Lecture 4 BIO 311D 1st Edition Outline of Last 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 Outline of Current Lecture I The Chromosome Theory of Inheritance II Morgan s Experimental Evidence III The Chromosomal Basis of Sex IV X Inactivation in Female Mammals V How Linkage Affects Inheritance VI Genetic Recombination Current Lecture The Chromosome Theory of Inheritance The theory states that o Mendelian genes have specific loci positions on chromosomes o Chromosomes undergo segregation and independent assortment Law of Segregation Two alleles for each gene separate during gamete formation Law of Independent Assortment Alleles of genes on nonhomologous chromosomes assort independently Morgan s Experimental Evidence Thomas Hunt Morgan an embryologist found evidence associating a specific gene with a specific chromosome He performed an experiment with fruit flies which provided evidence that chromosomes are the location of Mendel s heritable factors Advantages of breeding fruit flies o Fruit flies produce many offspring o A generation can be bred every two weeks o 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 eyes o F2 generation showed 3 1 ratio but only males had white eyes o This indicated that the white eyed mutant must be located on the x chromosome The 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 chromosome o 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 homozygous o A male needs only one copy of the allele heterozygous Examples of sex linked genes color blindness duchenne muscular dystrophy and hemophilia X Inactivation in Female Mammals Because females have XX one X becomes inactive Inactive x condenses into a barr body One of the two x chromosomes in each cell is randomly inactivated Example Female cats can have certain spots where she is brown and other spots where she is orange This is due to the inactivation of random X chromosomes How Linkage Affects Inheritance Morgan found that body color and wing size are usually inherited together Therefore he noted that these genes do not assort independently because they were on the same chromosome Genetic 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 phenotype o 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 produced
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