CBSE Class 10 Answered

Monohybrid Cross - A cross which involves only a single pair of contrasting characters.

• Consider pea plants with a pair of contrasting characters—tallness and dwarfness—with respect to the height of the stem. Because pea plants are self-fertilising, Mendel bred them for several generations till he obtained pure varieties. The parental generation P1 has the genotype TT for a pure tall parent plant and tt for a pure dwarf plant. The two parents are homozygous with one type of gamete each, i.e. T from tall parent and t from dwarf parent. These two plants are crossed with one another.
• Mendel found that all plants were tall. He called them first filial or F1 generation seeds. The F1 generation has genetic constitution Tt. It is genotypically a hybrid and a heterozygous plant with two different alleles. Phenotypically, the plant is tall because the allele or the gene T for tallness masks the effect of its corresponding recessive gene t.
• F1 plants are self-pollinated to obtain the F2 generation.
• The second filial generation F2 has a genotypic ratio of 1 TT:2 Tt:1 tt. In this case, because the allele T for tallness is dominant, the pea plants with genotype Tt will be tall. The phenotypic ratio is 3 tall:1 dwarf. Genotypically, it shows 3 types of plants: 1 TT, which is homozygous tall, 2 Tt which are heterozygous tall and 1 tt which is homozygous dwarf. Thus, the genotypic ratio is 1 TT:2 Tt:1 tt.