Principles of Importance breeding of Cross Pollinated Crops

Principles and Importance of breeding of Cross Pollinated Crops

    The most important methods of breeding cross-pollinated species are
    (1) mass selection;
    (2) development of F1hybrids and
    (3) development of synthetic varieties.
    • Since cross-pollinated species are naturally hybrid (heterozygous) for many traits and lose vigour as they become purebred (homozygous), a goal of each of these breeding methods is to preserve or restore heterozygosity.Hardy–Weinberg principle
    • The Hardy–Weinberg principle (also known by a variety of names: HWP, Hardy–Weinberg equilibrium, Hardy–Weinberg Theorem, HWE, or Hardy–Weinberg law) states that both allele and genotype frequencies in a population remain constant—that is, they are in equilibrium—from generation to generation unless specific disturbing influences are introduced.
    • Those disturbing influences include non-random mating, mutations, selection, limited population size, "overlapping generations", random genetic drift, gene flow and meiotic drive.
    • It is important to understand that outside the lab, one or more of these "disturbing influences" are always in effect. That is,

    Hardy–Weinberg equilibrium is impossible in nature.

    • Static allele frequencies in a population across generations assume: random mating, no mutation (the alleles don't change), no migration or emigration (no exchange of alleles between populations), infinitely large population size, and no selective pressure for or against any traits.
    • The Hardy-Weinberg model, named after the two scientists that derived it in the early part of this century, describes and predicts genotype and allele frequencies in a non-evolving population.
    • The model has five basic assumptions
    • The population is large (i.e., there is no genetic drift);
    • There is no gene flow between populations, from migration or transfer of gametes;
    • Mutations are negligible;
    • Individuals are mating randomly; and
    • Natural selection is not operating on the population. Given these assumptions, a population's genotype and allele frequencies will remain unchanged over successive generations, and the population is said to be in Hardy-Weinberg equilibrium. The Hardy-Weinberg model can also be applied to the genotype frequency of a single gene


    • The Hardy-Weinberg model enables us to compare a population's actual genetic structure over time with the genetic structure we would expect if the population were in Hardy-Weinberg equilibrium (i.e., not evolving).
    • If genotype frequencies differ from those we would expect under equilibrium, we can assume that one or more of the model's assumptions are being violated, and attempt to determine which one(s).
    • How do we use the Hardy-Weinberg model to predict genotype and allele frequencies? What does the model tell us about the genetic structure of a population?

Last modified: Monday, 2 April 2012, 9:19 PM