In this article we will discuss about how to select cross and self pollinated crops.
How to Select Cross Pollinated Crops?:
Cross pollinated crops are highly heterozygous due to free intermating of plants. Such crops are called random mating populations due to equal opportunity of mating in each individual. Random mating population is also called Mendelian population or Panmictic population.
To understand the genetic make-up of such population we need population genetics.
Fundamental law of population genetics is the Hardy- Weinberg law which provides the basis to study mendelian populations. This law was independently developed by Hardy (1908) in England and Weinberg (1909) in Germany.
According to Hardy-Weinberg law- “The gene and genotype frequencies in a Mendelian population remain constant generation after generation if there is no selection, mutation, migration or random drift”.
The frequencies of the three genotypes for a locus with two alleles (i.e., A & a) i.e., genotypic frequency or zygotic frequency (genotypic/zygotic frequency is the proportion of a genotype AA, Aa or aa in the population).
Such a population would be at equilibrium since genotypic frequencies would be stable i.e. would not change from one generation to the next. This equilibrium is called Hardy-Weinberg equilibrium A population is said to be at equilibrium when the frequencies of three genotypes AA, Aa & aa are p2, 2 pq and q2 respectively, whether a population is at equilibrium or not can be easily determined by a chi-square test.
Selection in Cross-Pollinated Species:
a) Increases the frequency of desirable alleles and genotypes.
b) Leads to production of new genotypes.
c) Changes the mean in the direction of selection.
d) May or may not reduce genetic variance.
Five different types of responses to selection:
i) Rapid gain followed by a period of slow gain is seen in characters governed by few major genes and several minor genes.
ii) Slow gain for a long time is the characteristic of traits governed by polygenes. The contribution of each gene is small but the heritability is high.
iii) Slow gain ending in a plateau, is found in such quantitative characters where the change due to selection beyond a limit is prevented by physiological limitations.
iv) No gain or little gain in such quantitative characters which have low heritability e.g., yield. The failure of selection is primarily due to low heritability.
v) Rapid gain – plateau – rapid gain, results from release of potential variability.
How to Select Self Pollinated Crops?:
Selection is essentially based on phenotype of the plants. The effectiveness of selection primarily depends upon the degree to which the phenotype reflects the genotype.
Basic Characteristics of Selection:
(1) Selection is effective only for heritable differences.
(2) Selection does not create new variation.
It means there are two requirements of selection:
(i) Variation must be present in the population.
(ii) Variation must be heritable.
Purpose of selection: to isolate desirable plant types from the population.
Two basic steps of any breeding programme:
(i) Creation of variation.
(ii) Selection.
Progeny Test:
The value of the plant is judged by the performance of its progeny. Evaluation of the plant on the basis of preformation of its progeny is known as progeny test. This test was developed by Louis de Vilmorin. Therefore it is also known as Vilmorin principle or Vilmorin Isolation Principle.
According to Vilmorin:
The real value of a plant can be known only by studying the progeny produced by it.
Two valuable functions of progeny test:
(i) To determine the breeding behaviour of a plant i.e. whether the plant is homozygous or heterozygous.
(ii) To find out whether the character for which the plant was selected is heritable. This is the most important function.
Effects of Self-Pollination:
Self-pollination increases homozygosity.
Self-pollination decreases heterozygosity.
Inbreeding:
Mating between individuals related by descent i.e., having a common parent or parents is called inbreeding.
Sibmating means brother-sister mating. Half sibmating means brother-stepsister mating. Self-pollination is the most intense form of inbreeding.
Homozygosity (i.e., frequency of genes in homozygous condition in the population) = Frequency of homozygotes =
Here homozyogosity is not affected by the number of segregating genes, and also by the linkage.
In a case of single gene, the frequency of homozygous plants in a generation is the same as the proportion of homozygosity. But when two or more genes are segregating, the proportion of homozygosity increases at a much faster rate than that of completely homozygous plants.
The proportion of completely homozygous plants:
When the no. of segregating genes increases the proportion of completely homozygous plants decreases. But selfing (Self- pollination) is so strong that even if 100 genes are segregating, more than 95% of population would be completely homozygous after only 12 generations.
(i) All the plants in the population become completely homozygous.
(ii) Population is a mixture of several homozygous genotypes.
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