* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download video slide
Genome evolution wikipedia , lookup
Artificial gene synthesis wikipedia , lookup
Gene expression profiling wikipedia , lookup
Quantitative trait locus wikipedia , lookup
Polycomb Group Proteins and Cancer wikipedia , lookup
Minimal genome wikipedia , lookup
Ridge (biology) wikipedia , lookup
Designer baby wikipedia , lookup
Biology and consumer behaviour wikipedia , lookup
Dominance (genetics) wikipedia , lookup
Gene expression programming wikipedia , lookup
Skewed X-inactivation wikipedia , lookup
Epigenetics of human development wikipedia , lookup
Genomic imprinting wikipedia , lookup
Microevolution wikipedia , lookup
Genome (book) wikipedia , lookup
Y chromosome wikipedia , lookup
Neocentromere wikipedia , lookup
Chapter 15 • The Chromosomal Basis of Inheritance Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Genes – Located on chromosomes Figure 15.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Researchers proposed in the early 1900s that genes are located on chromosomes • Behavior of chromosomes during meiosis accounts for Mendel’s laws of segregation and independent assortment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Chromosome theory of inheritance – Mendelian genes have specific loci on chromosomes – Chromosomes undergo segregation and independent assortment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chromosomal basis of Mendel’s laws P Generation Yellow-round Starting with two true-breeding pea plants, we follow two genes through the F1 and F2 generations. The two genes specify seed color (allele Y for yellow and allele y for green) and seed shape (allele R for round and allele r for wrinkled). These two genes are on different chromosomes. (Peas have seven chromosome pairs, but only two pairs are illustrated here.) seeds ( Green-wrinkled YYRR) seeds Y R Y y r R (yyrr) r y Meiosis Fertilization y Y R Gametes r All F1 plants produce yellow-round seeds R R y F1 Generation (YyRr) y r r Y Y Meiosis LAW OF SEGREGATION r R Y 1 The R and r alleles segregate at anaphase I, yielding two types of daughter cells for this locus. R y r R y Y y Y r y R R R Y Y r r 1 4 YR Y r r yr 2 Each gamete gets a long and a short chromosome in one of four allele combinations. y y Y Y ASSORTMENT Alleles at both loci segregate in anaphase I, yielding four types of daughter cells depending on the chromosome arrangement at metaphase I. Compare the arrangement of the R and r alleles in the cells on the left and right Metaphase II 1 4 3 Fertilization recombines the R and r alleles at random. y 1 r Y F2 Generation Y LAW OF INDEPENDENT r R Gametes R Anaphase I Y 2 Each gamete gets one long chromosome with either the R or r allele. Two equally probable arrangements of chromosomes at metaphase I r 1 4 yr y y R R 1 4 yR Fertilization among the F1 plants 9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings :3 :3 :1 3 Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. • Thomas Hunt Morgan – Convincing evidence that chromosomes are the location of Mendel’s heritable factors Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan’s Choice of Experimental Organism • Fruit flies (Drosophila) – Breed at a high rate – New generation every two weeks – 4 pairs of chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Morgan observed – Wild type, or normal, phenotypes (common in fly populations) • Alternatives to the wild type – Called mutant phenotypes Figure 15.3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) – F1 generation all had red eyes – F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Morgan determined that the white-eye mutant allele must be located on the X chromosome EXPERIMENT Morgan mated a wild-type (red-eyed) female with a mutant white-eyed male. The F1 offspring all had red eyes. P Generation X F1 Generation Morgan then bred an F1 red-eyed female to an F1 red-eyed male to produce the F2 generation. RESULTS The F2 generation showed a typical Mendelian 3:1 ratio of red eyes to white eyes. However, no females displayed the white-eye trait; they all had red eyes. Half the males had white eyes, and half had red eyes. F2 Generation Figure 15.4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CONCLUSION Since all F offspring had red eyes, the mutant 1 white-eye trait (w) must be recessive to the wild-type red-eye trait (w+). Since the recessive trait—white eyes—was expressed only in males in the F2 generation, Morgan hypothesized that the eye-color gene is located on the X chromosome and that there is no corresponding locus on the Y chromosome, as diagrammed here. P Generation W+ X X X X Y W+ W+ W W+ W Ova (eggs) F1 Generation Sperm W+ W W+ Ova (eggs) F2 Generation Sperm W+ W W+ W+ W+ W W W+ Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The first solid evidence indicating that a specific gene is associated with a specific chromosome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan’s symbols • The gene takes its symbol from the first mutant discovered • A (+) represents the wild allele • e.g. white eye allele is w red eye allele is w+ Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Linked genes • Linked genes tend to be inherited together located near each other on same chromosome, chromosome has hundreds or thousands of genes • 30,000 – 40,000 human genes/ genome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Linked genes P Generation (homozygous) EXPERIMENT Morgan first mated true-breeding wild-type flies with black, vestigial-winged flies to produce heterozygous F1 dihybrids, all of which are wild-type in appearance. He then mated wild-type F1 dihybrid females with black, vestigial-winged males, producing 2,300 F2 offspring, which he “scored” (classified according to phenotype). Wild type (gray body, normal wings) Double mutant (black body, vestigial wings) x Double mutant (black body, vestigial wings) b b vg vg b+ b+ vg+ vg+ F1 dihybrid (wild type) (gray body, normal wings) Double mutant (black body, vestigial wings) Double mutant TESTCROSS (black body, x vestigial wings) b b vg vg CONCLUSION If these two genes were on different chromosomes, the alleles from the F1 dihybrid would sort into gametes independently, and we would expect to see equal numbers of the four types of offspring. If these two genes were on the same chromosome, we would expect each allele combination, B+ vg+ and b vg, to stay together as gametes formed. In this case, only offspring with parental phenotypes would be produced. Since most offspring had a parental phenotype, Morgan concluded that the genes for body color and wing size are located on the same chromosome. However, the production of a small number of offspring with nonparental phenotypes indicated that some mechanism occasionally breaks the linkage between genes on the same chromosome. Figure 15.5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings b+ b vg+ vg RESULTS b vg b+ vg b vg+ 965 Wild type (gray-normal) 944 Blackvestigial 206 Grayvestigial 185 Blacknormal b+ b vg+ vg b b vg vg b+vg+ b vg Sperm Parental-type offspring b+ b vg vg b b vg+ vg Recombinant (nonparental-type) offspring • Morgan : – Linked genes do not assort independently – Unlinked genes are either on separate chromosomes or are far apart on the same chromosome, assort independently b+ vg+ Parents in testcross Most offspring X b vg b vg b vg b+ vg+ b vg or b vg Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings b vg Recombination of Unlinked Genes: Independent Assortment of Chromosomes • Mendel – Some offspring have combinations of traits that do not match either parent Gametes from yellow-round heterozygous parent (YyRr) YR Gametes from greenwrinkled homozygous recessive parent (yyrr) yr Yr yR Yyrr yyRr yr YyRr yyrr Parentaltype offspring Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recombinant offspring • Recombinant offspring – Show new combinations of the parental traits • When 50% of all offspring are recombinants – 50% frequency of recombination Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recombination of Linked Genes: Crossing Over • Morgan discovered linked genes – But due to appearance of recombinant phenotypes, the linkage incomplete Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Morgan: – Answer: Crossing over of homologous chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Linked genes – Recombination frequencies less than 50% Testcross parents b+ vg+ Gray body, normal wings b vg (F1 dihybrid) Replication of chromosomes b+ vg Meiosis I: Crossing over between b and vg loci produces new allele combinations. Black body, vestigial wings b vg (double mutant) Replication of chromosomes b vg b+vg+ vg b b vg vg b b vg b vg Meiosis II: Segregation of chromatids produces recombinant gametes with the new allele combinations. Gametes b vg Meiosis I and II: Even if crossing over occurs, no new allele combinations are produced. Recombinant chromosome Ova Sperm b+vg+ b vg b+ vg b vg+ b vg b+ vg+ Testcross offspring Sperm b vg Figure 15.6 b vg 944 965 BlackWild type (gray-normal) vestigial b+ vg+ b vg+ b vg b vg b+ vg 206 Grayvestigial b+ vg+ b vg b vg+ Ova 185 BlackRecombination normal b vg+ frequency b vg Parental-type offspring Recombinant offspring Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 391 recombinants = 2,300 total offspring 100 = 17% • A genetic map – Ordered list of the genetic loci along a particular chromosome – Developed using recombination frequencies Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Linkage map – Actual map of a chromosome based on recombination frequencies APPLICATION A linkage map shows the relative locations of genes along a chromosome. TECHNIQUE A linkage map is based on the assumption that the probability of a crossover between two genetic loci is proportional to the distance separating the loci. The recombination frequencies used to construct a linkage map for a particular chromosome are obtained from experimental crosses, such as the cross depicted in Figure 15.6. The distances between genes are expressed as map units (centimorgans), with one map unit equivalent to a 1% recombination frequency. Genes are arranged on the chromosome in the order that best fits the data. RESULTS In this example, the observed recombination frequencies between three Drosophila gene pairs (b–cn 9%, cn–vg 9.5%, and b–vg 17%) best fit a linear order in which cn is positioned about halfway between the other two genes: Recombination frequencies 9.5% 9% 17% Chromosome b cn vg The b–vg recombination frequency is slightly less than the sum of the b–cn and cn–vg frequencies because double crossovers are fairly likely to occur between b and vg in matings tracking these two genes. A second crossover Figure 15.7 would “cancel out” the first and thus reduce the observed b–vg recombination frequency. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The farther apart genes are on a chromosome the more likely they are to be separated during crossing over Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Many fruit fly genes were mapped using recombination frequencie I Y II X IV III Mutant phenotypes Short aristae Black body 0 Figure 15.8 Long aristae (appendages on head) Cinnabar Vestigial eyes wings 48.5 57.5 67.0 Gray body Red eyes Normal wings Wild-type phenotypes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Brown eyes 104.5 Red eyes Sex determination • Humans and other mammals – 2 sex chromosomes, X and Y 44 + XY 22 + X Sperm 44 + XX (a) The X-Y system Figure 15.9a Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 44 + XX Parents 22 + Y 22 + XY Ova Zygotes (offspring) 44 + XY • Different systems: 22 + XX 22 + X 76 + ZW 76 + ZZ (b) The X–0 system (c) The Z–W system Figure 15.9b–d 16 16 (Diploid) (Haploid) (d) The haplo-diploid system Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Sex chromosomes – Have genes for many characters unrelated to sex Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Sex-linked genes (on sex chromosomes) XaY XAXA (a) A father with the disorder will transmit the mutant allele to all daughters but to no sons. When the mother is a dominant Sperm Xa Y homozygote, the daughters will have the normal phenotype but will be carriers of Ova XA XAXa XAY the mutation. A a A XA X Y X Y XAXa (b) If a carrier mates with a male of normal phenotype, there is a 50% chance that each daughter will be a carrier like her mother, and a 50% chance that each son will have the disorder. XA XAY Y Sperm Ova XA XAXA XAY Xa XaYA XaY (c) If a carrier mates with a male who has the disorder, there is a 50% chance that each child born to them will have the disorder, regardless of sex. Daughters who do not have the disorder will be carriers, where as males without the disorder will be completely free of the recessive allele. Figure 15.10a–c Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings XAXa XaY Sperm Xa Y Ova XA XAXa XAY Xa XaYa XaY • Sex-linked disorders – Color blindness – Duchenne muscular dystrophy – Hemophilia Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings X inactivation in Female Mammals • One of the two X chromosomes in each cell is randomly inactivated during embryonic development Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • If a female is heterozygous for a particular gene located on the X chromosome mosaic for that character Two cell populations in adult cat: Active X Early embryo: X chromosomes Cell division Inactive X and X chromosome Inactive X inactivation Orange fur Black fur Allele for black fur Active X Figure 15.11 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Alterations of chromosome number or structure cause some genetic disorders • Can lead to spontaneous abortions or cause a variety of developmental disorders Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Abnormal Chromosome Number • e.g. nondisjunction – Chromosomes do not separate normally during meiosis – Gametes contain 2 copies or no copies of a particular chromosome Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n+1 Figure 15.12a, b n+1 n1 n+1 n –1 n–1 Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings n n (b) Nondisjunction of sister chromatids in meiosis II • Aneuploidy – Fertilization of gametes in which nondisjunction occurred – Abnormal chromosome number Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • trisomic zygote – 3 copies of a particular chromosome • monosomic – 1 copy Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Polyploidy – More than two complete sets of chromosomes Figure 15.13 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Alterations of chromosome structure (a) A deletion removes a chromosomal segment. (b) A duplication repeats a segment. (c) An inversion reverses a segment within a chromosome. (d) A translocation moves a segment from one chromosome to another, nonhomologous one. In a reciprocal translocation, the most common type, nonhomologous chromosomes exchange fragments. Nonreciprocal translocations also occur, in which a chromosome transfers a fragment without receiving a fragment in return. A B C D E F G H A B C D E F G H A B C D E F G H A B C D E F G H Deletion Duplication Inversion A B C E F G H A B C B C D E A D C B E F G H M N O C D E Reciprocal translocation M N O P Q Figure 15.14a–d Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings R A B P Q F G H R F G H Human Disorders Due to Chromosomal Alterations Trisomy 18 Trisomy 13 Deletion 3p syndrome Duplication 9p syndrome …………………… Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Down syndrome – Is usually the result of an extra chromosome 21, trisomy 21 Figure 15.15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Aneuploidy of Sex Chromosomes • Nondisjunction of sex chromosomes – Produces a variety of aneuploid conditions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Klinefelter syndrome – XXY male • Turner syndrome – monosomy X, (X0), female • XYY, XXXY, XXX, etc Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Disorders Caused by Structurally Altered Chromosomes • Cri du chat – Deletion in a chromosome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Certain cancers caused by translocationss Normal chromosome 9 Reciprocal translocation Translocated chromosome 9 Philadelphia chromosome Normal chromosome 22 Figure 15.16 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Translocated chromosome 22 • Genomic imprinting: phenotype depends on which parent gene comes from – Silencing of certain genes that are “stamped” with an imprint during gamete production Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance of Organelle Genes • Extranuclear genes found in organelles in the cytoplasm, e.g. mitochondrial genes “mitochondrial eve” Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Chloroplast or mitochondrial genes – Inheritance depends on the maternal parent zygote’s cytoplasm comes from the egg Figure 15.18 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings