孟德尔及其基因观念.ppt

Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,Mendel and the Gene Idea,孟德爾及其基因觀念,Overview:Drawing from the Deck of Genes,What,genetic principles,account for the passing of traits from,parents,to,offspring,?,The“,blending”hypothesis,is the idea that genetic material from the two parents blends together(like blue and yellow paint blend to make green),混生假說,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,The“,particulate”hypothesis,is the idea that parents pass on discrete heritable units(genes),Mendel,documented a particulate mechanism through his experiments with,garden peas,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Fig.14-1,Mendels Experimental,Quantitative Approach,Advantages of pea plants for genetic study:,There are many varieties with distinct heritable features,or,characters,特徵,(such as flower color);character variants(such as purple or white flowers)are called,traits,性狀,Mating of plants can be controlled,Each pea plant has sperm-producing organs(,stamens,)and egg-producing organs(,carpels,),Cross-pollination,(fertilization between different plants)can be achieved by dusting one plant with pollen from another,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Fig.14-2,TECHNIQUE,RESULTS,Parental,generation,(P),Stamens,Carpel,1,2,3,4,First,filial,gener-,ation,offspring,(F,1,),5,Fig.14-2a,Stamens,Carpel,Parental,generation,(P),TECHNIQUE,1,2,3,4,Mendel chose to track only those characters that varied in an,either-or manner,He also used varieties that were,true-breeding,(plants that produce offspring of the same variety when they self-pollinate),純系培養,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,In a typical experiment,Mendel mated two contrasting,true-breeding varieties,a process called,hybridization,雜交,The true-breeding parents are the,P generation,親代,The hybrid offspring of the P generation are called the,F,1,generation,第一子代,When F,1,individuals self-pollinate,the,F,2,generation,is produced,第二子代,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Fig.14-3-1,EXPERIMENT,P Generation,(true-breeding,parents),Purple,flowers,White,flowers,Fig.14-3-3,EXPERIMENT,P Generation,(true-breeding,parents),Purple,flowers,White,flowers,F,1,Generation,(hybrids),All plants had,purple flowers,F,2,Generation,705 purple-flowered,plants,224 white-flowered,plants,Mendel reasoned that,only the purple flower factor was affecting flower color,in the F,1,hybrids,Mendel called the purple flower color a,dominant trait,and the white flower color a,recessive trait,Mendel observed the same pattern of inheritance in six other pea plant characters,each represented by two traits,What Mendel called a“,heritable factor,”,遺傳因子,is what we now call a,gene,基因,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Mendels Model,Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F,2,offspring,Four related concepts make up this model,These concepts can be related to what we now know about,genes,and,chromosomes,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,The,first concept,is that,alternative versions,of genes,account for variations in inherited characters,For example,the gene for flower color in pea plants exists in two versions,one for purple flowers and the other for white flowers,These alternative versions of a gene are now called,alleles,對偶,Each gene resides at a,specific locus,on a,specific chromosome,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Fig.14-4,Allele for purple flowers,Homologous,pair of,chromosomes,Locus for flower-color gene,Allele for white flowers,The,second concept,is that for each character an organism inherits,two alleles,one from each parent,Mendel made this deduction without knowing about the role of chromosomes,The two alleles at a locus on a chromosome may be identical,as in the true-breeding plants of Mendels P generation,Alternatively,the two alleles at a locus may differ,as in the F,1,hybrids,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,The,third concept,is that if the two alleles at a locus differ,then one(the,dominant allele,)determines the organisms appearance,and the other(the,recessive allele,)has no noticeable effect on appearance,In the flower-color example,the F,1,plants had purple flowers because the allele for that trait is dominant,顯性性狀表現 隱性性狀不表現,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Mendels segregation model accounts for the 3:1 ratio he observed in the F,2,generation of his numerous crosses,The possible combinations of sperm and egg can be shown using a,Punnett,square,a diagram for predicting the results of a genetic cross between individuals of known genetic makeup,A,capital letter,represents a dominant allele,and a,lowercase letter,represents a recessive allele,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Fig.14-5-1,P Generation,Appearance:,Genetic makeup:,Gametes:,Purple flowers,White flowers,PP,P,pp,p,Fig.14-5-2,P Generation,Appearance:,Genetic makeup:,Gametes:,Purple flowers,White flowers,PP,P,pp,p,F,1,Generation,Gametes:,Genetic makeup:,Appearance:,Purple flowers,Pp,P,p,1,/,2,1,/,2,Useful Genetic Vocabulary,An organism with two identical alleles for a character is said to be,homozygous,for the gene controlling that character,同質基因,An organism that has two different alleles for a gene is said to be,heterozygous,for the gene controlling that character,異質基因,Unlike,homozygotes,heterozygotes,are not true-breeding,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Because of the different effects of dominant and recessive alleles,an organisms,traits do not always reveal its genetic composition,Therefore,we distinguish between an organisms,phenotype,表型,or physical appearance,and its,genotype,基因型,or genetic makeup,In the example of flower color in pea plants,PP,and,Pp,plants have the same phenotype(purple)but different genotypes,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,The Testcross,試交,How can we tell the genotype of an individual with the dominant phenotype?,Such an individual must have one dominant allele,but the individual could be either homozygous dominant or heterozygous,The answer is to carry out a,testcross,:breeding the,mystery individual with a,homozygous recessive individual,If any offspring display the recessive phenotype,the mystery parent must be heterozygous,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Fig.14-7,TECHNIQUE,RESULTS,Dominant phenotype,unknown genotype:,PP,or,Pp,?,Predictions,Recessive phenotype,known genotype:,pp,If,PP,If,Pp,or,Sperm,Sperm,p,p,p,p,P,P,P,p,Eggs,Eggs,Pp,Pp,Pp,Pp,Pp,Pp,pp,pp,or,All offspring purple,1,/,2,offspring purple and,1,/,2,offspring white,Fig.14-7a,Dominant phenotype,unknown genotype:,PP,or,Pp,?,Predictions,Recessive phenotype,known genotype:,pp,If,PP,If,Pp,or,Sperm,Sperm,p,p,p,p,P,P,P,p,Eggs,Eggs,Pp,Pp,Pp,Pp,Pp,Pp,pp,pp,TECHNIQUE,Fig.14-7b,RESULTS,All offspring purple,or,1,/,2,offspring purple and,1,/,2,offspring white,The Law of Independent Assortment,獨立分配率,Mendel derived the law of segregation by following a single character,The F,1,offspring produced in this cross were,monohybrids,individuals that are heterozygous for one character,A cross between such heterozygotes is called a,monohybrid cross,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Mendel identified his second law of inheritance by following two characters at the same time,Crossing two true-breeding parents differing in,two characters,produces,dihybrids,in the F,1,generation,heterozygous for both characters,A,dihybrid,cross,a cross between F,1,dihybrids,can determine whether two characters are transmitted to offspring as a,package or independently,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Fig.14-8,EXPERIMENT,RESULTS,P Generation,F,1,Generation,Predictions,Gametes,Hypothesis of,dependent,assortment,YYRR,yyrr,YR,yr,YyRr,Hypothesis of,independent,assortment,or,Predicted,offspring of,F,2,generation,Sperm,Sperm,YR,YR,yr,yr,Yr,YR,yR,Yr,yR,yr,YR,YYRR,YYRR,YyRr,YyRr,YyRr,YyRr,YyRr,YyRr,YYRr,YYRr,YyRR,YyRR,YYrr,Yyrr,Yyrr,yyRR,yyRr,yyRr,yyrr,yyrr,Phenotypic ratio 3:1,Eggs,Eggs,Phenotypic ratio 9:3:3:1,1,/,2,1,/,2,1,/,2,1,/,2,1,/,4,yr,1,/,4,1,/,4,1,/,4,1,/,4,1,/,4,1,/,4,1,/,4,1,/,4,3,/,4,9,/,16,3,/,16,3,/,16,1,/,16,Phenotypic ratio approximately 9:3:3:1,315,108,101,32,Fig.14-8a,EXPERIMENT,P Generation,F,1,Generation,Predictions,Gametes,Hypothesis of,dependent,assortment,YYRR,yyrr,YR,yr,YyRr,Hypothesis of,independent,assortment,or,Predicted,offspring of,F,2,generation,Sperm,Sperm,YR,YR,yr,yr,Yr,YR,yR,Yr,yR,yr,YR,YYRR,YYRR,YyRr,YyRr,YyRr,YyRr,YyRr,YyRr,YYRr,YYRr,YyRR,YyRR,YYrr,Yyrr,Yyrr,yyRR,yyRr,yyRr,yyrr,yyrr,Phenotypic ratio 3:1,Eggs,Eggs,Phenotypic ratio 9:3:3:1,1,/,2,1,/,2,1,/,2,1,/,2,1,/,4,yr,1,/,4,1,/,4,1,/,4,1,/,4,1,/,4,1,/,4,1,/,4,1,/,4,3,/,4,9,/,16,3,/,16,3,/,16,1,/,16,Fig.14-8b,RESULTS,Phenotypic ratio approximately 9:3:3:1,315,108,101,32,Using a,dihybrid,cross,Mendel developed the,law of independent assortment,The law of independent assortment states that each pair of,alleles segregates independently,of each other pair of alleles during gamete formation,Strictly speaking,this law applies only to genes on different,nonhomologous,chromosomes,Genes located near each other,on the same chromosome,tend to be,inherited together,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Concept 14.2:The laws of probability govern Mendelian inheritance,Mendels laws of,segregation,and,independent assortment,reflect the rules of probability,When tossing a coin,the outcome of one toss has no impact on the outcome of the next toss,In the same way,the alleles of one gene segregate into gametes independently of another genes alleles,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities,Probability in an F,1,monohybrid cross can be determined using the multiplication rule,Segregation in a heterozygous plant is like flipping a coin:Each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele,The Multiplication and Addition Rules Applied to Monohybrid Crosses,加乘法則,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Fig.14-9,Rr,Rr,Segregation of,alleles into eggs,Sperm,R,R,R,R,R,R,r,r,r,r,r,r,1,/,2,1,/,2,1,/,2,1,/,2,Segregation of,alleles into sperm,Eggs,1,/,4,1,/,4,1,/,4,1,/,4,The,rule of addition,states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities,The rule of addition can be used to figure out the probability that an F,2,plant from a monohybrid cross will be heterozygous rather than homozygous,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Solving Complex Genetics Problems with the Rules of Probability,We can apply the multiplication and addition rules to,predict,the outcome of crosses involving multiple characters,A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously,In calculating the chances for various genotypes,each character is considered separately,and then the individual probabilities are multiplied together,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Fig.14-UN1,Concept 14.3:Inheritance patterns are often more complex than predicted by simple Mendelian genetics,The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied,Many heritable characters are not determined by only one gene with two alleles,However,the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Extending Mendelian Genetics for a Single Gene,Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:,When alleles are not completely dominant or recessive,When a gene has more than two alleles,When a gene produces multiple phenotypes,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Degrees of Dominance,Complete dominance,occurs when phenotypes of the heterozygote and dominant homozygote are identical,In,incomplete dominance,the phenotype of F,1,hybrids is somewhere between the phenotypes of the two parental varieties,In,codominance,two dominant alleles affect the phenotype in separate,distinguishable ways,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Fig.14-10-1,Red,P Generation,Gametes,White,C,R,C,R,C,W,C,W,C,R,C,W,Fig.14-10-2,Red,P Generation,Gametes,White,C,R,C,R,C,W,C,W,C,R,C,W,F,1,Generation,Pink,C,R,C,W,C,R,C,W,Gametes,1,/,2,1,/,2,Fig.14-10-3,Red,P Generation,Gametes,White,C,R,C,R,C,W,C,W,C,R,C,W,F,1,Generation,Pink,C,R,C,W,C,R,C,W,Gametes,1,/,2,1,/,2,F,2,Generation,Sperm,Eggs,C,R,C,R,C,W,C,W,C,R,C,R,C,R,C,W,C,R,C,W,C,W,C,W,1,/,2,1,/,2,1,/,2,1,/,2,A dominant allele does not subdue a recessive allele;alleles dont interact,Alleles are simply variations in a genes nucleotide sequence,For any character,dominance/,recessiveness,relationships of alleles depend on the level at which we examine the phenotype,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,The Relation Between Dominance and,Phenotype,Tay-Sachs,disease,is fatal;a dysfunctional enzyme causes an accumulation of lipids in the brain,At the,organismal,level,the allele is recessive,At the,biochemical,level,the phenotype(i.e.,the enzyme activity level)is incompletely dominant,At the,molecular,level,the alleles are codominant,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Frequency of Dominant Alleles,Dominant alleles are not necessarily more common in populations than recessive alleles,For example,one baby out of 400 in the United States is born with extra fingers or toes,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage,In this example,the recessive allele is far more prevalent than the populations dominant allele,Copyright 2008 Pearson Education Inc.,publishing as Pearson Benjamin Cummings,Multiple Alleles,Most genes e