Lecture 4 BIO 311D 1st EditionOutline of Last 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 TestingOutline of Current Lecture I. The Chromosome Theory of InheritanceII. Morgan’s Experimental EvidenceIII. The Chromosomal Basis of SexIV. X Inactivation in Female MammalsV. How Linkage Affects InheritanceVI. Genetic RecombinationCurrent LectureThe Chromosome Theory of Inheritance:- The theory states that:o Mendelian genes have specific loci (positions) on chromosomeso Chromosomes undergo segregation andindependent assortment- Law of Segregation: Two alleles for each gene separateduring gamete formation- Law of Independent Assortment: Alleles of genes onnonhomologous chromosomes assort independentlyMorgan’s Experimental Evidence:- Thomas Hunt Morgan (an embryologist) found evidenceassociating a specific gene with a specific chromosome- He performed an experiment with fruit flies whichprovided evidence that chromosomes are the location ofMendel’s heritable factors- Advantages of breeding fruit flies:o Fruit flies produce many offspringo A generation can be bred every two weekso They only have four pairs of chromosomes- Wild Type: Normal phenotype common in fly populations(associated with a “+”)- Mutant phenotypes: Traits alternative to wild types- Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)o F1 generation all had red eyeso F2 generation showed 3:1 ratio but only males had white eyeso This indicated that thewhite eyed mutant must belocated on the xchromosomeThe Chromosomal Basis of Sex:- Females are XX and males are XY- The X chromosome is larger than the Y chromosome- Sex is determined by sperm in humans- Sex-linked genes are genes that are located on a sex chromosomeo Y-linked genes are less common than x-linked genes- X-linked genes follow specific patterns of inheritance for a recessive trait to be expressed:o A female needs two copies of the allele, therefore must be homozygouso A male needs only one copy of the allele (heterozygous)- Examples of sex linked genes: color blindness, duchenne muscular dystrophy, and hemophiliaX Inactivation in Female Mammals:- Because females have XX, one X becomesinactive- Inactive x condenses into a barr body- One of the two x chromosomes in eachcell is randomly inactivated- Example: Female cats can have certainspots where she is brown and other spotswhere she is orange. This is due to theinactivation of random X chromosomesHow Linkage Affects Inheritance:- Morgan found that body color and wingsize are usually inherited together- Therefore he noted that these genes donot assort independently because theywere on the same chromosomeGenetic Recombination:- Production of offspring with combinations of traits differing from either parent- Parental Types: Offspring with a phenotype matching one parents phenotype- Recombinant type: Offspring with nonparental phenotypeo 50% frequency of recombination is observed for any two genes on different chromosomes- Morgan discovered that genes can be linked, but linkage was incomplete because some recombinant phenotypes were observed (due to crossing over of homologous chromosomes)- Random fertilization increases the number of variant combinations that can be
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