Mendelian Genetics_6 PDF

@R.G., Hetauda/XII/Unit 2: Genetics/1 2.2 Mendelian genetics_6 General terminology, Mendel’s experiment and laws of inheritance, gene interactions (incomplete dominance, co-dominance). Some important terms a. Gene: It is a small segment of DNA, which determines a par...

@R.G., Hetauda/XII/Unit 2: Genetics/1 2.2 Mendelian genetics_6 General terminology, Mendel’s experiment and laws of inheritance, gene interactions (incomplete dominance, co-dominance). Some important terms a. Gene: It is a small segment of DNA, which determines a particular character of an organism. b. Chromosome: Chromosomes are thread like molecules found in the nucleus which carry hereditary information. c. Allele: One of the alternative forms of a gene. It is the particular form of gene. Eg: T and t are two allele of the gene for plant height. d. Genotype: It is the genetic composition of an organism, i.e. the combination of alleles it possesses. e. Phenotype: It is the external appearance or observable characteristics of an organism. f. Homozygous: The individuals with two identical alleles of the same gene are called homozygous. E.g TT (Pure tall) and tt(dwarf) g. Heterozygous: The individuals with two different alleles of the same gene are known as heterozygous. E.g Tt (Hybrid tall) h. Clone: Clones are organisms that are the exact genetic copies from their parents. i. Dominant and recessive: In heterozygous form, two contrasting genes are found. One of the gene or allele is able to express and other remains hidden. The gene which expresses is called dominant; and which unable to expresses is called recessive. j. Back-cross: The cross of F1 hybrid with either of the two parents is known as back cross. k. Test cross: The unknown genotype is crossed with homozygous recessive parents is known as test cross. l. Hybridization The mating or crossing of two plants or lines of dissimilar genotype are known as hybridization. Mendel’s experiment Gregor Johann Mendel (1822-1884) is considered as the pioneer in the field of genetics and called as ‘the father of genetics’. His conclusion constitutes the foundations of Modern Genetics. He concluded hybridization experiments on fruits, trees, flowers, vegetables, and more on garden peas. Selection of material The reason for selecting garden pea plant for his hybridization experiments, 1. Its life cycle is comparatively short. 2. Plants are annual and easy to cultivate. 3. It has many distinct, well-defined and easily observable morphological characters(traits). 4. Flowers are bisexual and naturally self-fertilizing, and also can be easily cross-fertilized. 5. The offspring of cross-fertilized plants are fertile.  @R.G., Hetauda/XII/Unit 2: Genetics/2 Seven pairs of contrasting characters in pea plant S. No. Character Dominant Recessive (1) Stem length Tall Dwarf (2) Seed shape Round Wrinkled (3) Seed colour Yellow Green (4) Pod shape Inflated Constricted (5) Pod colour Green Yellow (6) Flower colour Violet White (7) Flower position Axial Terminal Mendel's experiments 1. Monohybrid Cross: (Experiments for single pair of contrasting characters.) The cross which involves a single pair of contrasting characters is called monohybrid cross. For this Mendel used one of the seven pairs of contrasting traits. eg cross between pure tall plant (TT) and pure dwarf plant (tt) as parents. Only hybrid tall (Tt) plants are produced in F1- generation. F1 generation were allowed self pollination, both tall and dwarf plants were produced in F2 generation, the ratio 3:1, this is called phenotypic ratio and 1:2:1, this is called genotypic ratio. 2. Dihybrid Cross: (Crosses involving two pairs of contrasting traits) The cross which involves two pairs of contrasting characters is called dihybrid cross. For this Mendel used two pairs out of the seven pairs of contrasting traits. He crossed yellow round seed (YYRR) with green wrinkle (yyrr). In F1 –generation all hybrid appear yellow with round seeds (YyRr). During gametogenesis, these alleles are assorted independently to produced 4 types of gametes i.e. YR, Yr, yR, yr. these gametes unite randomly in the process of fertilization and produce four types of off-springs in F2- generation. They show 9:3:3:1 ratio.  @R.G., Hetauda/XII/Unit 2: Genetics/3 Mendel’s law of Inheritance 1] Law of dominance: It states that “When two pure plants with contrasting characters are crossed, only one form of the character appears in F1 generation, the other remains unexpressed”. The character which appears itself in F1 generation is called dominant; the alternative factor that fails to show itself in F1 generation is called recessive. 2] 1st law of inheritance or law of segregation or law of purity of gametes: It states that, “the characteristics of an organism are determined by internal factor which occurs in pairs, only one pair of such factor can be represented in a single gamete that separates at the time of gametogenesis”. It is carried out only in monohybrid cross. Phenotypic ratio: 3:1 Genotypic ratio: 1:2:1  @R.G., Hetauda/XII/Unit 2: Genetics/4 3] 2nd law of inheritance or law of Independent Assortment: It states that “when two pairs of independent alleles are brought together in the hybrid (F1), they, at the time of gamete formation, segregate or assort independent at random and freely”. This shows that genes are independently influence each other. Mendel formulated this law from the result of dihybrid cross. Yellow round seeds 9 Yellow wrinkled seed 3 Green round seeds 3 Green wrinkled seeds 1 Phenotypic ratio: 9:3:3:1 Genotypic ratio: 1:2:1:2:4:2: 1:2:1  @R.G., Hetauda/XII/Unit 2: Genetics/5 Patterns of inheritance Mendel’s laws do not apply universally and number of exception to them. 1. Incomplete dominance: the law of dominance is not always found to be correct. In many case, complete dominant is absent. In incomplete dominance, F1 phenotype is intermediate between the parental traits. In this condition dominant alleles cannot express fully. Example: Incomplete dominance is found in 4’O-clock plant (Mirabilis jalapa). When red flower plant (RR) crossed with white flower plant (rr), all F1 progenies have pink colour. When these F1-pink flowered plants are self pollinated to rise F2- generation, they produce red (RR), pink (Rr) and white (rr) flower plants giving 1:2:1 ratio. The phenotypic ratio is identical with genotypic ratio, such types of crossed show the incomplete dominance. 2. Co-dominance: In co-dominance, the genes of an allelomorphic pair are related as dominant and recessive, but both of them express themselves equally in F1 hybrid. It means that both the alleles are equally dominant and express equally in phenotype such condition is known as co- dominance. Example: blood group AB. Multiple alleles: Single character may be control by more than two alleles. It is known as multiple alleles. For example blood group in human.