Qualitative traits vs Quantitative traits
| Qualitative traits | Quantitative traits |
| Traits can be grouped | Traits can be measured |
| Determine by few gene | Determine by many genes |
| Each gene has a large contribution on traits | Each gene has small contribution on phenotype |
| Expression of the trait does not influence by the environment | Expression of the trait influenced by the environment |
| Discrete variation | Continuous variation |
Factors that plant breeders need to know about continuous variations.
- Heritability – Ratio of the genetic variance to phenotypic variance. for qualitative traits heritability is 100%, for quantitative traits heritability varies between 0% to 100%. if the heritability is higher that plants can be used for breeding programs to change the mean expression of characters of the next generations. IF heritability is close to zero there is no point to use for improve the mean expression of characters.
- Selection – it is the process of the separation of the best individuals for the heterogeneous unimproved population. Plant breeders always interest in extreme value of the distributions. most of the these extreme values have improved quantitative traits. Therefore with selection population mean get improve because only the best individuals contributed to next generation. Mainly there are two different selection types.
Intense selection – Small proportion of individuals are selected for breeding therefore it has a high level of selection pressure. rapid genetic improvement of that trait. however it leads to loss of the genetic diversity and reduce the overall fitness.
Mild selection – large number of individuals are selected for the breeding therefore it has a low level of selection pressure. slow genetic improvement for desired traits. However this reduce the loss of genetic diversity and reduce the risk of inbreeding depression.
- Environment factors – temperatures climate , nutrients, diseases and abiotic stress also affect the expression of the traits.
- Interaction between traits – selected plants have high yield producing characters however it also have susceptible to abiotic stress.