The grid method of mass selection was suggested by Gardner in 1962. Crosses between lines from different groups show greater levels of heterosis than crosses within groups. This is the most widely accepted hypothesis of heterosis. In Plants, Heterosis appears due to Developmental stimulation induced by the Union of Gametes coming from Two genetically complementing parents. Lower Birthrates Higher Infant Mortality Slower Growth rate.
Effects or Manifestations of Heterosis: Whatever may be the cause genetical or physiological , heterosis is a well known phenomenon. Fixation of hybrid vigour is needed for its commercial application which can be done by vegetative propagation, or by stable apomictic reproduction, or by transferring heterozygosity to polyploid or fixation by obtaining structural heterozygotes. This is termed as mutational heterosis. For example, in wheat higher heterosis is associated with crosses of more distantly related parents. Royal Family of Egypt Cleopatra was famous for inbreeding between Brothers and Sisters. Greater uptake followed by better utilisation of nutrients by hybrids. They are always high disease.
A hybrid between these two parents will have four dominant genes and exhibit superiority over both the parents which have two dominant genes each. It brings out the Superiority in F1 individuals but the vigour tends to Decrease from F2 generation onwards. Fresh seeds is to be2. What is negative mass selection? Optimal amount Models of gene to gene product interactions Eg. This theory is called by various names such as stimulation of heterozygosis, cumulative action of divergene alleles, single gene heterosis, super dominance and over dominance.
Current Category » Effects and Use of Heterosis in Plant Breeding Manifestation Effects of Heterosis: 1 Increase Yield: Heterosis is generally expressed as an increase in the yield of hybrid and may be measured in terms of grain, fruit, seed, leaf, tubers, etc. The following modifications of mass selection have been suggested by various workers: i Rejection of inferior pollen parents, ii Use of composite pollen for pollinating selected plants, iii Gardner 1962 suggested stratified method of mass selection in which field is divided into small; plats and 40-50 superior plants are selected in each plot. What is positive mass selection? Over dominance hypothesis of heterosis. They produce goods Purchased every with Uniform size. 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. Plant material is collected by following methods ways : i Through expeditions ii Through personal visit iii Through correspondence iv By exchange of material v By purchase, and vi As a gift. Dicliny — It refers to the unisexual flower.
Thus heterosis is directly proportional to the heterozygosis. In a heterozygous plant a 1, a 2 a combination of the effects is produced which is more favourable in the plant than the effect produced by either of the alleles alone. Fruits: In almost all the fruit trees. Fisher 1930 called it superdominance. It is also called as economic heterosis.
Types of Heterosis : Heterosis is of two types: True heterosis euheterosis and pseudo-heterosis. Heterosis or hybrid vigour have been commercially utilised in both cross pollinated and in some self-pollinated species. All these manifestations of heterosis can be traced at all levels of hybrid plant organisation. It is of two types, viz. This heterotic effect is called as mitochondrial complementation. Purelines have pure adaptability due to narrow genetic base. Improving the performance of various animals i.
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. Main features of purelines are given below: i Purelines are homozygous and homogeneous. A Dominance Hypothesis : This hypothesis was proposed by Davenport and further expanded by others. Fixation of heterosis in crops like potato, sweet potato, sugarcane, sugarbeet and many ornamental plants can be achieved by vegetative propagation as there seeds are not essential. The concept of pureline theory was developed by Johannsen in 1926 working with Princess Variety of common bean Phoseolus vulgaris. Cite examples of secondary introduction.
On the basis of his experiments Beal 1877-1882 concluded that F 1 hybrids yield as much as 40 percent more of the parental varieties. . Genetic Consequences of self-pollinated- 13 PowerPoint Presentation: 1 Bisexuality- Presences of male and female organs in the same flower. This phenomenon is commercially used to produce hybrid or synthetic varieties, which needs the maximisation of its expression and also fixation. Groups may pre-exist but even in their absence they can be selected for. Main sources of variation in a pureline variety are given below: i Natural outcrossing ii Spontaneous mutations and iii Mechanical admixtures.
According to this hypothesis is the result of superiority of heterozygote over its both homozygous parents. In these cases due to presence of divergent alleles the hybrids have the capacity to perform different functions which is not possible by any of the heterozygotes. When they are combined together; i. It is also called luxuriance. Use of Heterosis in Plant Breeding: Heterosis is exploited through the development of hybrid. To explain all these processes different hypotheses have been put forwarded: 1.
Advantages of pureline selection are as follows: i It helps in isolating best genotypes for disease resistance, insect resistance, yield, quality- earliness etc. Why clones are more prone to new race of a pathogen? It has practical importance in plant breeding. They are listed below: i Plant introduction ii Pureline selection iii Mass selection iv Progeny selection v Synthetic and composite breeding etc. Progeny possess superiority over parents is in vegetative growth, but not in yield and adaptation, usually sterile or poorly fertile. The term clone was first used by Webber in 1903. Heterosis is due to masking of deleterious recessive alleles by dominant alleles 4.