelic polymorphism is very widespread in every population. For example, there are 3 versions of the gene coding for the blood type, i.e., there are 3 possible alleles for this gene: A, B, O. Another example, there are thousands of versions of the HLA system genes (Human Leukocyte Antigen). That explains the incompatibility problems which may occur in the case of transplants.
Nevertheless, even though a gene has many different versions, we cannot establish a “normal” or an “abnormal” version. There are only different alleles which may lead to different phenotypic characteristics.

        b) Origin of allelic polymorphism : genetic mutation.
A gene pool can be hereditarily be transmitted through generations. Allelic polymorphism is mainly due to genetic mutations, which have been passed down through generations from the first individual in which the mutation occurred.
Before going any further, more explanations about what a mutation is are needed. A mutation is a localized modification of DNA, which may have an influence on a greater or lesser part of the genetic program, by changing the nucleotide succession. A mutation may either cause a different expression of the characteristic controlled by the "diseased" gene or have no consequences whatsoever, in this case, the mutation is said to be “silent”. Mutations which have occurred all along the human evolutionary process are the cause of allelic polymorphism.
There are different kinds of mutations:
   Mutations by substitution consist of the replacement of one or many DNA nucleotides by another. Such a mutation may cause no changes in the phenotype since genetic code is redundant; A 3-nucleotide block of DNA undergoing such a mutation may correspond to an unchanged RNA-codon and thus to the same amino acid on the future synthesized polypeptide. However, it may also cause severe damage at the phenotype level. Indeed it could cause changes during protein synthesis or simply deter it from processing.
   Mutation by deletion is a genetic modification consisting of a suppression of one or many DNA-nucleotides. It may affect several DNA-genes because it can cause a shift in the DNA-transcription and then a bad RNA-translation into polypeptide. Yet an insertion may make up for a deletion, reducing the scope of the consequences.
   Mutation by insertion consists of an insertion of one or several DNA-nucleotides. As with the mutations by deletion, mutation by insertion may have a great influence on phenotype expression by causing a shift in the nucleotide succession and may severe affect protein synthesis.
Also, mutations may cause more or less damage which could be of greater or lesser consequences. These mutations imply the creation of new characteristics, new alleles, which will be passed down to future generations only if they do not affect the individuals too severely; otherwise they would be eliminated by natural selection.
We observe that a mutation can be hereditarily transmitted only if it affects a germinal cell (ovum and spermatozum for animals).

    III) A specific allele combination characterizes each organism:
        a) Genetic polymorphism in population :
Mutations of a gene are statistically rare. But because of the large number of nucleotides in DNA (many billions), mutations frequently occur. Taken collectively, mutations which have occurred in all populations are the cause of allelic polymorphism and genotype diversity. When a gene has only a few versions (a few allele), it may be because mutations were too harmful for the organism and so unlikely to be preserved.

        b) Genetic diversity of organisms :
In each individual's genetic program, there are two versions (alleles), among all possible ones, for each gene. The two alleles of a gene are each carried by one chromosome of a pair on the same locus.
When the two alleles are identical, the individual is said to be “homozygous”. When the two alleles are different, the individual is “heterozygous”.
It has been estimated that one person chosen at random among 100 000 is highly likely to be heterozygous.
It is very unlikely for children of the same couple to have the same gene pool; they will combine the many alleles of their two parents; this combination, called genetic mixing, will ensure the uniqueness of each organism. Identical twins are the exception to this rule.

        c) Genetic fingerprint :
Modern technology allows us to “cut “ DNA molecules into specific segments through specific enzymes which enable us to work on specific nucleotide sequences. These enzymes are called restriction enzymes.
We can then differentiate between the different DNA segments by electrophoreses, which enables us to establish a kind of “bar code”, which will be different in each individual.
This “bar code” constitutes the genetic fingerprint which scientists, the police and judges use increasingly to carry out their investigations.

        d) Phenotype results from genotype expression :
Thus phenotype diversity is due to protein diversity. Protein synthesis is controlled by a genetic program. Protein polymorphism has been observed during studies among populations. This protein polymorphism is due to allelic polymorphism.
The expression of genes occurs according to specific rules. The two alleles of a gene in a homozygous individual control the synthesize of the same protein.
When an individual is heterozygous, that is to say when the two alleles of a gene are different, either the two gene versions express and then the alleles are said to be codominants (e.g. HLA genes ); or one expresses at the expense of the other, then the first is said to be dominant and the second is recessive.
We can notice that although a person has a genetic disease, he can nonetheless carries one “normal” allele; This means that it is the “abnormal” allele which has expressed.

Finally, we can conclude that the phenotype depends on genotype expression. Rules governing genotype expression can quickly become very complex.