Week Two - Extensions to Mendel's Laws of Inheritance
Week Two - Extensions to Mendel's Laws of Inheritance Bio 0041-01
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This 13 page Class Notes was uploaded by Sarina Scott on Sunday September 20, 2015. The Class Notes belongs to Bio 0041-01 at Tufts University taught by Ekaterina Mirkin in Summer 2015. Since its upload, it has received 76 views. For similar materials see General Genetics in Biology at Tufts University.
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Date Created: 09/20/15
General Genetics Ekatarina Mirkin Lecture 3 091 51 5 Extensions to Mendelian Genetics Pt 1 Extensions refer to observed deviations from Mendel s straightforward phenotypic ratios and their molecular explanations in all cases alleles are still inherited according to Mendel s laws they are not exceptions Extensions for Single Gene Inheritance a Incomplete Dominance hybrid progeny does not resemble either purebreeding parents and thus both parental alleles contribute to the phenotype of the F1 generation ie intermediate blend 1 F2 ratio 1 2 1 1 Phenotypic ratios re ect genotypic ratios no collapse of genotypes ii Ex ower color in snapdragons 1 AA l red 100 pigment Aa I pink 50 pigment aa I White 0 pigment 2 Normal A allele encodes for an enzyme in involved in pigment production mutant a allele encodes for the absence of function enzyme ie no pigment is produced 3 Heterozygous individual contains only one functional copy of the enzyme therefore it makes half the normal amount of pigment causing the phenotype to present as pink Marla A FI39LEFEI39EIHTI shims Fr irmr rmpek i i li l l F 3 31 uvialmg g t rt flu y ri Fa LEIEl Iiiamaailu 2 gag Elia H 39 quot a a l F51 n hr 1 if a a 2 n 1 All 439 ti 4 h 7 1 EH Ea Ire ll QUEQ tutorial rll in le i lte u I 8 Codominance cross between two purebreeding parents produces a heterozygous hybrid progeny equally displaying both parental alleles F2 ratio 1 2 1 b EX coat patterns in lentils i CSCS I spotted CSCD I spotted m dotted CDCD I dotted a Codmninant lentil neat patterns ametes 3935 F1 Jail Newman a CECE a if 57 a 1 r if L FE v g F 55 a 1 marinaale 2 I is p i magma 1 c c m em 1 Multiple alleles 8 b b Every individual can only carry two alleles however a gene has the ability to have more than two alleles possible within a population this leads to pairwise dominance relationships between pairs of alleles i Dominance patterns are determined by the phenotype of the heterozygote EX seed coat patterns in lentils i Five alleles possible for C gene spotted CS dotted CD clear CC marbled1 CW and marbled2 CM 1 marbled1 gt marbled2 gt spotted dotted gt clear ii Dominance relations can eXhibit complete dominance incomplete dominance or codominance EX 2 coat color in mice i This example highlights the ability to follow gene transmission via the effects of mutations heritable alternations of genetic material induced in a lab or occurring spontaneously resulting in phenotypic variation ii A l agouti wild type atat I black back yellow belly lab mutant aa I black lab mutant iii Dominance series is determined experimental via crosses Ell Mus imisrsiims house mouse colors in the lab quot 7 43h AllElias D39ll39 theagnn gene GermWE Phenotype 31 agouii 3 Lil Hillardi gel low mi Math 355 lilac felhgiiri39 1 Ex 3 human major histocompatibility complex MHC a MHC are molecules present on every cell in the body excluding RBCs and sperm which allow the immune system to properly react to the presence of foreign substances b MHC gene contains more than 10 genes polygenic and each gene has 400 1200 alleles polymorphic i No two individuals exhibit the same two alleles therefore the variation of histocompatibility complexes present in individuals is incredibly variant ii Polymorphism increases the probability that individuals within the population will express an allele that will allow them to survive effects of pathogens b Alleles exhibit codominant relationships I Recessive lethal alleles a One trait is dominant to the alternative trait for its particular characteristic but recessive for lethality b F2 ratio 2 1 0 Ex coat color in mice i The Agouti gene previously mentioned above expresses a third allele AY which is dominant to A for coat color but recessive to A for lethality 1 AA l agouti grey AYA l yellow AYAY l individual dies in utero 2 2 phenotypes are present 1 coat color and 39 2 viability a All yellcuw imiEE are hetrar gyg tea jquot mat Isam 1 Molecular explanation of this phenomenon a The Agouti gene is present on the chromosome in close proximity to Raly an RNAbinding protein essential for embryogenesis b Expression of the AY allele causes a deletion of the Raly gene however the promoter of Raly is still present and causes an overexpression of the Agouti gene resulting in the dominant yellow phenotype of an AYA individual c A heterozygote is able to function with only one copy of the Raly gene however a homozygous AYAY individual lacks both copies of the essential Raly gene and thus is not viable I Recessive lethality is a special case of pleiotropy in which two seemingly unrelated characteristics are effected by one allele 1 Pleiotropy a A single gene determines a number of distinct and seemingly unrelated characteristics due to the ability of a protein to exhibit multiple functions and thus effect multiple systems b Ex recessive mutation in a gene causes many aboriginal Maori men in New Zealand to express both respiratory problems and sterility i Mutated gene causes nonfunctioning cilia producing a failure to clear one s lungs and agella producing immotile sperm 1 Extensions for Multifactorial Inheritance a Novel phenotypes i Ex seed coat color in lentils 1 Lentil seeds have an opaque outer layer that if colored will cover the green chlorophyll present in the inner layer 2 E2 ratio 9 3 3 1 expressing a novel phenotype a This ratio is familiar from Mendel s law indicating two independently assorting genes for two different traits b Only a single trait seed color is expressed in this example so we can derive that two independently sorting genes for one trait are interacting to produce a phenotype 3 Molecular explanation two genes for seed color function in two independent parallel pathways to produce four separate phenotypes a Each allele begins with a colorless precursor b Allele A encodes enzyme A while allele a does not produce any enzyme presence of homozygous dominant or heterozygous genotype for allele A results in the expression of tan pigment i AA Aa Enzyme A l tan pigment b Allele B of a second gene encodes enzyme B while allele b does not produce any enzyme presence of homozygous dominant or heterozygous genotype for allele B results in the expression of gray pigment i BB Bb Enzyme B l gray pigment ii A B I brown seed color both tan and gray pigments are produced b Presence of homozygous recessive for either allele a g b aa ibb results in the absence of the enzyme and no pigment will be produced 1 A bb l tan seed color ii aa B I gray seed color b Presence of homozygous recessive for both allele A mB aa bb results in the absence of both enzymes and the seed coat is unpigmented so the green chlorophyll in the cotyledon will show b Complementary gene action b i A particular twogene interaction in which multiple genotypes encode for the same phenotype and thus the four F2 genotypic classes produce fewer than four observable phenotypes ii EX ower color in sweet peas 1 F2 ratio 9 7 a 916 purple A B b 716 white 3 A bb 3 aa B 1 aa bb i Represents the collapse of three genotypic classes into one phenotypic class b 2 Molecular explanation one pathway has two sequential reactions catalyzed by different enzymes a At least one dominant allele of both genes is required to produce the enzymes necessary for pigment production if either or both genes is homozygous recessive the will not produce any pigment ie white b ii EX 2 ocularcutaneous albinism OCA in humans 1 Albinism is a recessive disease however two affected parents can produce unaffected children due to complementation if the children have at least one copy of the dominant allele of both genes heterozygous for gene 1 and gene 2 they will not eXpress the recessive phenotype 2 Mutations are located in two separate genes 3 Pedigree showing a recessive mutation that is able to skip generations is evidence of complementation Redundant genes i EX leaf development in maize 1 F2 ratio 15 1 a 1516 normal 9 A B 3 A bb 3aa B i Represents the collapse of three genotypic classes into one phenotypic class b 116 skinny 1 aa bb 2 Molecular explanation two enzymes perform the same function a Both protein A and protein B recruit precursor cells to become part of the leaf b One dominant allele of either gene or both results in enough protein to produce the normal phenotype only a homozygous recessive individual eXpresses the mutant phenotype Extensions to Medelian Genetics for Multifactorial Inheritance Pt 2 I Epistasis a relationship between two genes in which one allele epistatic allele masks the effects of another gene hypostatic allele a Recessive epistasis epistatic allele must by homozygous recessive to mask the hypostatic allele i EX coat color in labradore retrievers 1 Homozygous recessive genotype for allele ee of gene E is epistatic to gene B gene B is hypostatic to allele e of gene E 2 Molecular pathway EEK EH SE Ell V F riu teian E Fraulein E Umelanln L IWIEb b Eumelaniin dap j 7 r 39 precursorquot densely EEK Ea 1 I 1 Eumemnln THC F mlem B mmrle Pr leiin EK E IV I l V A MET a precursorquot 777 W 777 quotmm am quot EWEaitel Hanser EB EllTi bl BE 39 3 pvrrulein E MD prulein E Err nu protein B culurlEsu 39 precursor E 5 b Protein E makes eumelanin i ee cannot make eumelanin gt yellow B allele is irrelevant if individual is homozygous recessive b Protein B deposits eumelanin i B deposits densely gt black coat ii bb deposits less densley gt brown 1 F2 ratio 934 a 4 16 represents the ability of genotype ee to maks the effect of all B genotypes causing the collapse of two genotypic classes into one phenotypic class Dominant epistasis I one copy of the epistatic allele is sufficient to mask the hypostatic allele indicates an antagonistic function a EX color in summer squash i Dominant allele B prevents pigment deposition and is epistatic to gene A both alleles A and a in the presence of allele B the squash will be white regardless of its genotype for gene A As EB Eh EF39EFEFI Enzyme quotgiji allt hlhin Nil ganEni ingmunu depua ed an EH HE C Wu awyrrie fir Protein E N t lumen E1 5 great I E van plgr Er n EJEQITFIEHT ngth Idaapnsited AAA3 Eb Green Emlynleg fbll h39iv NU Prmeilg I i 39 39 4 PF 1 m quoti a 1 3 39 pkyneni imilf Hg g an Eb H Wu Etienne A MEI P rutein EJFEEH Ef e T39rquot 1 aan Pkwnent pm mnl 39 gmam 39 9 quot depusited I F2 ratio 12White 3yellow 1green a 12 16 represents the ability of genotype B to mask the effect of all A genotypes causing the collapse of 2 genotypic classes into one phenotypic class Dominant epistasis II the presence of at least one copy of the epistatic allele masks the hypostatic allele however the initial precursor is colorless and thus three genotypic classes collapse to form one phenotypic class a EX feather color in chickens i Dominant allele B of gene B is epistatic to allele A of gene A and prevents pigment deposition A3 HE E 1 391 a l39zatt39 lElr izvir riE Fl le i E Guinness y 1 F H H In Numgimeni39 prenursnxr 3 39 emailed 5152 ED Ifquot f i u if E I I hueuyme M Frn ein E H 1 VJUFE ES x L to xtrlizilsgmBLW 1 Jquot 1 Iquot precursnr E puqmem smelled Ala EDIWIEES Enzyme ND prmmnjh Firecur snr 1 H 1 39 an 67151 li m A 7 lac ewwne A Ital F rulmn E q a E lnrles 1 Hit I Nu nIglrnenl precursor pigment I deposited I B individuals are white due to their inability to deposit the pigment Whether or not the pigment was even made While aa bb individuals are White due to their inability to make the pigment they are able to deposit however this is irrelevant I F2 ratio 13white 3brown a Genotype B masks the effect of allele A causing the collapse of three genotypic classes into one phenotypic class I Redundant genes Duplicate dominant epistasis a Alleles A and B are epistatic to one another a dominant allele for either gene produces one phenotype i If one A allele is present B is irrelevant if one B allele is present A is irrelevant I Complementation Duplicate recessive epistasis a Alleles a and b are epistatic to one another a recessive allele for either gene results in one phenotype i If one a allele is present B is irrelevant if one b allele is present A is irrelevant a 52 ailL ab 339 SEE E 1333 iii Review i in ll39l i i l gma e in ma If gang liiii fii il limgi39l jhl H iiHII Ful llquot IItllli39lillr fb IEITJI39Ed lira IErl l39llzl ljl39l nil15311 I rang a Sumsrim of ine Elmammonia F1 Miamiigm ian il i Hiram LE iihaihirii Em E Ham E Ilnwmml39m Exith Ea AF M a E m innit Huangii Emr diamzquot F3 phenmgmii Lennil cm mainti see Fig ELIE5 a 1 gl ig umplmr ar nl jdtl 39ll39l i allale Emmet pea ml Mia5 of we gene5 i3 IlE39CESiiil Ii gIazn lime Fig 3IJI EMF wrdiite ml39enniwe hm iwEFEMM iInrrrrir mli Labraglan hammer Izmir semihivent mu genr mailis iIIE1 calm Ilsee Flg 31de HEMl1 alelies n5 adminname H mailman Emilails I lemma Ii Simmen SE LIElil39i wigr airin n jnncqmn hide5 EHEEIZLE Cal Iii 1 Fiqi L1H 5 13321 FlirtIii alleles b El itlw agem r ii39ll iH lit ll m39lilalE 39lli El39lifl g i39i39l IEEEIU IETEE Hells5 megslie Iii13m E HEEH of E l fi 39E Fgin 31Eaa3 9a 133 damnani allele If nilum gen I iEetiumdnmg E39l39ll39 ned mnam lie5 Hirsch5mm meleefather 31 We vegaim is Heinz535 I39EH39 Fig 521 151 ll Eli39lzl ir 39 Elh 39ii 39 I Same Genotype Producing Alternate Phenotypes a Incomplete penetrance the percentage of individuals with a particular genotype that Show the expected phenotype is less than 100 i With respect to a pedigree dominant autosomal diseases with incomplete penetrance are able to 39skip39 a generation b Variable expressivity expressivity is the degree or intensity with which a particular genotype is expressed in a phenotype and can be unvarying uniform or varying intermediate blend Eumplete penetrance unuarying expressivity lyncnmplete penetrance A 00000 falsifiable expressiiuity 000000000 Eulh 0000000 I EX Retinoblastoma is caused by an autosomal dominant mutation in which 75 of children who inherited thus mutation develop eve cancer In some children a tumor develops in one eye in others in both eyes a Retinoblastoma involves both incomplete penetrance 75 penetrance m variable eXpressivity presence of tumor in eye II Environmental effects on phenotype a EX Temperature sensitive ts allele gt effects of temperature on coat color of Siamese cats conditional mutation i Extremities of Siamese cats are darker in color than their body due to a temperature sensitive allele of the enzyme which produces melatonin ii Mutation causes melaninproducing enzyme to be partially destabilized and thus at lower temperatures the enzyme is functional and produces melanin but if the temperature becomes elevated the enzyme will unfold and denature M f lE y39T l l r H nonfunctinnal a 7vmIPEirtzum calmness L 7 r W DW FJEWUSEWE I I precursnr melanin nnn i t n r en yams Gamer calmness fwmm u melanim UWWEEWE ll temperature precursnr mnditms I EX 2 Conditional lethal alleles gt effects of temperature on viability of Drosophila y a Under permissive conditions lt 29C a speci c protein required for signal transmission in neurons is present and thus the y is viable I b Under restrictive conditions gt29C the protein is absent and the y is inviable dead ABO blood types in humans example of multiple alleles codominance and recessive epistasis a Gene 1 controls which speci c sugar polymer will be present on the surface of the RBC and contains three different alleles IA IB and i i IA and IB are codominant alleles and both will be expressed ii IA and IB are both completely dominant to i allele iii These relationships correspond to 6 genotypes and 4 phenotypes 1H n ll39l pr telm 39egjrieieeelee en H39Btilgx ll Hfquot 391 v Gene ty pe Pheeelyee 1 WA bleed type A g A W ifquot allele I quot I 2 39 a 391 r 439 lg l1 hleedtype a i FEE I EWPE J1 m N H Pile bleed type 153 sugar Sugar 5 euger 3 39 bleeeieee I Medical implications of ABO blood group genetics a One39s body produces antibodies against the antigens NOT present in the body and thus there are only certain types of blood one can give and receive depending on the antibodies present bl eed relerier le Fiates REG 5 a 7 I leetl type anti heel i e e in ee rum ram 9 ient 3 E 1 B C A agei n e t E e E egei H et A B AS H e it her 355 D agai n e t e e we 5 1 1 I Example of recessive epistasis in ABO blood group genetics Rare Bombay phenotype apparent O a Particular sugars present on the surface of RBCs are attached to a particular protein called protein H The gene for protein H is epistatic to the ABO gene meaning that Without at least one dominant H allele protein H will be absent and the sugars will not be able to attach to the cell surface hh genotype appears to be an O genotype regardless of actual ABO genotype because there are no sugars present and thus would have difficulty with givingreceiving blood
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