1. Many varieties were available with observable and contrasting characters.
2. Peas normally self pollinate so pure lines could be obtained for experimental purpose
3. It was easily available.
4 The life span of pea is short and it produced large number of offsprings
5. They were easy to cultivate and did not require much care.
(a) Dominance and Recessive
(b) Homozygous and heterozygous
(c) Monohybrid and dihybrid.
| Dominance | Recessive |
| 1. When the allele expresses itself in both heterozygous as well as homozygous condition. | 1. When the allele expresses itself only in homozygous condition. |
| 2. The allele is unmodified and functional. | 2. The allele is modified and non-functional. |
| Homozygous | Heterozygous |
| 1.Condition where both the alleles of the gene are same. | 1.Condition where the alleles are different. |
| 2.The gametes formed from them are same. | 2. The gametes formed from them are different. |
| eg. TT or tt | eg. Tt |
| Monohybrid | Dihybrid |
| 1. When an individual is heterozygous for genes controlling one character it is called monohybrid | 1. When an individual is heterologous for genes controlling two characters it is called dihybrid. |
| 2. In F2 generation it produces a phenotypic ratio of 3:1. | 2. In F2 generation it produces a phenotypic ratio of 9:3:3:1 |
Explain the law of dominance using a monohybrid cross.
According to this law, when two factors of a character are unlike, one of them will manifest in the body and is called dominant while the other remains hidden and is termed recessive factor.
The law can be well explained by the monohybrid cross by studying the following crosses :
(i) Pure tall = TT, Hybrid tall = Tt
The 50% are pure tall and 50% hybrid tall. Then pure tall plants will produce 100% tall in F2 generation and hybrid plants will produce in ratio of 1 : 2 : 1 in the F2 generation.
(ii) When the cross is made between pure tall and pure dwarf, we get results as follows (Fig. 1.2)
Fig. 1.1. A Punnett square used to understand a typical monohybrid cross conducted by Mendel between true-breeding tall plants and true-breeding dwarf plants
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