a) Pure breed: These are the plants that expresses a selected character over several generations. Such plants according to Mendel were pure breed for that character. 

b) Phenotype: The characters which can be seen is known as phenotype. We cannot determine the internal factors by phenotype. It tells about only the dominating char-acters which express externally. The phenotype ratio in monohybrid cross is 3 : 1. 

c) Genotype: The genetic make up of an individual is known as genotype. Genotype itself is the indication of internal factors. It tells about both dominant and recessive characters present within. The genotype ratio in monohybrid cross is 1 : 2: 1. 

d) Alleles: Alleles are corresponding pairs of genes located at specific positions in chromosomes. Together they determine the genotype of their host organism. Every individual possesses a pair of alleles for any particular trait and that each parent passes a rondomly selected copy of only one of these to an offspring. The offspring then receives its own pair of alleles for that trait one each from both parents.