Medelian Genetic Inheritance
Dominant & Recessive
Based on this data, Mendel was the first to hypothesize what we now refer to as Mendelian genetic inheritance. He proposed that parents (mothers and fathers) each contribute half of their offspring's genetic material. We now know (although Mendel did not at the time), that this genetic material is deoxyribonucleic acid, or DNA. DNA makes up genes, which are units of heredity that can determine specific physical characteristics. That is, this genetic material controls the phenotype that we can observe; we refer to the genetic basis of the phenotype as the genotype.
To put it simply, a genotype is the instructions in your DNA, while a phenotype is the trait you have because of those instructions.
In many organisms, including humans, there are two copies of every gene (one inherited from our mother, the other from our father). We refer to each version of a gene as an allele. For example, for a particular gene, let's say that one allele is A and the other allele is a. Then the genotype of that individual, for this particular gene, could be any combination of these alleles - AA, Aa, or aa. If an individual has two copies of the same allele (i.e. AA or aa), then that individual is said to be a homozygote, whereas an individual that has two different alleles (i.e. Aa) is called a heterozygote. These terms can be easily remembered, as "homo" means "same" and "hetero" means "different." Two different alleles can correspond to different phenotypes, e.g. green versus yellow.