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The Chromosomal Basis of Inheritance PowerPoint Lectures for Biology, Seventh Edition Chapter 15 Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Overview: Locating Genes on Chromosomes • Genes – Are located on chromosomes – Can be visualized using certain techniques Figure 15.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes • Several researchers proposed in the early 1900s that genes are located on chromosomes • The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The chromosomal basis of Mendel’s laws P Generation Starting with two true-breeding pea plants, we follow two genes through the F 1 and F2 generations. The two genes specify seed color (allele Y for yellow and allele y for Y 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.) Yellow-round seeds (YYRR) Green-wrinkled seeds (yyrr) Y R r R r y Meiosis Fertilization y R Y Gametes y r All F1 plants produce yellow-round seeds (YyRr) R R y F1 Generation y r r Y Y Meiosis LAW OF SEGREGATION r R Y 1 The R and r alleles segregate R at anaphase I, yielding two types of daughter cells for this locus. Y y Y y LAW OF INDEPENDENT ASSORTMENT r y Y 1 Alleles at both loci segregate in anaphase I, yielding four types of daughter cells R depending on the chromosome arrangement at metaphase I. Compare the arrangement of the R and r alleles in the cells y on the left and right r R y y Metaphase II Y y Y Y R R r Y r r 1 yr 4 F2 Generation 3 Fertilization recombines the R and r alleles at random. Y Y r 1 YR 4 Figure 15.2 R Anaphase I r Y Gametes r r R 2 Each gamete gets one long chromosome with either the R or r allele. Two equally probable arrangements of chromosomes at metaphase I 1 yr 4 2 Each gamete gets a long and a short chromosome in one of four allele combinations. y y R R 1 yR 4 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. Scientists build on the discoveries of their predecessors: mid-1800’s Gregor Mendel - Laws of Inheritance; named "factors" as hereditary information 1879- Walter Fleming - stained cells and identified chromosomes 1902 Walter Sutton- "Chromosomal Theory of Heredity" • chromosomes are the carriers of traits • each chromosome carries many traits Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Scientists build on the discoveries of their predecessors: 1905 E.B Wilson - American biologist identified sex chromosomes in insects Human: total 23 pairs of chromosomes • 1 pair of sex chromosomes XX or XY; (inherit 1 from each parent) • your 22 other pairs are called autosomes, the body chromosomes that carry most of your traits All the chromosomes of an individual cell can be visualize with a karyotype. how to make one and what they look like Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Chromosomal Basis of Sex • An organism’s sex – Is an inherited phenotypic character determined by the presence or absence of certain chromosomes • In humans and other mammals – There are two varieties of sex chromosomes, X and Y 44 + XY 22 + Sperm X 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 of sex determination – Are found in other organisms 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 Inheritance of Sex-Linked Genes • The sex chromosomes – Have genes for many characters unrelated to sex • A gene located on either sex chromosome – Is called a sex-linked gene Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan’s Experimental Evidence: Scientific Inquiry • Thomas Hunt Morgan – Provided convincing evidence that chromosomes are the location of Mendel’s heritable factors won the Nobel Prize in Physiology or Medicine 1933 "for his discoveries concerning the role played by the chromosome in heredity" Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan’s Choice of Experimental Organism • Morgan worked with fruit flies Drosophila melanogaster – Because they breed at a high rate – A new generation can be bred every two weeks – They have only four pairs of chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Morgan first observed and noted – Wild type, or normal, phenotypes that were common in the fly populations • Traits alternative to the wild type – Are called mutant phenotypes Figure 15.3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair • In one experiment Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) – The F1 generation all had red eyes – The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes mutant wt 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. CONCLUSION 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 Since all F1 offspring had red eyes, the mutant 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. • Morgan’s discovery that transmission of the X chromosome in fruit flies correlates with inheritance of the eye-color trait – Was the first solid evidence indicating that a specific gene is associated with a specific chromosome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Some recessive alleles found on the X chromosome in humans cause certain types of disorders – Color blindness – Duchenne muscular dystrophy – Hemophilia Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan did other experiments with fruit flies – To see how the inheritance of two different characters is affected by gene location • Morgan determined that – Genes that are close together on the same chromosome are linked and do not assort independently – Unlinked genes are either on separate chromosomes of are far apart on the same chromosome and 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 • When Mendel followed the inheritance of two characters – He observed that some offspring have combinations of traits that do not match either parent in the P generation Gametes from yellow-round heterozygous parent (YyRr) Independent assortment Gametes from greenwrinkled homozygous recessive parent (yyrr) YR yr Yr yR Yyrr yyRr yr YyRr yyrr Parentaltype offspring Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recombinant offspring Due to the appearance of recombinant phenotypes, – Some process must occasionally break the physical connection between genes on the same chromosome – Crossing over of homologous chromosomes was the mechanism behind the recombination – Crossover animation, narrated Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • “Recombinant “offspring – Are those that show new combinations of the parental traits • When 50% of all offspring are recombinants – Geneticists say that there is a 50% frequency of recombination Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene Linkage • Each chromosome – Has hundreds or thousands of genes • Linked genes tend to be inherited together because they are located near each other on the same chromosome • http://highered.mcgrawhill.com/sites/dl/free/0072835125 /126997/animation5.html Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Linked genes – Exhibit 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% Linkage Mapping: Using Recombination Data: Scientific Inquiry • A genetic map – Is an ordered list of the genetic loci along a particular chromosome – Can be developed using recombination frequencies Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A linkage map – Is the 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 Crossing Over is a normal part of meiotic division • The farther apart genes are on a chromosome – The more likely they are to be separated during crossing over CROSSING OVER: alleles on chromosomes become rearranged during meiosis (click for animation) "Chromosome Mapping" is now possible. The crossover % is directly related to a gene's position on the chromosome. example: the relative abundance of four genes on a chromosome can be mapped from the following data on crossover frequencies: GENES Frequency of Crossover B and D 5% C and A 15% A and B 30% C and B 45% C and D 50% Which of the following represents the relative positions of the four genes on the chromosome? a. ABCD b. ADCB c. CABD d. CBAD e. DBCA HUMAN: chromosome maps online Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene Mapping • Many fruit fly genes – Were mapped initially using recombination I frequencies Y II Try this online gene mapping tutorial, if you’re feeling ambitious http://www.dnaftb.org/11/pr oblem.html Figure 15.8 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings X IV III Mutant phenotypes Short aristae Black body 0 Long aristae (appendages on head) Cinnabar Vestigial eyes wings 48.5 57.5 67.0 Gray body Red eyes Normal wings Wild-type phenotypes Brown eyes 104.5 Red eyes