Mechanism of Heredity
The mechanism of heredity is dependent upon the behaviour of chromosome and the gene they carry some facts regarding characters.
I) A mixed population of plant species having heredity variation is used by the breeder to select plants with traits or characters for development of improved variety. E.g. Seed colours, size, plant height, leaf size, shape, disease resistant etc. Heritable variation results when different plants exhibit contrasting form of these characters. The contrasting traits are determined by alternative form of gene and their interaction.
II) The genes are located on the chromosome and are determine of the characters of a plant. These genes are having specific position on the chromosome and are duplicated when the chromosome divide.
The alternative form of a gene called alleles, which determine contrasting form of characters. The genes may be dominant or recessive (R or r). The plant with similar identical genes at a given locus on homologous chromosome are said to be homozygous and with unlike genes are said to be heterozygous. The appearance of plant is phenotype, while genetic constitution is genotype. The chromosomes are red shape body present in a nucleus of cell. They carry genes hence important in heredity. A chromosome occurs singly (haploid) in spores and in pairs (diploid) in body cell. The chromosome number is constant in any species and it divides longitudinally during mitosis and homologue chromosome separate during moysis. The mechanism of heredity can be explained with a single gene characters.
Phenotype: Red X White
Genotype: RR X rr
Gametes: R X r
F1: Rr – Red (dominant and heterozygous)
F2: F1 X F1
Genotype: Rr X Rr
Gametes: R r R r
F2: RR Rr Rr rr
Genotypic Ratio: 1:2:1
Phenotypic Ratio: 3:1
The tendency of genes to be inherited in a group on to the next generation is known as linkage which occurs due to the residence of genes on the same chromosome. The string of gene in a chromosome is linkage group. The number of linkage group in any species is equal to the pairs of chromosome. If the genes are completely linked on a chromosome, there would be no combination of genes within the same linkage group, and due to this there would be certain restriction on breeders to obtain new recombination of linked genes.
Recombination of linked genes occurs due to crossing over (Exchange of segment between nonsister chromatides of homologous chromosome). A breeder may select plants with recombination of linked genes for desirable characters.
Linkage may be coupling phase or repulsion phase. In coupling phase two dominant genes are linked with recessive gene (AB/ab). In repulsion phase one dominant and recessive genes are linked with one recessive and dominant genes ( Ab/aB).
IV) Gene Interaction:
The phenomenon of two or more gene affecting the expression of each other in a various ways for the development of a single character of an organism is known as gene interaction or it is the phenomenon in which two pairs of nonalleic gene affect the same character. In gene interaction when two dominant genes come together, they usually produce different phenotype than their own and it result into the modification of normal F2 phenotypic ratio. Gene interaction effect the expression of one character of an individual. It adds variability by producing new phenotype.
V) Heterosis or Hybrid Vigour:
When two homozygous inbred of genet cally unlike constitution are crossed together the resulting hybrid are usually vigorous, productive taller and sturdier than either parents. This increased productivity or superiority of the hybrid over the parents is known as heterosis or hybrid vigour. The term heterosis was coin by G.H. shull (1914). He derived from the Greek word heterosis mean different and Osis means condition, therefore , literally means a different condition.
VI) Polyploidy and Plant Breeding:
Polyploidy is the condition in which an organism having more than two sets of chromosome or genomes in their somatic cell. In contrast to the normal of ploid they may be triploid (3n), tetraploid (4n) , penataploid ( Sn) and so on. Polyploids may be Euploids ( An organism having the exact multiple of the basic chromosome number in 3n, 4n, etc) or Aneuploid ( An organism having unequal number of chromosome i.e monosomic ( 2n-1), nullisomic ( 2n-1) , Trisomic ( 2n +1) etc.
When polyploidy are developed from two or more sp by crossing known as allopolyploids. They are sterile but it can be overcome by doubling the chromosome number by colchicines treatments. Ex. Triticale Wheat.
Polyploidy is of special significance in plant breeding because it adds genetic diversity in the plant kingdom. It increases the complexity of genetic ratios . E.g hexaploid wheat. Therefore, it has an important factor in the evolution of plant species.
VII) Male Sterile and Self- incompatibility:
It refers to the absence of functional male gametes (Pollen grains), while female gametes functional gametes functional normally. The ability to set seed after self or cross pollination within the species ore genera must be known to breeder. Sterility is the inability to obtain seed set due to either failure of pollen grain or ovues. Male sterility is recorded in many crops in which, male sexual organ (Stamens) are malformed or aborted and non-viable pollen grains are produced.
The male sterility may be occurs due to gene, cytoplasm or combination of both. On the basis of this, it is classified in to genetic male sterility, cytoplasmic male sterility and genetic cytoplasmic male sterility.
Self – incompatibility:
It refers to the failure of pollen grain to fertilize the same flower or other flower on the same plants. In this case both pollen and ovule are functional, but they could not fertilize the flower and set seeds due to some physiological hindrances. Incompatibility may occur due to the lot of season.
1) Pollen grain fails to germinate on the stigma.
2) Pollen grain germinates but the pollen tube fails to enter the stigma.
3) Some times pollen tube enters the style but growth is very slow to effect fertilization.
4) Pollen tube enters the ovule but there is no fertilization due to degeneration of egg cell.
5) Fertilization is effected but embryo degenerate at very early stage.