Monohybrid cross:
It is the cross between two plants or animals differing in only one trait. In this cross, only a pair of contrasting characters are observed, for example red flower and white flower character; and smooth with wooly character of hair etc. These types of control sating traits or characters are said to be alleles. Alleles are the two alternative genes at the same locus on homologous chromosomes.
Red flowered pea plants are crossed with white flowered pea plants. The parental plants are referred as homozygous. Since they possess red flower character R in two doses: Similarly white flowered plant is homozygous.
During the formation of gametes, reduction division takes place and gametes carry only one gene or factor. Thus F plant receives R and r from each of the parents. F plants bears only red flowers, it is because the red flower character R is dominant and white flower character, i.e. the red flower gene is dominant over white flower gene is recessive.
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F1 plant is represented as Rr-two different factors are present hence it is referred to as heterozygous. The factors R and r separate during the formation of gametes. Random fertilization takes place resulting in the plants which show 3:1 ratio, as shown in the figure.
Out of 3/4 red flowered plants only 1/3 are homozygous (RR) and so breed true; remaining 2/3 plants are heterozyous (Rr) and yield again 3:1 ratio, remaining 2/3 plants are homozygous (rr) since they breed true. By the above observation. 3:1 ratio can be split into 1:2:1 ratio according to their genetical nature or genotype.