What You ll Learn. You will identify the basic concepts of genetics. Chapter 11 Introduction to Genetics. You will examine the process of meiosis.

Transcription

1 Wha You ll Learn Chaper 11 Inroducion o Geneics You will idenify he basic conceps of geneics. You will examine he process of meiosis. Secion Objecives: Relae Mendel s wo laws o he resuls he obained in his experimens wih garden peas. Predic he possible offspring of a geneic cross by using a Punne square. Why Mendel Succeeded I was no unil he mid-nineeenh cenury ha Gregor Mendel, an Ausrian monk, carried ou imporan sudies of herediy he passing on of characerisics from parens o offspring. Characerisics ha are inheried are called rais. Why Mendel Succeeded Mendel was he firs person o succeed in predicing how rais are ransferred from one generaion o he nex. A complee explanaion requires he careful sudy of geneics he branch of biology ha sudies herediy. Mendel chose his subjec carefully Mendel chose o use he garden pea in his experimens for several reasons. Garden pea plans reproduce sexually, which means ha hey produce male and female sex cells, called gamees. 1

2 Mendel chose his subjec carefully he male gamee forms in he pollen grain, which is produced in he male reproducive organ. he female gamee forms in he female reproducive organ. In a process called ferilizaion, he male gamee unies wih he female gamee. he resuling ferilized cell, called a zygoe (ZI goh), hen develops ino a seed. Mendel chose his subjec carefully he ransfer of pollen grains from a male reproducive organ o a female reproducive organ in a plan is called pollinaion. Mendel chose his subjec carefully Remove male pars When he waned o breed, or cross, one plan wih anoher, Mendel opened he peals of a flower and removed he male organs. Mendel chose his subjec carefully He hen dused he female organ wih pollen from he plan he wished o cross i wih. ransfer pollen Female par Pollen grains Male pars Cross-pollinaion Mendel chose his subjec carefully his process is called cross-pollinaion. By using his echnique, Mendel could be sure of he parens in his cross. Mendel was a careful researcher He sudied only one rai a a ime o conrol variables, and he analyzed his daa mahemaically. he all pea plans he worked wih were from populaions of plans ha had been all for many generaions and had always produced all offspring. 2

3 Mendel was a careful researcher Such plans are said o be rue breeding for allness. Likewise, he shor plans he worked wih were rue breeding for shorness. Mendel s Monohybrid Crosses A hybrid is he offspring of parens ha have differen forms of a rai, such as all and shor heigh. Mendel s firs experimens are called monohybrid crosses because mono means one and he wo paren plans differed from each oher by a single rai heigh. he firs generaion Mendel seleced a six-foo-all pea plan ha came from a populaion of pea plans, all of which were over six fee all. He cross-pollinaed his all pea plan wih pollen from a shor pea plan. All of he offspring grew o be as all as he aller paren. he second generaion Mendel allowed he all plans in his firs generaion o self-pollinae. Afer he seeds formed, he planed hem and couned more han 1000 plans in his second generaion. hree-fourhs of he plans were as all as he all plans in he paren and firs generaions. he second generaion One-fourh of he offspring were as shor as he shor plans in he paren generaion. In he second generaion, all and shor plans occurred in a raio of abou hree all plans o one shor plan. P 1 Shor pea plan F 1 F 2 All all pea plans 3 all: 1 shor all pea plan he second generaion he original parens, he rue-breeding plans, are known as he P 1 generaion. he offspring of he paren plans are known as he F 1 generaion. When you cross wo F 1 plans wih each oher, heir offspring are he F 2 generaion. 3

4 Dominan rai he second generaion Seed Seed shape color round yellow Flower color purple Flower posiion Pod color Pod shape Plan heigh axial (side) green inflaed all he second generaion In every case, he found ha one rai of a pair seemed o disappear in he F 1 generaion, only o reappear unchanged in one-fourh of he F 2 plans. Recessive rai wrinkled green whie erminal (ips) yellow consriced shor he rule of uni facors Mendel concluded ha each organism has wo facors ha conrol each of is rais. We now know ha hese facors are genes and ha hey are locaed on chromosomes. he rule of uni facors An organism s wo alleles are locaed on differen copies of a chromosome one inheried from he female paren and one from he male paren. Genes exis in alernaive forms. We call hese differen gene forms alleles. he rule of dominance he rule of dominance Mendel called he observed rai dominan and he rai ha disappeared recessive. Mendel concluded ha he allele for all plans is dominan o he allele for shor plans. When recording he resuls of crosses, i is cusomary o use he same leer for differen alleles of he same gene. all plan F 1 Shor plan All all plans 4

5 he rule of dominance An uppercase leer is used for he dominan allele and a lowercase leer for he recessive allele. he dominan allele is always wrien firs. all plan Shor plan F 1 All all plans he law of segregaion he law of segregaion saes ha every individual has wo alleles of each gene and when gamees are produced, each gamee receives one of hese alleles. During ferilizaion, hese gamees randomly pair o produce four combinaions of alleles. F 2 Phenoypes and Genoypes Law of segregaion all F 1 cross all plan all plan all all Shor 3 1 wo organisms can look alike bu have differen underlying allele combinaions. Phenoypes and Genoypes he way an organism looks and behaves is called is phenoype. he allele combinaion an organism conains is known as is genoype. An organism s genoype can always be known by is phenoype. Phenoypes and Genoypes An organism is homozygous for a rai if is wo alleles for he rai are he same. he rue-breeding all plan ha had wo alleles for allness () would be homozygous for he rai of heigh. Phenoypes and Genoypes An organism is heerozygous for a rai if is wo alleles for he rai differ from each oher. herefore, he all plan ha had one allele for allness and one allele for shorness () is heerozygous for he rai of heigh. 5

6 Mendel s Dihybrid Crosses Mendel performed anoher se of crosses in which he used peas ha differed from each oher in wo rais raher han only one. Such a cross involving wo differen rais is called a dihybrid cross. he firs generaion Mendel ook rue-breeding pea plans ha had round yellow seeds (RRYY) and crossed hem wih rue-breeding pea plans ha had wrinkled green seeds (rryy). He already knew he round-seeded rai was dominan o he wrinkled-seeded rai. He also knew ha yellow was dominan o green. P 1 F 1 F 2 he firs generaion Dihybrid Cross Round yellow round yellow x wrinkled green Wrinkled green All round yellow Round yellow Round green Wrinkled yellow Wrinkled green he second generaion Mendel hen le he F 1 plans pollinae hemselves. He found some plans ha produced round yellow seeds and ohers ha produced wrinkled green seeds. He also found some plans wih round green seeds and ohers wih wrinkled yellow seeds. he second generaion He found hey appeared in a definie raio of phenoypes 9 round yellow: 3 round green: 3 wrinkled yellow: 1 wrinkled green. he law of independen assormen Mendel s second law saes ha genes for differen rais for example, seed shape and seed color are inheried independenly of each oher. his conclusion is known as he law of independen assormen. 6

7 Punne Squares In 1905, Reginald Punne, an English biologis, devised a shorhand way of finding he expeced proporions of possible genoypes in he offspring of a cross. his mehod is called a Punne square. Punne Squares If you know he genoypes of he parens, you can use a Punne square o predic he possible genoypes of heir offspring. Monohybrid crosses Heerozygous all paren Heerozygous all paren A Punne square for his cross is wo boxes all and wo boxes wide because each paren can produce wo kinds of gamees for his rai. Monohybrid crosses Heerozygous all paren Heerozygous all paren he wo kinds of gamees from one paren are lised on op of he square, and he wo kinds of gamees from he oher paren are lised on he lef side. Monohybrid crosses I doesn maer which se of gamees is on op and which is on he side. Each box is filled in wih he gamees above and o he lef side of ha box. You can see ha each box hen conains wo alleles one possible genoype. Afer he genoypes have been deermined, you can deermine he phenoypes. Gamees from RrYy paren Punne Square of Dihybrid Cross Gamees from RrYy paren RY Ry ry ry RRYY RRYy RrYY RrYy RY RRYy RRYy RrYy Rryy Ry RrYY RrYy rryy rryy ry RrYy Rryy rryy rryy ry Dihybrid crosses A Punne square for a dihybrid cross will need o be four boxes on each side for a oal of 16 boxes. 7

8 Gamees from RrYy paren Punne Square of Dihybrid Cross Gamees from RrYy paren RY Ry ry ry RRYY RRYy RrYY RrYy RY RRYy RRYy RrYy Rryy Ry RrYY RrYy rryy rryy ry RrYy Rryy rryy rryy ry Dihybrid crosses F 1 cross: RrYy RrYy round yellow round green wrinkled yellow wrinkled green Probabiliy In realiy you don ge he exac raio of resuls shown in he square. ha s because, in some ways, geneics is like flipping a coin i follows he rules of chance. he probabiliy or chance ha an even will occur can be deermined by dividing he number of desired oucomes by he oal number of possible oucomes. Probabiliy Probabiliy A Punne square can be used o deermine he probabiliy of geing a pea plan ha produces round seeds when wo plans ha are heerozygous (Rr) are crossed. R RR Rr R Rr rr r he Punne square shows hree plans wih round seeds ou of four oal plans, so he probabiliy is 3 / 4. r Probabiliy Punne Square R r RR Rr R Rr rr r I is imporan o remember ha he resuls prediced by probabiliy are more likely o be seen when here is a large number of offspring. Click image o view movie. 8

9 Secion Objecives Analyze how meiosis mainains a consan number of chromosomes wihin a species. Infer how meiosis leads o variaion in a species. Relae Mendel s laws of herediy o he evens of meiosis. Genes, Chromosomes, and Numbers Genes do no exis free in he nucleus of a cell; hey are lined up on chromosomes. ypically, a chromosome can conain a housand or more genes along is lengh. Diploid and haploid cells In he body cells of animals and mos plans, chromosomes occur in pairs. A cell wih wo of each kind of chromosome is called a diploid cell and is said o conain a diploid, or 2n, number of chromosomes. Diploid and haploid cells his pairing suppors Mendel s conclusion ha organisms have wo facors alleles for each rai. Organisms produce gamees ha conain one of each kind of chromosome. A cell conaining one of each kind of chromosome is called a haploid cell and is said o conain a haploid, or n, number of chromosomes. Diploid and haploid cells Chromosome Numbers of Common Organisms Organism Body Cell (2n) Gamee (n) Frui fly 8 4 Garden pea 14 7 Corn omao Leopard Frog Apple 34 Human 46 Chimpanzee 48 Dog 78 Adder s ongue fern his fac suppors Mendel s conclusion ha paren organisms give one allele for each rai o each of heir offspring. Diploid and haploid cells Chromosome Numbers of Common Organisms Organism Body Cell (2n) Gamee (n) Frui fly 8 4 Garden pea 14 7 Corn omao Leopard Frog Apple 34 Human 46 Chimpanzee 48 Dog 78 Adder s ongue fern his able shows he diploid and haploid number of chromosomes of some species. 9

10 Homologous chromosomes he wo chromosomes of each pair in a diploid cell are called homologous chromosomes. Each pair of homologous chromosomes has genes for he same rais. Homologous chromosomes On homologous Homologous Chromosome 4 chromosomes, hese a A genes are arranged in he same order, bu because erminal Axial here are differen Inflaed possible alleles for he same gene, he wo chromosomes in a D d Consriced homologous pair are no always idenical o Shor each oher. all Why meiosis? When cells divide by miosis, he new cells have exacly he same number and kind of chromosomes as he original cells. Imagine if miosis were he only means of cell division. Each pea plan paren, which has 14 chromosomes, would produce gamees ha conained a complee se of 14 chromosomes. Why meiosis? he F 1 pea plans would have cell nuclei wih 28 chromosomes, and he F 2 plans would have cell nuclei wih 56 chromosomes. Why meiosis? here mus be anoher form of cell division ha allows offspring o have he same number of chromosomes as heir parens. his kind of cell division, which produces gamees conaining half he number of chromosomes as a paren s body cell, is called meiosis. Why meiosis? Meiosis consiss of wo separae divisions, known as meiosis I and meiosis II. Meiosis I begins wih one diploid (2n) cell. By he end of meiosis II, here are four haploid (n) cells. 10

11 Why meiosis? hese haploid cells are called sex cells gamees. Male gamees are called sperm. Female gamees are called eggs. When a sperm ferilizes an egg, he resuling zygoe once again has he diploid number of chromosomes. Why meiosis? Haploid gamees (n=23) Egg Cell Ferilizaion Diploid zygoe (2n=46) Sperm Cell Miosis and Developmen Meiosis Meiosis Mulicellular diploid aduls (2n=46) his paern of reproducion, involving he producion and subsequen fusion of haploid sex cells, is called sexual reproducion. he Phases of Meiosis he Phases of Meiosis During meiosis, a spindle forms and he cyoplasm divides in he same ways hey do during miosis. However, wha happens o he chromosomes in meiosis is very differen. Click image o view movie. Inerphase Prophase I During inerphase, he cell replicaes is chromosomes. he chromosomes coil up and a spindle forms. Afer replicaion, each chromosome consiss of wo idenical siser chromaids, held ogeher by a cenromere. Inerphase As he chromosomes coil, homologous chromosomes line up wih each oher gene by gene along heir lengh, o form a four-par srucure called a erad. Prophase I 11

12 Prophase I he chromaids in a erad pair ighly. In fac, hey pair so ighly ha non-siser chromaids from homologous chromosomes can acually break and exchange geneic maerial in a process known as crossing over. Prophase I Prophase I Crossing over can occur a any locaion on a chromosome, and i can occur a several locaions a he same ime. Prophase I Prophase I Siser chromaids erad Homologous chromosomes Nonsiser chromaids Crossing over in erad Gamees I is esimaed ha during prophase I of meiosis in humans, here is an average of wo o hree crossovers for each pair of homologous chromosomes. Prophase I Siser chromaids erad Homologous chromosomes Nonsiser chromaids Crossing over in erad Gamees Crossing over resuls in new combinaions of alleles on a chromosome. Meaphase I During meaphase I, he cenromere of each chromosome becomes aached o a spindle fiber. he spindle fibers pull he erads ino he middle, or equaor, of he spindle. Meaphase I Anaphase I Anaphase I begins as homologous chromosomes, each wih is wo chromaids, separae and move o opposie ends of he cell. his criical sep ensures ha each new cell will receive only one chromosome from each homologous pair. Anaphase I 12

13 elophase I elophase I Evens occur in he reverse order from he evens of prophase I. he spindle is broken down, he chromosomes uncoil, and he cyoplasm divides o yield wo new cells. elophase I Each cell has half he geneic informaion of he original cell because i has only one chromosome from each homologous pair. elophase I he phases of meiosis II he second division in meiosis is simply a mioic division of he producs of meiosis I. Meiosis II consiss of prophase II, meaphase II, anaphase II, and elophase II. Meiosis II he phases of meiosis II During prophase II, a spindle forms in each of he wo new cells and he spindle fibers aach o he chromosomes. Prophase II he phases of meiosis II he phases of meiosis II he chromosomes, sill made up of siser chromaids, are pulled o he cener of he cell and line up randomly a he equaor during meaphase II. Meaphase II Anaphase II begins as he cenromere of each chromosome splis, allowing he siser chromaids o separae and move o opposie poles. Anaphase II 13

14 he phases of meiosis II Finally nuclei, reform, he spindles break down, and he cyoplasm divides during elophase II. he phases of meiosis II A he end of meiosis II, four haploid cells have been formed from one diploid cell. hese haploid cells will become gamees, ransmiing he genes hey conain o offspring. elophase II Meiosis Provides for Geneic Variaion Cells ha are formed by miosis are idenical o each oher and o he paren cell. Crossing over during meiosis, however, provides a way o rearrange allele combinaions. hus, variabiliy is increased. Geneic recombinaion Reassormen of chromosomes and he geneic informaion hey carry, eiher by crossing over or by independen segregaion of homologous chromosomes, is called geneic recombinaion. MEIOSIS I MEIOSIS II Geneic recombinaion I is a major source of variaion among organisms. Meiosis explains Mendel s resuls he segregaion of chromosomes in anaphase I of meiosis explains Mendel s observaion ha each paren gives one allele for each rai a random o each offspring, regardless of wheher he allele is expressed. Possible gamees Possible gamees Chromosome A Chromosome B Chromosome a Chromosome b 14

15 Meiosis explains Mendel s resuls he segregaion of chromosomes a random during anaphase I also explains how facors, or genes, for differen rais are inheried independenly of each oher. Nondisjuncion he failure of homologous chromosomes o separae properly during meiosis is called nondisjuncion. Nondisjuncion Recall ha during meiosis I, one chromosome from each homologous pair moves o each pole of he cell. In nondisjuncion, boh chromosomes of a homologous pair move o he same pole of he cell. Nondisjuncion he effecs of nondisjuncion are ofen seen afer gamees fuse. When a gamee wih an exra chromosome is ferilized by a normal gamee, he zygoe will have an exra chromosome. his condiion is called risomy. Nondisjuncion Alhough organisms wih exra chromosomes ofen survive, organisms lacking one or more chromosomes usually do no. When a gamee wih a missing chromosome fuses wih a normal gamee during ferilizaion, he resuling zygoe lacks a chromosome. Nondisjuncion An example of monosomy ha is no lehal is urner syndrome, in which human females have only a single X chromosome insead of wo. his condiion is called monosomy. 15

16 Male paren (2n) Female paren (2n) Meiosis Nondisjuncion Nondisjuncion Abnormal gamee (2n) Meiosis Nondisjuncion Abnormal gamee (2n) Zygoe (4n) Nondisjuncion When a gamee wih an exra se of c chromosomes is ferilized by a normal haploid gamee, he offspring has hree ses of chromosomes and is riploid. he fusion of wo gamees, each wih an exra se of chromosomes, produces offspring wih four ses of chromosomes a eraploid. Chromosome Mapping Crossing over produces new allele combinaions. Geneiciss use he frequency of crossing over o map he relaive posiions of genes on a chromosome. A 50 B A 10 D B 5 C or or D 10 A C 5 B D 35 C or C 35 D Chromosome Mapping Genes ha are farher apar on a chromosome are more likely o have crossing over occur beween hem han are genes ha are closer ogeher. A 50 B A 10 D B 5 C or or D 10 A C 5 B D 35 C or C 35 D Chromosome Mapping Suppose here are four genes A, B, C, and D on a chromosome. A 10 D 35 C 5 B 50 Chromosome Mapping Geneiciss deermine ha he frequencies of recombinaion among hem are as follows: beween A and B 50%; beween A and D 10%; beween B and C 5%; beween C and D 35%. he recombinaion frequencies can be convered o map unis: A-B = 50; A-D = 10; B-C = 5; C-D =

17 Chromosome Mapping hese map unis are no acual disances on he chromosome, bu hey give relaive disances beween genes. Geneiciss line up he genes as shown. Chromosome Mapping he genes can be arranged in he sequence ha reflecs he recombinaion daa. his sequence is a chromosome map. A 10 D 35 C 5 B A 10 D 35 C 5 B Polyploidy Polyploidy Organisms wih more han he usual number of chromosome ses are called polyploids. Polyploidy is rare in animals and almos always causes deah of he zygoe. However, polyploidy frequenly occurs in plans. Many polyploid plans are of grea commercial value. Gene Linkage and Maps If genes are close ogeher on he same chromosome, hey usually are inheried ogeher. hese genes are said o be linked. Gene Linkage and Maps Linked genes may become separaed on differen homologous chromosomes as a resul of crossing over. When crossing over produces new gene combinaions, geneiciss can use he frequencies of hese new gene combinaions o make a chromosome map showing he relaive locaions of he genes. 17