Genetic variability can also be created by sexual hybridization, mutagenesis, polyploidy, somaclonal variation, somatic hybridization through protoplast fusion, genetic engineering etc.
Transgressive segregation is possible in - Polygenic trait
Phenomenon through which we get variation in F2 or later generation outside the range of both the parents is called as Transgression or transgressive segregation.
Transgressive segregants are only possible from the cross between two parents with mean values for a quantitative trait
Transgressive segregation is the production of plants in F, generation that are superior to both the parents for one or more characters
Quantitative character are expressed in terms of Degree rather than kind
A gene, which has major effect on the expression of a trait and whose effect is not substantially affected by the environment is called as oligogene.
One or few genes governing the same character - oligogene
In general, selection will be easy in case of Monogenic characters
Several gene governing the same character - Polygene
Genes that have small and usually additive effects on the expression of a trait and whose effect is strictly affected by the environment is called as - Polygenes
Polygenic characters show variable magnitudes of heritability and environment plays an important role in the development of such traits
Polygenic character shows continuous variation hence classification into clear-cut groups is not possible.
Oligogenic character show discontinuous variation hence classification into clear-cut groups is possible.
The linkage among polygenes leads to slow release of potential variability Major genic traits are also known as - Qualitative character
Minor genic traits are also known as - Quantitative character
A trait that shows some features of qualitative and other features of quantitative trait is known as - Quasi-quantitative trait.
Transfer of a monogenic trait through backcross procedure is much easier as compared to that of a polygenic trait.
During the transfer of a polygenic trait through backcross method, F, and F, generations are raised after each backcross generations.
AAbb x aaBB crosses will produce transgressive segregation in the F2 generation
The appearance in F2 individuals with a higher or lower intensity of character than those present in the parents involved in the cross is called as-Transgressive segregation
Polygenes have small and cumulative effects on the characters.
When magnitude of effect of the environment on phenotype increases, the phenotypic classes progressively overlap each other and form continuous variation
Phenotype is the result of a joint action of genotype and environment
Total variation present in a population arises due to genotypic and environmental effects
The sum of genetic and environmental variances gives an estimate of the phenotypic variance.
The continuous variation in quantitative characters are due to the cumulative effect of several genes and marked effect of environment on the expression of the characters
According to Mendel, plant height is qualitative characters, but usually plant height is quantitative characters and highly sensitive to the environmental factors
Mendel proposed the fundamental laws of inheritance based on his studies on qualitative characters
Yule first suggested that, genes controlling quantitative traits are transmitted according to the laws of Mendel
Johannsen first suggested that the development of a character is influenced by both genotype and environment
Inheritance of both qualitative and quantitative characters follows the laws of segregation and independent assortment.
The ability of a gene to express itself in all the individuals carrying it in the appropriate genotype is called as - Complete penetrance.
The ability of a gene which does not express itself in all the individuals carrying it in the appropriate genotypes is called as penetrance Incomplete.
Variable degrees of expression of an oligogene in different individuals having it in the appropriate genotypes is called as – Variable expressivity
The ability of a gene to express itself uniformly in all the individuals that carry it in the appropriate genotypes is called as - Uniform expressivity
The variance in P1 P2 and F1 due to - Environment only.
The variance in F2 due to - Both genetic and environmental components.
Using replicated data from a trial conducted at one location, the phenotypic, genotypic and environmental variance can be estimated
G x E interaction - Study of differential performance of genotypes over environments
Quantitative or non-crossover G x E interaction - Ranking of varieties does not change over the environments (only based on differential response of genotypes)
Qualitative or crossover GE interaction - Ranking of genotypes changes with the environment
In case of most of the quantitative characters, the contribution of environment varies from 10 to 50 percent
The F2 variance has both genotypic and environmental components
Incomplete penetrance is the result of environment effects
Incomplete penetrance and variable expressivity are consequences of the effects of environmental factors on the expression of concerned genes.
When a gene influences the expression of more than one character is called as Pleiotropy
A gene that affects more than one trait is known as - Pleiotropic gene
Over-dominance and pleiotropy are the chief cause of heterozygotes for sickle cell anaemia are more resistant to malaria than the two homozygotes
In 1911, Morgan discovered the phenomenon of linkage.
Linkage between two dominant genes - Coupling phase
When one dominant and one recessive gene are linked together Repulsion phase.
Pleiotropy may arise due to the pleiotropic gene action or it may due to tight linkage
Production of new combinations of genes as a result of independent assortment of unlinked genes, crossing over between linked genes or intragenic crossing over – Recombination.
The F2 and subsequent generations obtained through continued selfing of a hybrid is known as - Segregating generations
The segregation of one pair of alleles being independent of that of another pair of alleles is called as - Independent assortment
Independent assortment of genes is based on random assortment of non- homologous chromosomes
In case of incomplete dominance, genotype is readily known from the phenotype.
A single gene with variable expressivity may produce a number of phenotypes as if more than one gene were involved in the control of the trait.
Xeroderma pigmentosum, disease resistance, allergy etc. are examples of threshold character
Multiple factor hypothesis deals with a character are governed by several genes that have small and cumulative effect; give rise to polygenic inheritance.
Number of genes involved in the control of quantitative traits can be estimated by two methods: 1. Castle-Wright segregation index given by Wright in 1934.
2. Inbred-backcross and genotype assay method.
Coefficient of variation does not have unit.
A small circle which depicts the position of a genotype on the graph in metroglyph analysis – Glyph.
The rays on the graph in metroglyph analysis indicates - Variation in various character.
A line diagram which is drown in D2 statistics to depict genetic diversity for various characters - Cluster diagram.
Analysis of D2 statistics is difficult as compared to metroglyph technique.
Burton 1952 divided polygenic variation into phenotypic, genotypic and environment components.
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