BIOL 1441 1st Edition Lecture 26 Outline of Last Lecture I. EpistasisII. Polygenic inheritanceIII. Multifactorial traitsIV. Recessively inherited disordersV. InbreedingVI. Dominantly inherited disordersVII. Multifactorial disordersVIII. Appling mendelian genetics to human inheritanceIX. Dominant/recessive inheritanceX. Pedigree analysisOutline of Current Lecture I. Chromosome Theory of InheritanceII. Morgan’s Experimental EvidenceIII. Sex-Linked GenesIV. Gene LinkageV. Linkage MappingCurrent LectureI. Chromosome Theory of Inheritancea. Genes have specific loci (location) along chromosomesThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.b. Chromosomes undergo segregation & independent assortmenti. Law of Segregation: only pass on 1 alleleii. Law of Independent Assortment: making sperm or egg/ mix up mom anddad’s chromosomesII. Morgan’s Experimental Evidencea. Thomas Hunt Morgan – provided the 1st solid evidence associating a specific gene with a specific chromosomeb. Drosophila melanogaster, fruit flyi. Very good model for geneticsii. Hundreds of offspringiii. Breed every 2 weeksiv. Small, easy to maintainv. 4 chromosomes- 3 autosomes, 1 sex chromosomec. Wild Typei. Phenotypes most commonly observed (normal)- wild typeii. Different from the wild type- mutant phenotypesiii. USED INTEAD OF DOMINANT/RECESSIVEiv. Letter comes from the first mutation found-lower casev. Wild type is labeled with same letter but with a “+” afterIII. Sex-Linked Genesa. Sex-linked- genes on a sex chromosome (X)i. Follow inheritance pattern based on sexb. Examples of Sex-Linked Genesi. Color blindnessii. Muscular dystrophyiii. Certain types of baldnessiv. Hemophilac. Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)i. F1 generation all had red eyesii. F2 generation showed the 3:1 red:white eye ratio, but only males had white eyesd. Morgan determined that the white-eye mutant allele must be located on the X chromosome, thus supporting the chromosome theory of inheritancei. Unique pattern of inheritancee. Chromosomal Basis of Sexi. XX- femaleii. XY- maleiii. X & Y chromosome are homologues- even though they have different genes on the chromosomes & obvious different phenotypesf. Y Chromosome- 78 genesi. Y chromosome has short regions at either end homologous (the same) with corresponding regions of Xii. Anatomical signs of sex ~2 monthsiii. SRY gene- sex determining region of Y1. Absence of SRY, gonads develop into ovaries2. SRY codes for protein that regulates other genesiv. *smaller b/c only has genes that makes boys into boys/doesn’t turn on right awayg. Inheritance of Sex-Linked Genesi. Many genes on X chromosomes that do not code for sexii. Fathers pass X sex-linked alleles to daughters, not to sons- why not?!iii. Mothers can pass to son or daughteriv. *much bigger-has many genes for anatomical development for males/femalesh. Gene found on sex chromosome (either X or Y)i. *mostly refers to X b/c Y doesn’t have genes for male and femalei. Daughters- normal inheritance b/c they have 2 copies of Xj. Sons- unique pattern of inheritance, only have 1 XIV. Gene Linkagea. Each chromosome has thousands of genesb. Genes located on the same chromosome that tend to be inherited together- linked genesc. There is no physical connection or linkd. “Linked genes”- when 2 genes are so close together on a chromosome, they are inherited as one unit- not independentlye. Independent Assortmenti. Follow Mendel’s Laws of independent assortment and segregationii. Some genes on same chromosome still exhibit independent assortment1. Crossing over- genetic recombinationiii. These genes are usually far apart on chromosomef. Genetic Recombinationi. Crossing over- prophase Iii. “swap” genesg. Determining if genes are linkedi. Look at percent of recombinationii. Independent assortment- high % recombinationiii. Linked- low % recombination1. >50% = independent assortment2. <50% = linkediv. Example:1. Body Color & Wing Size Crossa. b+ = gray body (WT)b. b = black body (mutant)c. vg+ = normal wings (WT)d. vg = vestigial wings (mutant)2. In order to determine if genes are linked do a test cross dihybrid to double mutant3. Predictions…a. Independent assortmenti. 9:3:3:1 pattern of inheritanceii. 9- dihybrid wildtype parentiii. 1- double mutant parentiv. 3 & 3- recombinants of parents (>50% recombination)b. Linkedi. Majority of offspring (50% or more) look like one parent or otherii. Crossing over- <50% offspring look like recombinantsh. Genetic Recombinationi. Recombinant- phenotype that neither parent exhibitsii. An organism with recombined DNA from both parentsiii. Recombination of Unlinked Genes1. 50% frequency of recombination is observed for any 2 genes on different chromosomesa. In a 9:3:3:1 pattern- 56% look like 1 parent, 6% like the other parent, 36% recombinants b. With recombination- higher % recombinants2. We expect 50% of the offspring to have recombined DNA for all the traits on different chromosomea. Follow Mendel Laws- independently assortb. Randomly line up at metaphase I in meiosisiv. Recombination of Linked Genes1. Linked genes- expect offspring to look like parentals (half look like one parent, half like the other)a. Small percent of recombinants- crossing overi. Less than 50%b. Crossing over of homologous chromosomes in prophase 1 produces a small subset of recombinantsi. Look like neither parent v. Are the genes linked?1. Number of flies for each genotype divided by the total number of flies2. Added the percents together for the flies that phenotypically resemble parents and the flies that phenotypically are recombinants (recombinant frequency)3. Is recombination higher than 50%?a. Yes- independently assortedb. No- linked genesvi. Conclusions- linked1. From the results, Morgan reasoned that body color & wing size are usually inherited together because the genes are on the same chromosomevii. Recombinant Frequency: (recombinants ÷ total offspring) × 1001. How often do recombinant occurV. Linkage Mappinga. Linkage map- genetic map of a chromosome predicts where genes are located (loci)i. *Based on recombination frequenciesb. The farther apart 2 genes are on a chromosome, the higher the probability that acrossover will occur between themi. Therefore the farther apart 2 genes, the higher the recombination frequencyii. (A