Lec 3 Bio 308
Lec 3 Bio 308 Bio 308
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This 4 page Class Notes was uploaded by Kiara Reyes on Sunday September 4, 2016. The Class Notes belongs to Bio 308 at Northern Illinois University taught by M. Johns in Summer 2016. Since its upload, it has received 23 views. For similar materials see Genetics in Biology at Northern Illinois University.
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Date Created: 09/04/16
Lecture 3: Extension to Mendel’s Genetic Model Partial (Incomplete) Dominance Partial Dominance- heterozygous has phenotype intermediate between the two homozygous Co-Dominance Co-Dominance- both parental alleles are expressed in the heterozygous Ex] Blood Groups. A glycoprotein antigen on the surface of red blood cells cause them to clot in the presence of the presence of the corresponding antibody Electrophoresis Electrophoresis- way of separating DNA or proteins on the basis of how fast they move in an electric field The speed the molecules move in proportional to their charge and inversely proportional to their size: small, highly charged molecules move faster then large, less charged molecules Ex] consider hemoglobin, the oxygen-carrying protein in blood. Hemoglobin has 4 polypeptide subunits: 2 alpha and 2 beta Multiple Alleles Mendel used only 2 alleles per gene. Any change in the DNA sequence of a gene is different allele, millions of possible alleles of any gene Polymorphic- gene have several common alleles; several different forms Monomorphic- genes are constrained by natural selection to have only a single allele Complete- heterozygous looks like dominant Genes with multiple alleles can have a variety of dominance patterns between the alleles “ABO Blood Group” ABO blood group is a common multiple allele system. The gene are called I having 3 alleles: Major Histocompatibility Locus (MHC) Major Histocompatibility Locus (MHC)- primary determinant of human tissue type, which determines whether organs can be transplanted between people without rejection by the immune system Consist of 6 major genes lying close together on one chromosome. These genes are usually inherited as a single unit called haplotype. Taken together, MHC genes are probably the most polymorphic region of the human genome Lethal Alleles Many alleles cause genetic diseases are called “dominant” because heterozygous are affected Ex] Achondroplasia- form of dwarfism Manx Cat- cat doesn’t have tail Genes would be better described as partially dominant because the heterozygous are quite different from the heterozygous: homozygous are lethal Lethal alleles give an unusual inheritance ratio Note: as time passes, the dominate TT slowly dies off, leaving the partial and recessive alleles to break and build the missing part “Phenotype Ratio for Multiple Genes” To calculate the ratio offspring by using the Fork-line Approach: 1) Determine the ratio of phenotype for each gene separately. These ratios depend on the type of dominance as well as the genotype of the 2 parents 2) Draw a fork-line diagram of the offspring, splitting each line for each separate gene, showing the ratio of phenotype 3) Combine the phenotype and multiply the phenotype ratios along each branch to get the find proportions of each type of offsprings Interaction Between Two Genes More then one genes can affect the same trait. Lots of possible interaction: using only 2 genes and complete dominance for both 1. Two dominant alleles necessary for the trait 2. Duplicates Genes- dominant allele from at least one of the two genes is needed to give the dominant phenotype Ex] Shepherds Purse: to get a triangular seed pod, you must have a dominant allele from either the A gene or B gene 3. Two genes with different effects on the traits Epistasis- one gene controls the expression of another gene Penetrance and Expressivity Expression of many gene is affected by the environment or by “background” genetic influences. Penetrance- percentage of offspring with mutant genotype that express the mutant phenotype Expressivity- Degree to which the mutant phenotype is expressed Ex] Recessive trait is 30% pentant, 2 heterozygous mate. What percentage of offsprings shows mutant phenotype? Environmental Effect Many traits are affected by the environment as well as by genetics Ex] Hydrangea flower color is controlled flirst by flower color genes similar to those in the pea -Purple vs White with complete dominance -But Pink vs Purple is controlled by acidity of soil in which plans grow Phenocopies Phenocopies- organism that has a mutant phenotype but a normal (wild type). Mutant appearance through an environmental cause Ex] Drug that causes birth defects are common cause. Nothing is genetically wrong with the child, but it can be exposed in utero to toxic chemicals -nothing genetically wrong, something physically happens Pleotropy Pleotropy- one gene affecting several times Quite common: genes make proteins and often affect the overall phenotype in subtle ways that affect many different body system Ex] sickle cells anemia causes enlarged spleen, muscle pain, low red blood cells count, resistance to malaria and early death. All of this is caused by single mutations in one of the hemoglobin genes
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