Non-Mendelian Genetics PDF

Non-Mendelian Genetics Non-Mendelian Genetics Why non-mendelian genetics? Some characters don’t follow Mendel’s law. In those cases where Mendel's proposals are not valid and the expected phenotype ratio differ. Inheritance patterns are not-distributed in accordance with Mendel's rule...

Non-Mendelian Genetics Non-Mendelian Genetics Why non-mendelian genetics? Some characters don’t follow Mendel’s law. In those cases where Mendel's proposals are not valid and the expected phenotype ratio differ. Inheritance patterns are not-distributed in accordance with Mendel's rules. The expected phenotypic ratios in these cases where Mendel's proposals are not applicable: 1. Multiple alleles 2. Lethal genotypes 3. Incomplete dominance 4. Co-dominance 5. Penetrance 6. Expressivity 7. Pleiotropy 8. Epistasis Mendel’s law Multiple alleles Multiple alleles: having more than two alleles that are possible to code for any one characteristic. Lethal genotypes Dominant lethal genes? Recessive lethal genes? Which can be detected in the population??? Incomplete Dominance Incomplete dominance is the blending of traits expressed by the alleles that combine for any given characteristic. Co-dominance Instead of creating a new feature, in co-dominance, both alleles are equally expressed and their features are both seen in the phenotype. Sex-linked Traits Sex-linked traits are found on the sex chromosomes of the species and are passed down in that manner. Sex-limited Traits Sex-limited genes are genes that are present in both sexes of sexually reproducing species but are expressed in only one sex and remain 'turned off' in the other. In other words, sex-limited genes cause the two sexes to show different traits or phenotypes, despite having the same genotype. These traits are carried on autosomal chromosomes. Sex-influenced Traits These characters are expressed differently in males and females. Penetrance Penetrance : the percentage of individuals expressing the character for a particular genotype. Types: Complete penetrance and incomplete penetrance. Complete penetrance: If all the individuals express the character for a particular genotype. In complete penetrance, the character is expressed in 100% individuals, eg. Mendel’s tall and dwarf plants. TT: tall character; Tt: tall character; tt: Dwarf character Incomplete penetrance: If a few individuals do not express the character even though they contain the necessary genes eg. Blue eye The genes for blue eyes BB produce blue eyes only in 90% human beings. About 10% people have white eyes even though they contain the genes BB for blue eyes. Penetrance is influenced by environmental factors such as food, light, temperature etc., Expressivity The variation in the degree of expression of a particular gene is called expressivity. A particular gene may produce varying degrees of expression in different individuals. Expressivity is due to the influence of environmental factors on the genes. Eg. Vestigial wing in Drosophila vv, recessive flies, 100 % expression It shows variation in the degree of expression Pleiotropy Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. Therefore, a mutation in a pleiotropic gene may have an effect on several traits  PHENYLKETONURIA The genes pp for Phenylketonuria produce accumulation of phenylalanine in the blood. In addition, it produces many other characters such as mental retardation, widely spaced incisors, pigmented patches on skin, excessive sweating, non-pigmented hairs and eyes etc., Similarly, the Potato mutant gene suppresses the growth of meristematic tissue, axillary shoot and petals. It produces apocarpous pistil and dialatory anthers. MODIFIED DIHYBRID CROSS RATIOS Complementary gene interaction: 9:7 Supplementary gene interaction: 9:3:4 (Recessive epistasis) Epistasis: 12:3:1 Duplicate gene interaction: 15:1 THANK YOU