Poweri 1944, 45 reported that the crossing, however, may result in either weak or vigorous hybrids as compared to parental inbreeds. Other genetic factors can augment the magnitude of heterosis. Greater Adaptability: Hybrids are generally more adapted to environmental changes than the inbred lines due to heterozygosity. These Parents differe in Only one Character. It prevents self-pollination and promote cross-pollination.
This is the most widely accepted hypothesis of heterosis. Genetic Causes: There are two possible causes of heterosis viz. What are means of Plant Introduction? He suggested that there may be a linkage between some favourable dominant genes and some un-favourable recessive genes and as a result it is not possible to obtain true breeding homozygous individual for all dominant genes in F 2 generation. . Greater uptake followed by better utilisation of nutrients by hybrids. What are purposes of plant introduction? In another explanation it can be showed that if the number of genes governing the quantitative characters is large, symmetrical distribution would be obtained even without linkage. These varieties were released after selection from the material received from Mexico.
Selection which operates in nature without human interference is called natural selection. Introduction that can be used for commercial cultivation without any change in the original genotype is called primary introduction. Thus, there are three possible genetic causes of heterosis viz. Each allele produces favourable yet different effects in the plant. Perhaps no more than a 1% improvement could be looked for from this cause. Use of Heterosis in Plant Breeding: Heterosis is exploited through the development of hybrid. Various, important crops such as maize, potato, tomato, brinjal, groundnut, papaya, pineapple, triticale etc.
According to him trait like yield is governed by large number of genes or polygenes, which exhibit continuous variation resulting in symmetrical distribution of genes. A hybrid between these two parents will have four dominant genes and exhibit superiority over both the parents which have two dominant genes each. Those breeding methods that are rarely or less commonly used for crop improvement are known as special breeding methods. Fruits: In almost all the fruit trees. If new classes of genetic and epi-genetic variation contribute to trait variation and also show dominance, their contribution to heterosis is assured. Lower Birthrates Higher Infant Mortality Slower Growth rate. Both the parents differ for dominant genes.
More branches per panicle and more grains per branch. He suggested that there may be linkage between some favourable dominant genes and some unfavourable recessive genes and as a result. Overdominance hypothesis Hull 1945, Crow 194 8 : Heterosis is caused by overdominance at many loci influencing the trait. East in 1936 further elaborated this theory by proposing a series of alleles a1,a2,a3,a4 ----- of gradually increasing divergence in function. It may be either morphological or physiological in nature.
According to this hypothesis, heterosis is the result of the superiority of dominant alleles, when recessive alleles are deleterious; here the deterious recessive genes of one parent are hidden by the dominant genes of another parent and the hybrid exhibits heterosis. Examples of primary introduction in wheat and rice are given below: i Wheat: Sonora 64 and Lerma Rojo. Examples of secondary introduction are given below: i Wheat: Kalyan sona and Sonalika. The latter are very important as they are firm-footed animals that can carry heavy loads on steep Himalayan mountain terrain. Who coined the term Selection? Organism Level: Heterosis is expressed as increased growth and differentiation of vegetative organs, synthesis and accumulation of nutritional substances and utilisation of metabolic process for yield formation.
In case of maize, the gene ma affects maturity. Such inbreeds would be as vigorous as the F 1 hybrids. Progeny possess superiority over parents is in vegetative growth, but not in yield and adaptation, usually sterile or poorly fertile. There is another objection against over-dominance hypothesis that there are many examples where the homozygotes are superior to the heterozygotes. Who developed the concept of pureline theory? Genetic Basis of Heterosis : There are two main theories to explain the genetic cause of heterosis. The epistasis is also considered to be associated with heterosis.
Main features of purelines are given below: i Purelines are homozygous and homogeneous. Individual heterosis: Heterosis is retained in the breeding of crossbred animal and is related to the probability of alleles from different parental lines joining together. The initial growth activities include the different physiological processes during germination: a Efficient water absorption, b Better activity of enzymes, c Rapid mobilization and utilization of stored food matter, d Transformation and building up of active protoplasmic synthesis. What is negative mass selection? How will you classify plant breeding methods? Epistasis can be detected or estimated by various biometrical models. Compare mass selection and pureline selection.
Dispersion of favourable alleles is common: in its absence genetic progress is not possible since progeny with better performance than parents will not be found. This objection was removed by Collins 1921. Mode of pollination The magnitude of heterosis differs depending upon the mode of pollination of a species. Estimation of Heterosis : Estimation of Heterosis Useful heterosis The term useful heterosis was used by Meredith and Bridge 1972. Introductors which are immediately adapted to the changed environment are called direct introductions. Vegetable Crops: Tomato, brinjal, onions, cucurbits, etc.