CBSE Class 12-science Answered

Mendel selected true breeding tall (TT) and dwarf (tt) pea plants.When a tall pea plant is crossed with a short (dwarf) pea plant, all the F1 hybrids are tall. (i.e., in this case, the gene causing tallness is dominant while the gene causing dwarfness is recessive.).The trait expressing itself in the hybrid is the dominant one. 

(Mendel’s first law of inheritance states that when a pair of contrasting factors is brought in a hybrid, one factor inhibits the appearance of the other. The one which inhibits is the dominant one and which is inhibited is recessive.)  

• In Mendel’s experiment of breeding tall (TT) and dwarf (tt) plants, Mendel found that all plants in the F1 generation were tall.
• The F1 generation had genetic constitution Tt. It is genotypically a hybrid and a heterozygous plant having two different alleles, T and t.
• Phenotypically, the plant is tall because the allele or the gene T for tallness masks the effect of its corresponding recessive gene t.
• So, dwarf plants with genotype (tt) were not obtained in the F1 generation. This implies that out of two, one of the contrasting characters for a particular trait is dominant which masks the expression of the other character which then becomes recessive.
• This experiment showed that traits may be dominant or recessive, depending on their expression in the F1 generation.