Chapter 2 Notes
Chapter 2 Notes Bio 230
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This 0 page Class Notes was uploaded by Elizabeth Notetaker on Sunday January 24, 2016. The Class Notes belongs to Bio 230 at West Chester University of Pennsylvania taught by Dr. Donze- Reiner in Spring 2016. Since its upload, it has received 24 views. For similar materials see Genetics in Biology at West Chester University of Pennsylvania.
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Date Created: 01/24/16
gene discovery guring out the part of the genome that a certain property arises from One way to do this is by using single gene inheritance patterns genetics works like this guring out how a machine works by taking out speci c parts to see what happens wild type normal type found in nature mutants abnormal variants whose traits are heritable These arise from mutation heritable change in DNA polymorphisms coexistence of 2 common phenotypes or wild types in nature steps of gene discovery 1 collect mutants of a speci c trait a sometimes treat population with radiation to increase mutation rate 2 mate the mutants to nd out if the gene that is mutated is singlegene inheritance a genetic dissectionbased on the cross of mutants vs wild type scientists can gure out if the trait is controlled by 1 gene or multiple based on the of offspring of each type b many times the trait is controlled by a few different genes and these genes are named as a set 3 gure out what the gene does at the cellular level how the protein is different from wildtype 4 gure out how this mutation results in the physical difference of the organism quotuseful in agriculturehealthcare as well forward genetics using mutants to gure out DNA sequence amp biological function reverse genetics genomic analysis rst then induces mutation by altering DNA to see result 21 Single Gene Inheritance Patterns Mendel chose pea plants to study they are easy to breed His experiments led to our understanding of single gene inheritance For all 7 characteristics Mendel found plants that showed contrasting phenotypes pure lines all offspring produced by matings within members of that line were identical Basically all yellow seeded peas will produce only yellow seeded peas when mated with other yellow seeded peas sel ng allowing pollen from a ower to fall on itself mating with itself when crossing yellow amp green pure lines parental generation all offspring are yellow rst llial generation second lial generationF2 was produced by sel ng the F1 generation and resulted in a 31 ratio the green phenotype which was not seen in F2 reappeared in F2 What does this mean The genetic component for the green seeds was not seen in F1 but must have still been there It was being masked by the yellow component but was still being carried heterozygous when the F2 generation was selfed this happened 0 F2 green sel ng ONLY produced green seeds 0 13 F2 yeow sel ng produced ONLY yeows 0 23 of F2 sel ng produced 34 yellow and 14 green OVERALL F2 was 14 pure breeding greens 14 pure breeding yellows 12 yellows that acted like F1 yellows Mendel39s Study Results 5 1 genes are hereditary factors that control traits 2 every plant has a pair of genes 3 4 if there is even one dominant allele then the phenotype will be the dominant a gene comes in 2 forms alleles with Y as dominant and y as recessive phenotype law of segregation or Mendel39s rst law a gamete only contains one allele of the gene pair monohybrid heterozygote for one gene 22 The Chromosomal Basis of SingleGene Inheritance Patterns Diploids mitosis somatic cells one cell becomes 2 genetically identical ces sister chromatids identical copies of a chromosome one goes to each daughter ce meiocytes produce sex cells by meiosis produces 4 cells that are haploid 2 divisions S phase when DNA replication occurs production of sister chromatids synapsis joining of homologous pairs in meiosis bivalent tetrad ini l I39 Ia III In II I II i Iquot H Cent39omai e DNA Ir FunafutiInna prominf I I lel n39 I i I 5 i 1 H El I 1 is l I i 1 i i M F 395 I T at V I rf f39ufI E l H E 7 wrj I mthh Ei EEf Tammi ahtnmatidm chtumaticls iii ivaienl H3 mmw my in Fair or Homologous Ehrnm sm FIRST CELL DIVISION one dyad of the bivalent moves to each side of the cell SECOND CELL DIVISION one member of the dyad moves to each side of the cell chromosomes are pulled apart by spindle bers when they are depolymerized and shortened this information supports Mendel39s genetic research Haploid we can examine one meiocyte in haploid organisms 0 when 2 cells of opposite mating type fuse a diploid cell is formed This diploid cell is the meiocyte o haploid cells that are the result of the diploid cell dividing stay in a sac called an ascus haploid meiosis only requires you to look at one meiocyte where in diploid organisms you must look at meiocytes of both parents Also haploids don39t mask recessive alleles with dominant aees because they only have one This makes haploid organisms excellent model organisms 23 The Molecular Basis of Mendelian Inheritance Patterns alleles are different versions of the same gene only a handful out of thousands of nucleotides are different between wild type and mutant alleles Mutations can happen at any spot on a gene 0 DNA replication occurs before both meiosis and mitosis and is extremely accurate so if a mutation is present on a parental DNA strand the same mutation will be in the same spot of the daughter DNA strand 0 S phase produces 2 copies of each allele 2 per gene A and a so that each daughter ce gets a full copy of the DNA 0 why do different aees produce different phenotypes They produce different proteins 0 example enzymesproteins that are made from a mutated DNA sequence may have a different shape of their active site thus making the enzyme useless or its function is different than the wild type enzyme 0 if a mutation happens in an exon coding region of DNA it is much more likely that the protein will have a difference in its ability to function like the wild type protein 0 null alleles mutated alleles that completely destroy the protein39s ability to function leaky mutations mutations that don39t make a protein completely useless the wild type function of the protein quotleaksquot into the mutated protein sometimes a mutated protein still functions as a wildtype depending on the mutation dominance and recessiveness of genesproteins o dominant haplosufficient one copy of the normal gene is enough to have a normal function example if a cell needs 10 normal proteins to function then even a heterozygote with one dominant allele can produce 10 proteins so the cell functions normally This is a haplosuf cient gene 0 recessive haploinsufficient when the single wild type allele cannot produce enough normal protein to make the cell functional example a cell needs 20 proteins to function AA produces 24 12 per A but Aa produces only 12 not enough to have the cell function normally sometimes a mutation makes a protein have a completely new function Sometimes a heterozygote cannot mask the new function with the wild type allele so the new function is present in the cell 24 Some Genes Discovered by Observing Semation Rations the standard procedure is to cross the mutant and the wild type 3 examples 1 ower color a wild type red crossed with mutant white the offspring follow the 31 ratio so we can assume that the mutation is single gene recessive 2 wing development in ies a the cross between wild type and mutant yielded a 11 ratio This means that it is a dominant mutation 3 mold with excessive branchinghapoid a cross of wild type with hyper branching yielded a 11 ratio this means that the hyper branching is caused by 1 gene if you know the genotypes of parents then you can deduce what the ratios will be for their offspring test cross if an individual has a dominant phenotype you don39t know if it is homozygous dominant or heterozygous To nd out you can cross it with a homozygous recessive and look at the ratios the recessive individual is the tester also another strategy is to self the unknown if it is 31 then you know it is heterozygous 2 ways that the law of equal segregation can be used 1 guessing genotypes from phenotype ratios 2 predicting the phenotypes from parents whose genotype is known 25 Sex Linked SingleGene Inheritance Patterns sex chromosomes determines sex in organisms that are dimorphs male female homogametic sex gametes are all the same in females every egg has an X chromosome heterogametic sex 2 types of gametes sperm with X and sperm with Y sometimes females are the heterogametic sex dioecious species plants that show animallike sexual dimorphism having ovaries and testes hemizygous where the differential regions part of chromosome with genes have no counterparts on the other sex X chromosome has 1005 of genes and the Y chromosome has dozens Most genes on the Y chromosome don39t have counterparts on the X chromosome The ones that DON T have counterparts on the X chromosome are responsible for male sexual function sex linkage genes that are in the differential region of sex chromosomes which tend to be passed on with a speci c gender there is X linkage and Y linkage sex linked genes have different phenotypic ratios for each sex amp are different in phenotypic ratios compared to autosomal genes pseudoautosomal regions 1 and 2 regions on the X and Y chromosome that are identical and thus act as if they are autosomal Xlinked inheritance Crossing wild type with mutant type results in F1 being all wild type and F2 being 31 However every mutant is male lf males get one copy of the mutant allele they express it because they don39t have a second X chromosome to mask the recessive trait ln reciprocal crossesdominant female x recessive male vs recessive female x dominant male ratios are different 26 Human Pedigree Analysis pedigree analysis looking at medical records of human matings propositus the person that comes to the attention of the geneticist rst in a family This person usually has a medical disorder and thus a reason to go to a geneticist Because humans usually have few offspring many pedigrees have to be combined to see ratios Autosomal recessive disorders pedigree features 1 progeny with mutant trait usually occurs from unaffected parents 2 there is no association with sex there are usually few affected individuals in a family tree mating between relatives creates a higher chance of affected children The rare recessive allele will be present in one family and so it is more likely that 2 individuals from the same family will be heterozygous and thus can produce an affected child It is much less likely that two separate families will have the same recessive allele examples cystic brosis albinism Autosomal dominant disorders dominance does NOT mean common In some dominant diseases DD two dominant alleles is fatal so if an individual is affected by the disease they are most likely Dd pedigree features 1 phenotype tends to appear in every generation of the pedigree 2 mothers amp fathers pass the trait on to their sons amp daughters equally if Ala and aa are crossed they have a 11 ratio of affected to unaffected children examples Huntington disease polydactyly Autosomal polymorphism polymorph alleles are common in populations so when doing a pedigree one can not assume that people who marry into an affected family are homozygous normal polymorphic traits are extremely common in natural populations and this fact has confused geneticists for a long time Why hasn39t the quotbetterquot trait become the more common one Xlinked recessive disorders pedigree features 1 many more males than females show the uncommon phenotype this is because males only have one X chromosome so they cannot mask the recessive allele with a second X chromosome 2 none of the offspring of an affected male show the phenotype but all daughters are carriers This is because with rare recessive diseases it is common that the female with the affected male will be homozygous dominant as most in the population are Half of the sons of the affected daughters will show the phenotype 3 no sons of an affected daughter will show the phenotype This is because males can ONLY get their x chromosome from their mother because they get their Y chromosome from their fathers Thus male to male transmission is an indicator of autosomal conditions examples redgreen colorblindness hemophilia Duchenne muscular dystrophy Xlinked dominant disorders pedigree features 1 affected males always pass condition to females but not to any of their sons 2 heterozygous females and unaffected males pass the condition to half their sons amp daughters example hypophosphatemia vitamin D resistance hypertrichosis excessive body hair Y linked inheritance only males can inherit the genes fathers ALWAYS transmit to their sons Mare sterility is not heritable because it cannot be passed down but is caused by mutations in males one possible example is hairy ear rims Calculating risks in pedigree analysis Parents often want to know the risk of having a child affected by a life threatening disease such as TaySachs disease in order to calculate risks you can use the product rule which states that the probability of 2 independent events happening is the product of the probabilities of those two events by themselves example 2 parents each have a 13 chance of being heterozygous Tt If they are both heterozygous their child has a 14 chance of being tt Overall the child39s chance of being tt is 13 x 13 x 14 136 If the tt genotype is fatal and causes a life of pain an suffering this would be an extremely important piece of information for parents to have when deciding if sometimes in populations there is a higher rate of recessive alleles such as in jewish populations and TaySachs disease This is because people only mate with people from the same population so the recessive allele keeps getting passed down
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