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The terminology of Mendelian inheritance
Gregor Mendel is famous for discovering “particulate inheritance” or the idea that hereditary elements are passed on in discrete units rather than “blended” together at each new generation. Today we call those discrete units genes.

A gene is a hereditary factor that determines (or influences) a particular trait. A gene is comprised of a specific DNA sequence and is located on a specific region of a specific chromosome. Because of its specific location, a gene can also be called a genetic locus.
An allele is a particular variant of a gene, in the same way that chocolate and vanilla are particular variants of ice cream.
An organism’s genotype is the particular collection of alleles found in its DNA. An organism with two of the same alleles for a particular gene is homozygous at that locus; an organism with two different alleles for a particular gene is heterozygous at that locus.
An organism’s phenotype is its observable traits. An organism can have a heterozygous at a particular locus but have a phenotype that looks like only one of the two alleles. This is because some alleles mask the appearance of others in a dominant/recessive pattern.
A dominant allele produces its phenotype whether the organism is homozygous or heterozygous at that locus. For example, in humans the allele for brown eyes is dominant to the allele for blue eyes, so a person who is heterozygous at the eye color locus will have brown eyes.
A recessive allele produces its phenotype only when homozygous at the locus; its phenotype is masked if the locus is heterozygous. For example, a person must have two copies of the blue eye color allele to have blue eyes.
Sometimes specific recessive alleles are associated with diseases. A person who is heterozygous for the gene will be phenotypically normal, but carry a copy of the recessive, disease-associated allele. This person is said to be a carrier and can pass on the disease allele to his or her offspring.