The genotype is the genetic constitution of a cell, an organism, or an individual (i.e. the specific allele makeup of the individual) usually with reference to a specific character under consideration. For instance, the human albino gene has two recessive alleles, recessive a and recessive a. It is a generally accepted theory that inherited genotype, transmitted epigenetic factors, and non-hereditary environmental variation contribute to the phenotype of an individual. Non-hereditary DNA mutations are not classically understood as representing the individual's genotype. Hence, scientists and physicians sometimes talk for example about the (geno)type of a particular cancer, that is the genotype of the disease as distinct from the diseased.
The distinction between genotype and phenotype is commonly experienced when studying family patterns for certain hereditary diseases or conditions, for example, haemophilia. Due to the diploidy of humans (and most animals), there are two alleles for any given gene. These alleles can be the same (homozygous) or different (heterozygous), depending on the individual (see zygote). With a dominant allele, the offspring is guaranteed to inherit the trait in question irrespective of the second allele.
With a recessive allele, the phenotype depends upon the other allele. In the case of haemophilia and similarly recessive diseases a heterozygous individual is a carrier. This person has a normal phenotype but runs a 50-50 risk of passing his or her abnormal gene on to offspring. A homozygous dominant individual has a normal phenotype and no risk of abnormal offspring. A homozygous recessive individual has an abnormal phenotype and is guaranteed to pass the abnormal gene onto offspring.