(N/A) Mendel proposed that something was being passed down unchanged from parents to offspring through the gametes over successive generations. He called these things 'factors'. Today,we call them 'genes'. Therefore,genes are the units of inheritance.
Genes that code for a pair of contrasting traits are known as alleles.
In alphabetical symbols,capital letters are used for the trait expressed in the $F_1$ generation,and small letters are used for the other trait. For example,$T$ is used for tallness and $t$ for dwarfness. Now,$T$ and $t$ are alleles of each other,which can be expressed as $TT, Tt,$ or $tt$.
If both alleles are identical,the organism is homozygous ($TT$ or $tt$).
$TT$ and $tt$ represent the genotype of the plants,while the terms 'tall' and 'dwarf' represent the phenotype.
In a pair of dissimilar alleles,one dominates the other and is expressed in the $F_1$ generation. This is called the dominant factor,and the one that is not expressed is called the recessive factor.
In a homozygous condition,the alleles are identical ($TT$ or $tt$),but in a heterozygous condition,they are dissimilar,such as $Tt$. Crosses between $Tt$ and $tt$ are known as monohybrid experiments.