Lecture 3 BIO 311D 1st EditionOutline of Last Lecture I. Mendel’s Experimental Quantitative ApproachII. The Law of SegregationIII. Mendel’s ModelIV. The Test CrossV. The Law of Independent AssortmentOutline of Current Lecture I. The Laws of ProbabilityII. The Multiplication and Addition Rules Applied to Monohybrid CrossesIII. Exceptions to Mendelian GeneticsIV. Degrees of DominanceV. Tay-Sachs diseaseVI. Multiple AllelesVII. PleiotropyVIII. EpistasisIX. Polygenetic InheritanceX. The Environmental Impact on PhenotypeXI. Why humans are not good subjects for genetic researchXII. Pedigree AnalysisXIII. Cystic FibrosisXIV. Sickle-CellXV. Fetal TestingCurrent LectureThe 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 allelesThe 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 probabilitieso This is used to figure out the probability if a F2 plant will be heterozygous or homozygous• We can apply the multiplicationand addition rules to predict theoutcome of crosses involvingmultiple charactersExceptions to Mendelian Genetics:- When alleles are not completely dominant or recessive- When a gene has more than two alleles- When a gene produces multiple phenotypesDegrees 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 allelesMultiple Alleles:- Example is the four phenotypes ofthe ABO blood group aredetermined by three alleles for theenzyme I that attaches A or Bcarbohydrates to red blood cellsPleiotropy:- Most genes have multiplephenotypic effects known asPleiotropy- This is responsible for multiplesymptoms of certain hereditarydiseases- Examples include cystic fibrosisand sickle cell diseaseEpistasis:- 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 coloro One gene determines the pigment color (either black or brown)o The other gene determines if the color is shownPolygenic Inheritance:- Quantitative characters are those that vary in population- Quantitative variation indicates polygenic inheritance- Example: Skin colorThe 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 phenotypeWhy are humans not good subjects for genetic research?- Generation time is too long- Parents produce relatively few offspring- Breeding experiments are unacceptableSo 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 mannero Example: Albinism is a recessive condition- Consanguineous is the mating between close relativeso This type of mating increases the chance of mating between two carriersCystic 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 intestineSickle-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 symptomso 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
View Full Document
Unlocking...