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MENDEL LAW’S OF HEREDITARY AND LINKAGES What is heredity The passing on of characteristics (traits) from parents to offspring Genetics is the study of heredity Mendelian Laws of Inheritance Gregor Mendel, an Austrian . monk, in 1865 carried out important studies of heredity - the passing on of characteristics from parents to offspring. Mendel was the first person to succeed in predicting how traits are transferred from one generation to the next. Mendel’s Laws of Heredity Mendel studied garden pea plants because: They reproduce sexually They have two distinct, male and female, sex cells called gametes Their traits are easy to isolate He came to three important conclusions from the experimental results: 1. That the inheritance of each trait is determined by "units" or "factors" that are passed on to descendents unchanged (these units are now called genes ) 2. That an individual inherits one such unit from each parent for each trait 3. That a trait may not show up in an individual but can still be passed on to the next generation Mendel's observations from the experiments can be summarized in THREE principles: Law Definition Law of Segregation During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene. Law of Independent Assortment Genes for different traits can segregate independently during the formation of gametes. Law of Dominance Some alleles are dominant while others are recessive; an organism with at least one dominant allele will display the effect of the dominant allele. Law of Segregation The hybrids of F1 generation have two contrasting characters (dominant and recessive nature). These characters, remain together but do not intermingle with each other and separates during gametogensis so that each gamete will have only one character i.e. dominant or recessive. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype. Ratio of the monohybrid cross is 3:1 Law of Independent Assortment Separate genes for separate traits are passed independently from parents to offspring. More precisely, the law states that alleles of different genes assort independently of one another during gamete formation Ratio of the dihybrid cross is 9:3:3:1. Law of Dominance It states that recessive alleles will always be masked by dominant alleles. Therefore, a cross between a homozygous dominant and a homozygous recessive will always express the dominant phenotype, while still having a heterozygous genotype. The one which expresses in the F1 generation is called Dominant and the one which is suppressed (not expressed) is called "recessive." Phenotype: Tall (TT) x Dwarf (tt) Gamete : T X t F1 Generation : Tt (Tall) Linkages Dihybrid cross is considered. And we assume both the genes to be: 1. Autosomal 2. One allele of each gene is dominant 3. No epistasis is involved Parental Genotypes YYRR Parental Gametes F1 Genotypes X YR yyrr yr YyRr F1 Progeny is now TEST CROSSED i.e. heterozygous parent crossed to a fully homozygous recessive plants Test cross phenotype: Test cross genotype: Gametes: Test cross progeny: Ratio Heterozygous F1 dihybrid Homozygous recessive parent YellowRound YyRr GreenWrinkled yyrr YR Yr yR yr YR Yr yr YyRr (25%) Yyrr (25%) YR yR yr yyRr ( 25%) yyrr (25%) Yellow Round Yellow Wrinkled Green Round Green Wrinkled PARENTAL COMBINATION RECOMBINANT COMBINATION RECOMBINANT COMBINATION PARENTAL COMBINATION 1 1 1 1 Now we take gene Z located on the same chromosome as gene Y. Dihybrid cross of gene Y and Z are as follows: Parental Genotypes Parental Gametes YYZZ YZ yyzz yz YyZz F1Genotypes Test cross genotype X YyZz X yyzz The expected progeny was 100% parental and no recombination. But this was only possible if the two genes were completely linked. This is a rare case. The actual result for the genes located on the same chromosome was somewhere between the result expected for complete linkage and independent assortment. i.e. greater than 50% of he progeny with parental type phenotype and less than 50% with recombinant Genetic markers located on the same chromosome thus tend to remain together during sexual reproduction. i.e. they do not exhibit independent assortment. Such genetic markers are said to be linked and phenomenon is known as linkage And genetic markers are said to be linked whenever 50% of the gametes produced contain parental combination of the markers and less than 50% of the gametes contain recombinant combinations of the markers Effect of linkage were first evident in the result of a dihybrid cross in sweet peas W.Bateson and R.C. Punnett T.H. Morgan was the first to relate linkage to the segregation of homologues chromosomes The effect of linkage was illustrated by considering the result of two crosses involving pairs of some alleles of two genes located on second chromosome of D.melanogaster Example in case of DROSOPHLIA 41% 41% Parental 9% 9% Recombinant