BIOL 1441 1st Edition Lecture 26 Outline of Last Lecture I Epistasis II Polygenic inheritance III Multifactorial traits IV Recessively inherited disorders V Inbreeding VI Dominantly inherited disorders VII Multifactorial disorders VIII Appling mendelian genetics to human inheritance IX Dominant recessive inheritance X Pedigree analysis Outline of Current Lecture I Chromosome Theory of Inheritance II Morgan s Experimental Evidence III Sex Linked Genes IV Gene Linkage V Linkage Mapping Current Lecture I Chromosome Theory of Inheritance a Genes have specific loci location along chromosomes These 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 II III b Chromosomes undergo segregation independent assortment i Law of Segregation only pass on 1 allele ii Law of Independent Assortment making sperm or egg mix up mom and dad s chromosomes Morgan s Experimental Evidence a Thomas Hunt Morgan provided the 1st solid evidence associating a specific gene with a specific chromosome b Drosophila melanogaster fruit fly i Very good model for genetics ii Hundreds of offspring iii Breed every 2 weeks iv Small easy to maintain v 4 chromosomes 3 autosomes 1 sex chromosome c Wild Type i Phenotypes most commonly observed normal wild type ii Different from the wild type mutant phenotypes iii USED INTEAD OF DOMINANT RECESSIVE iv Letter comes from the first mutation found lower case v Wild type is labeled with same letter but with a after Sex Linked Genes a Sex linked genes on a sex chromosome X i Follow inheritance pattern based on sex b Examples of Sex Linked Genes i Color blindness ii Muscular dystrophy iii Certain types of baldness iv Hemophila c Morgan mated male flies with white eyes mutant with female flies with red eyes wild type i F1 generation all had red eyes ii F2 generation showed the 3 1 red white eye ratio but only males had white eyes d Morgan determined that the white eye mutant allele must be located on the X chromosome thus supporting the chromosome theory of inheritance i Unique pattern of inheritance e Chromosomal Basis of Sex i XX female ii XY male iii X Y chromosome are homologues even though they have different genes on the chromosomes obvious different phenotypes f Y Chromosome 78 genes i Y chromosome has short regions at either end homologous the same with corresponding regions of X IV ii Anatomical signs of sex 2 months iii SRY gene sex determining region of Y 1 Absence of SRY gonads develop into ovaries 2 SRY codes for protein that regulates other genes iv smaller b c only has genes that makes boys into boys doesn t turn on right away g Inheritance of Sex Linked Genes i Many genes on X chromosomes that do not code for sex ii Fathers pass X sex linked alleles to daughters not to sons why not iii Mothers can pass to son or daughter iv much bigger has many genes for anatomical development for males females h Gene found on sex chromosome either X or Y i mostly refers to X b c Y doesn t have genes for male and female i Daughters normal inheritance b c they have 2 copies of X j Sons unique pattern of inheritance only have 1 X Gene Linkage a Each chromosome has thousands of genes b Genes located on the same chromosome that tend to be inherited togetherlinked genes c There is no physical connection or link d Linked genes when 2 genes are so close together on a chromosome they are inherited as one unit not independently e Independent Assortment i Follow Mendel s Laws of independent assortment and segregation ii Some genes on same chromosome still exhibit independent assortment 1 Crossing over genetic recombination iii These genes are usually far apart on chromosome f Genetic Recombination i Crossing over prophase I ii swap genes g Determining if genes are linked i Look at percent of recombination ii Independent assortment high recombination iii Linked low recombination 1 50 independent assortment 2 50 linked iv Example 1 Body Color Wing Size Cross a 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 mutant 3 Predictions a Independent assortment i 9 3 3 1 pattern of inheritance ii 9 dihybrid wildtype parent iii 1 double mutant parent iv 3 3 recombinants of parents 50 recombination b Linked i Majority of offspring 50 or more look like one parent or other ii Crossing over 50 offspring look like recombinants h Genetic Recombination i Recombinant phenotype that neither parent exhibits ii An organism with recombined DNA from both parents iii Recombination of Unlinked Genes 1 50 frequency of recombination is observed for any 2 genes on different chromosomes a In a 9 3 3 1 pattern 56 look like 1 parent 6 like the other parent 36 recombinants b With recombination higher recombinants 2 We expect 50 of the offspring to have recombined DNA for all the traits on different chromosome a Follow Mendel Laws independently assort b Randomly line up at metaphase I in meiosis iv Recombination of Linked Genes 1 Linked genes expect offspring to look like parentals half look like one parent half like the other a Small percent of recombinants crossing over i Less than 50 b Crossing over of homologous chromosomes in prophase 1 produces a small subset of recombinants i Look like neither parent v Are the genes linked 1 Number of flies for each genotype divided by the total number of flies 2 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 V a Yes independently assorted b No linked genes vi Conclusions linked 1 From the results Morgan reasoned that body color wing size are usually inherited together because the genes are on the same chromosome vii Recombinant Frequency recombinants total offspring 100 1 How often do recombinant occur Linkage Mapping a Linkage map genetic map of a chromosome predicts where genes are located loci i Based on recombination frequencies b The farther apart 2 genes are on a chromosome the higher the probability that a crossover will occur between them i Therefore the farther apart 2 genes the higher the recombination frequency ii A recombination event will occur c Distances between genes can be expressed as map units i Centimorgan one map unit represents a 1 recombination frequency 1 1 CM 1 recombination ii Map units indicate relative distance