Genetics and Plant breeding Pointers 25 ✓

 

• 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.