Q. 9. In a monohybrid cross of plants with red and white-flowered plants. Mendel got only red flowered plants. On self-pollinating these plants got both red and white flowered plants in 3 : 1 ratio. Explain the basis of using RR and rr symbols to represent the genotype of plants of the parental generation.
Phenotypic ratio Red flower : White flower 3 : 1
Genotypic ratio RR : Rr : rr 1 : 2 : 1
Generally, upper case letters are used as symbols for dominant and lower case for recessive traits of the same gene (alleles). Experiment shows that it is a monohybrid cross with 3 : 1 ratio in -generation.
This shows parents must be true-breeds. As parents are diploid and homologous chromosomes carry alleles with a similar type they are represented with RR and rr.
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