BIOLOGY 101. CHAPTER 15: The Chromosomal Basis of Inheritance: Locating Genes Along Chromosomes

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1 BIOLOGY 101 CHAPTER 15: The Chrmsmal Basis f Inheritance: Lcating Genes Alng Chrmsmes

2 Lcating Genes Alng the Chrmsme CONCEPTS: 15.2 Sex-linked genes exhibit unique patterns f inheritance 15.3 Linked genes tend t be inherited tgether because they are lcated near each ther n the same chrmsme 15.4 Alteratins f chrmsme number r structure cause sme genetic disrders 15.5 Sme nrmal inheritance patterns are exceptins t the standard Mendelian inheritance

3 Lcating Genes Alng the Chrmsme 15.1 Mendel s theries vindicated and accepted Mendel s hereditary factrs were purely abstract when first prpsed Tday we can shw that the factrs genes are lcated n chrmsmes The lcatin f a particular gene can be seen by tagging islated chrmsmes with a flurescent dye that highlights the gene

4 Lcating Genes Alng the Chrmsme 15.1 Mrgan shwed that Mendelian inheritance has its physical basis in the behavir f chrmsmes The first slid evidence assciating a specific gene with a specific chrmsme came in the early 20th century frm the wrk f Thmas Hunt Mrgan ( ) These early experiments prvided cnvincing evidence that the chrmsmes are the lcatin f Mendel s heritable factrs

5 Lcating Genes Alng the Chrmsme 15.2 Sex-linked genes exhibit unique patterns f inheritance Mrgan s discvery f a trait that crrelated with the gender f flies was key t the develpment f the chrmsme thery f inheritance In humans and sme ther animals, there is a chrmsmal basis f sex determinatin

6 Sex-linked genes exhibit unique patterns f inheritance 15.2 The Chrmsmal Basis f Sex In humans and ther mammals, there are tw varieties f sex chrmsmes: a larger X chrmsme and a smaller Y chrmsme A persn with tw X chrmsmes develps as a female, while a male develps frm a zygte with ne X and ne Y

7 Sex-linked genes exhibit unique patterns f inheritance 15.2 The Chrmsmal Basis f Sex In humans and ther mammals, there are tw varieties f sex chrmsmes: a larger X chrmsme and a smaller Y chrmsme A persn with tw X chrmsmes develps as a female, while a male develps frm a zygte with ne X and ne Y Only the ends f the Y chrmsme have regins that are hmlgus with crrespnding regins f the X chrmsme

8 Sex-linked genes exhibit unique patterns f inheritance 15.2 The Chrmsmal Basis f Sex In humans and ther mammals, there are tw varieties f sex chrmsmes: a larger X chrmsme and a smaller Y chrmsme A persn with tw X chrmsmes develps as a female, while a male develps frm a zygte with ne X and ne Y Only the ends f the Y chrmsme have regins that are hmlgus with crrespnding regins f the X chrmsme Other rganisms have different methds f sex determinatin

9 Sex-linked genes exhibit unique patterns f inheritance 15.2 The SRY gene is n the Y chrmsme Shrt segments at the ends f the Y chrmsmes are hmlgus with the X, allwing the tw t behave like hmlgues during meisis in males A gene n the Y chrmsme called SRY (sex-determining regin n the Y) is respnsible fr develpment f the testes in an embry In individuals lacking the SRY gene, the generic embrynic gnads develp int varies.

10 A gene lcated n either sex chrmsme is called a sex-linked gene 15.2 Inheritance f X-Linked Genes Genes lcated n nly ne f the sex chrmsmes are called sex-linked genes Genes nly n the X chrmsme are called X-linked genes

11 A gene lcated n either sex chrmsme is called a sex-linked gene 15.2 Inheritance f X-Linked Genes Genes lcated n nly ne f the sex chrmsmes are called sex-linked genes Genes nly n the X chrmsme are called X-linked genes Genes nly n the Y chrmsme are called Y-linked genes; there are few f these

12 A gene lcated n either sex chrmsme is called a sex-linked gene 15.2 Inheritance f X-Linked Genes X chrmsmes have genes fr many characters unrelated t sex, whereas mst Y-linked genes are related t sex determinatin X-linked genes fllw specific patterns f inheritance Fr a recessive X-linked trait t be expressed A female needs tw cpies f the allele (hmzygus) A male needs nly ne cpy f the allele (hemizygus) X-linked recessive disrders are much mre cmmn in males than in females The transmissin f X-linked recessive traits

13 A gene lcated n either sex chrmsme is called a sex-linked gene 15.2 Inheritance f X-Linked Genes Example f disrders caused by recessive alleles n the X chrmsme in humans: Clr blindness (mstly X-linked) Duchenne muscular dystrphy Hemphilia Males can nly inherit X-linked traits frm their mther

14 Sex-linked genes exhibit unique patterns f inheritance 15.2 Althugh female mammals inherit tw X chrmsmes, nly ne X chrmsme is active X-Inactivatin in Female Mammals In mammalian females, ne f the tw X chrmsmes in each cell is randmly inactivated during embrynic develpment Murray Barr nticed that in the nucleus f females, but nt males, a darkly staining bdy is visible. It was hypthesized that this was an inactivated X chrmsme in females s that there wuld nly be 1 functinal cpy f X genes as in males. The inactivated X chrmsme is called a Barr bdy Females with XXX gentype have 2 Barr bdies XXY Males have ne Barr bdy

15 Sex-linked genes exhibit unique patterns f inheritance 15.2 Althugh female mammals inherit tw X chrmsmes, nly ne X chrmsme is active X-Inactivatin in Female Mammals In mammalian females, ne f the tw X chrmsmes in each cell is randmly inactivated during embrynic develpment Barr bdies: If a female is heterzygus fr a particular gene lcated n the X chrmsme, she will be a msaic fr that character As a cnsequence, females cnsist f a msaic f tw types f cells, sme with an active paternal X chrmsme and thers with an active maternal X chrmsme. If a female is heterzygus fr a X-linked trait, apprximately half her cells will express ne allele, and the ther half will express the alternate allele X inactivatin and the trtiseshell cat

16 Sex-linked genes exhibit unique patterns f inheritance 15.2 Althugh female mammals inherit tw X chrmsmes, nly ne X chrmsme is active X inactivatin invlves a set f genes that eventually cver ne X chrmsme with cpies f RNA mlecules A particular regin f each X chrmsme cntains several genes invlved in the inactivatin prcess. One f the genes called XIST (X-inactive specific transcript) is activated n the Barr-bdy chrmsme. Multiple cpies f the XIST RNA prduct attach t the X chrmsme n which they are made, almst cvering it. Interactin f this RNA with the chrmsme initiates X inactivatin

17 Lcating Genes Alng the Chrmsme 15.3 Linked genes tend t be inherited tgether because they are lcated near each ther n the same chrmsme Each chrmsme has hundreds r thusands f genes (except the Y chrmsme) Genes lcated n the same chrmsme that tend t be inherited tgether are called linked genes Gene lci that are clse tgether are mre tightly linked

18 Lcating Genes Alng the Chrmsme 15.3 Linked genes tend t be inherited tgether because they are lcated near each ther n the same chrmsme Each chrmsme has hundreds r thusands f genes (except the Y chrmsme) Genes lcated n the same chrmsme that tend t be inherited tgether are called linked genes Gene lci that are clse tgether are mre tightly linked This is because gene lci that are clse t each ther have less chance f having a crss-ver event (during prphase) than thse that are far apart These lci are clse tgether, leaving less chance f a crss-ver event

19 Linked genes tend t be inherited tgether because they are lcated near each ther n the same chrmsme 15.3 Independent assrtment f chrmsmes prduces genetic recmbinatin f unlinked genes The genetic findings f Mendel and Mrgan relate t the chrmsmal basis f recmbinatin Offspring with a phentype matching ne f the parental phentypes are called parental types Offspring with nnparental phentypes (new cmbinatins f traits) are called recmbinant types, r recmbinants

20 Linked genes tend t be inherited tgether because they are lcated near each ther n the same chrmsme 15.3 Crssing ver prduces genetic recmbinatin f linked genes Mrgan discvered that genes can be linked, but the linkage was incmplete, because sme recmbinant phentypes were bserved He prpsed that sme prcess must ccasinally break the physical cnnectin between genes n the same chrmsme That mechanism was the crssing ver f hmlgus chrmsmes Chrmsmal basis fr recmbinatin f linked genes

21 Linked genes tend t be inherited tgether because they are lcated near each ther n the same chrmsme 15.3 New cmbinatins f alleles prvide variatin fr natural selectin Recmbinant chrmsmes bring alleles tgether in new cmbinatins in gametes Randm fertilizatin and Independent Assrtment further increase the number f variant cmbinatins that can be prduced This abundance f genetic variatin is the raw material upn which natural selectin wrks

22 Lcating Genes Alng the Chrmsme 15.4 Alteratins f chrmsme number r structure cause sme genetic disrders Large-scale chrmsmal alteratins in humans and ther mammals ften lead t spntaneus abrtins (miscarriages) r cause a variety f develpmental disrders Plants tlerate such genetic changes better than animals d

23 Alteratins f chrmsme number r structure cause sme genetic disrders 15.4 Nndisjunctin leads t abnrmal chrmsme number In nndisjunctin, pairs f hmlgus chrmsmes (r sister chrmatids) d nt separate nrmally during meisis

24 Alteratins f chrmsme number r structure cause sme genetic disrders 15.4 Nndisjunctin leads t abnrmal chrmsme number In nndisjunctin, pairs f hmlgus chrmsmes (r sister chrmatids) d nt separate nrmally during meisis Sister Chrmatids d nt separate

25 Alteratins f chrmsme number r structure cause sme genetic disrders 15.4 Nndisjunctin leads t abnrmal chrmsme number In nndisjunctin, pairs f hmlgus chrmsmes (r sister chrmatids) d nt separate nrmally during meisis As a result, ne gamete receives tw f the same type f chrmsme, and anther gamete receives n cpy

26 Alteratins f chrmsme number r structure cause sme genetic disrders 15.4 Nndisjunctin leads t abnrmal chrmsme number Aneuplidy results frm the fertilizatin f gametes in which nndisjunctin ccurred Offspring with this cnditin have an abnrmal number f a particular chrmsme A mnsmic zygte has nly ne cpy f a particular chrmsme A trismic zygte has three cpies f a particular chrmsme Dwn Syndrme results frm trismy f chrmsme 21

27 Alteratins f chrmsme number r structure cause sme genetic disrders 15.4 Human disrders are due t chrmsme alteratins Dwn Syndrme (Trismy 21) Dwn syndrme is an aneuplid cnditin that results frm three cpies f chrmsme 21 It affects abut ne ut f every 700 children brn in the United States The frequency f Dwn syndrme increases with the age f the mther, a crrelatin that has nt been explained

28 Alteratins f chrmsme number r structure cause sme genetic disrders 15.4 Human disrders are due t chrmsme alteratins Klinefelter Syndrme (KS) Klinefelter Syndrme is an aneuplid cnditin that results frm tw cpies f the X chrmsme, and ne cpy f the Y chrmsme. The primary features are sterility and small testicles

29 Sme inheritance patterns are exceptins t the standard Mendelian inheritance 15.5 The phentypic effects f sme mammalian genes depend n whether they are inherited frm the mther r the father There are tw nrmal exceptins t Mendelian genetics One exceptin invlves genes lcated in the nucleus The ther exceptin invlves genes lcated utside the nucleus In bth cases, the gender f the parent cntributing an allele is a factr in the pattern f inheritance

30 Sme inheritance patterns are exceptins t the standard Mendelian inheritance 15.5 The phentypic effects f sme mammalian genes depend n whether they are inherited frm the mther r the father Genmic Imprinting: Fr a few mammalian traits, the phentype depends n which parent passed alng the alleles fr thse traits Such variatin in phentype is called genmic imprinting Genmic imprinting invlves the silencing f certain genes depending n which parent passes them n Silenced (imprinted) genes are inactivated thrugh DNA methylatin during egg r sperm frmatin IGF2 gene is lcated n chrmsme 11p15.5, a regin which cntains numerus imprinted genes In sme cases, histnes are methylated instead f DNA Abut 70 different instances f genmic imprinting have been identified in humans IGF2 inactivatin may play a rle in the develpment f Brwn Fat

31 Sme inheritance patterns are exceptins t the standard Mendelian inheritance 15.5 The phentypic effects f sme mammalian genes depend n whether they are inherited frm the mther r the father Genmic Imprinting: Fr a few mammalian traits, the phentype depends n which parent passed alng the alleles fr thse traits Mammals shuld nly have ne cpy f genes that are cntrlled by imprinting The imprinted gene is generally nly f either maternal r paternal rigin this prevents bth cpies frm being inactivated IGF2 gene is lcated n chrmsme 11p15.5, a regin which cntains numerus imprinted genes IGF2 inactivatin may play a rle in the develpment f Brwn Fat

32 Sme inheritance patterns are exceptins t the standard Mendelian inheritance 15.5 Extranuclear genes exhibit a nn-mendelian pattern f inheritance. Inheritance f Organelle Genes: Extranuclear genes (r cytplasmic genes) are fund in rganelles in the cytplasm Mitchndria, chlrplasts, and ther plant plastids carry small circular DNA mlecules Extranuclear genes are inherited maternally because the zygte s cytplasm cmes frm the egg In humans, the 16,569 base pairs f mitchndrial DNA encde 37 genes.

33 Sme inheritance patterns are exceptins t the standard Mendelian inheritance 15.5 Extranuclear genes exhibit a nn-mendelian pattern f inheritance. Sme defects in mitchndrial genes prevent cells frm making enugh ATP and result in diseases that affect the muscular and nervus systems Fr example, mitchndrial mypathy and Leber s hereditary ptic neurpathy

34 Sme inheritance patterns are exceptins t the standard Mendelian inheritance 15.5 Extranuclear genes exhibit a nn-mendelian pattern f inheritance. Inheritance f Organelle Genes: Because mitchndrial DNA is always inherited frm the mther, it is very useful in determining ancestral lineage