Lecture 3 BIO 311D 1st Edition Outline of Last Lecture I Mendel s Experimental Quantitative Approach II The Law of Segregation III Mendel s Model IV The Test Cross V The Law of Independent Assortment Outline of Current Lecture I The Laws of Probability II The Multiplication and Addition Rules Applied to Monohybrid Crosses III Exceptions to Mendelian Genetics IV Degrees of Dominance V Tay Sachs disease VI Multiple Alleles VII Pleiotropy VIII Epistasis IX Polygenetic Inheritance X The Environmental Impact on Phenotype XI Why humans are not good subjects for genetic research XII Pedigree Analysis XIII Cystic Fibrosis XIV Sickle Cell XV Fetal Testing Current Lecture The Laws of Probability The outcome of one toss of a coin has no impact on the outcome of the next toss Similarly alleles of one gene segregate into gametes independently of another gene s alleles The Multiplication and Addition Rules Applied to Monohybrid Crosses Multiplication Rule The probability that two or more independent events will occur together is the product of their individual probabilities Addition Rule The probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities o This is used to figure out the probability if a F2 plant will be heterozygous or homozygous We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters Exceptions to Mendelian Genetics When alleles are not completely dominant or recessive When a gene has more than two alleles When a gene produces multiple phenotypes Degrees of Dominance Complete Dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical example brown eyes are dominant over blue eyes Incomplete Dominance Phenotype of F1 hybrids is somewhere between phenotypes of two parental varieties example a white and red flower breed to produce a pink flower Codominance Two dominant alleles affect the phenotype in separate distinguishing ways example a flower is half red and the other half is yellow Tay Sachs Disease A very fatal disease A dysfunctional enzyme causes an accumulation of lipids in the brain At the organismal level the allele is recessive At the biochemical level the phenotype is incompletely dominant the phenotype is at the enzyme activity level At the molecular level alleles are codominant Dominant alleles are not necessarily more common than recessive alleles Multiple Alleles Example is the four phenotypes of the ABO blood group are determined by three alleles for the enzyme I that attaches A or B carbohydrates to red blood cells Pleiotropy Most genes have multiple phenotypic effects known as Pleiotropy This is responsible for multiple symptoms of certain hereditary diseases Examples include cystic fibrosis and sickle cell disease Epistasis Some traits may be determined by two or more genes One gene at a locus alters the phenotypic expression of a gene at a second locus In other words one gene determines whether or not that trait is shown Example Labrador Retrievers and their coat color o One gene determines the pigment color either black or brown o The other gene determines if the color is shown Polygenic Inheritance Quantitative characters are those that vary in population Quantitative variation indicates polygenic inheritance Example Skin color The Environmental Impact on Phenotype The Norm of Reaction phenotypic range of a genotype influenced by environment Example Hydrangea flower colors depend on soil acidity where the color ranges from blue violet to pink Multifactorial When genetic and environmental factors both influence phenotype Why are humans not good subjects for genetic research Generation time is too long Parents produce relatively few offspring Breeding experiments are unacceptable So Pedigree Analysis is used instead of breeding humans Pedigree Analysis Family tree that shows inheritance patterns Used to make predictions about future offspring Many genetic disorders are inherited in a recessive manner o Example Albinism is a recessive condition Consanguineous is the mating between close relatives o This type of mating increases the chance of mating between two carriers Cystic Fibrosis The most common lethal genetic disease Results in defective or absent chloride transport channels in plasma membranes leading to a buildup of chloride ions outside the cell Symptoms Mucus buildup in internal organs and abnormal absorption of nutrients in the small intestine Sickle Cell Caused by the substitution of one amino acid in the hemoglobin protein in red blood cells In homozygous individuals all hemoglobin is abnormal Symptoms physical weakness pain organ damage paralysis Heterozygous individuals are carriers and sometimes suffer symptoms o However an advantage of heterozygous individuals is that they are less susceptible to the malaria parasite Note that some human disorders are caused by dominant alleles example dwarfism Fetal Testing Amniocentesis When the liquid that bathes the fetus is removed and tested Chorionic Villus Sampling CVS Sample of the placenta is removed and tested
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