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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece.

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Presentation on theme: "Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece."— Presentation transcript:

1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 15 The Chromosomal Basis of Inheritance

2 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

3 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

4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The chromosomal basis of Mendel’s laws Figure 15.2 Yellow-round seeds (YYRR) Green-wrinkled seeds (yyrr) Meiosis Fertilization Gametes All F 1 plants produce yellow-round seeds (YyRr) P Generation F 1 Generation Meiosis Two equally probable arrangements of chromosomes at metaphase I LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT Anaphase I Metaphase II Fertilization among the F 1 plants 9: 3 : YR yr yR Gametes Y R R Y y r r y R Y yr R y Y r R y Y r R Y r y rR Y y R Y r y R Y Y R R Y r y r y R y r Y r Y r Y r Y R y R y R y r Y F 2 Generation Starting with two true-breeding pea plants, we follow two genes through the F 1 and F 2 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.) The R and r alleles segregate at anaphase I, yielding two types of daughter cells for this locus. 1 Each gamete gets one long chromosome with either the R or r allele. 2 Fertilization recombines the R and r alleles at random. 3 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 1 Each gamete gets a long and a short chromosome in one of four allele combinations. 2 Fertilization results in the 9:3:3:1 phenotypic ratio in the F 2 generation. 3

5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Scientists build on the discoveries of their predecessors: mid-1800’s Gregor Mendel - Laws of Inheritance; named "factors" as hereditary information 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

6 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 likehow to make onewhat they look like

7 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 Figure 15.9a (a) The X-Y system 44 + XY 44 + XX Parents 22 + X 22 + Y 22 + XY SpermOva 44 + XX 44 + XY Zygotes (offspring)

8 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Different systems of sex determination – Are found in other organisms Figure 15.9b–d 22 + XX 22 + X 76 + ZZ 76 + ZW 16 (Haploid) 16 (Diploid) (b) The X–0 system (c) The Z–W system (d) The haplo-diploid system

9 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

10 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"

11 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

12 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

13 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 F 1 generation all had red eyes – The F 2 generation showed the 3:1 red:white eye ratio, but only males had white eyes wt mutant

14 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 15.4 The F 2 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. Morgan then bred an F 1 red-eyed female to an F 1 red-eyed male to produce the F 2 generation. RESULTS P Generation F 1 Generation X F 2 Generation Morgan mated a wild-type (red-eyed) female with a mutant white-eyed male. The F 1 offspring all had red eyes. EXPERIMENT Morgan determined – That the white-eye mutant allele must be located on the X chromosome Since all F 1 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 F 2 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. CONCLUSION

15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

16 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

17 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan did other experiments with fruit flies 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 Parents in testcross b + vg + b vg b + vg + b vg Most offspring X or – To see how the inheritance of two different characters is affected by gene location

18 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 green- wrinkled homozygous recessive parent (yyrr) Gametes from yellow-round heterozygous parent (YyRr) Parental- type offspring Recombinant offspring YyRryyrrYyrryyRr YR yr Yr yR yr Independent assortment

19 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings – 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 Crossover animation Due to the appearance of recombinant phenotypes,

20 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

21 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 hill.com/sites/dl/free/ /126997/animation5.htmlhttp://highered.mcgraw- hill.com/sites/dl/free/ /126997/animation5.html

22 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 15.6 Testcross parents Gray body, normal wings (F 1 dihybrid) b+b+ vg + bvg Replication of chromosomes b+b+ vg b+b+ vg + b vg Meiosis I: Crossing over between b and vg loci produces new allele combinations. Meiosis II: Segregation of chromatids produces recombinant gametes with the new allele combinations.  Recombinant chromosome b + vg + b vg b + vg b vg + b vg Sperm b vg Replication of chromosomes vg b b b b vg Meiosis I and II: Even if crossing over occurs, no new allele combinations are produced. OvaGametes Testcross offspring Sperm b + vg + b vg b + vgb vg Wild type (gray-normal) b + vg + b vg b vg + b + vg + b vg Black- vestigial 206 Gray- vestigial 185 Black- normal Recombination frequency = 391 recombinants 2,300 total offspring  100 = 17% Parental-type offspring Recombinant offspring Ova b vg Black body, vestigial wings (double mutant) b Linked genes – Exhibit recombination frequencies less than 50%

23 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

24 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A linkage map – Is the actual map of a chromosome based on recombination frequencies Recombination frequencies 9% 9.5% 17% bcn vg Chromosome 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 would “cancel out” the first and thus reduce the observed b–vg recombination frequency. 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: RESULTS A linkage map shows the relative locations of genes along a chromosome. APPLICATION 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 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. Figure 15.7

25 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.click for animation example: the relative abundance of four genes on a chromosome can be mapped from the following data on crossover frequencies: GENESFrequency of Crossover B and D5% C and A15% A and B30% C and B45% C and D50% 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 onlinechromosome maps online

26 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene Mapping Many fruit fly genes – Were mapped initially using recombination frequencies Figure 15.8 Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial wings Brown eyes Long aristae (appendages on head) Gray body Red eyes Normal wings Red eyes Wild-type phenotypes II Y I X IV III Try this online gene mapping tutorial, if you’re feeling ambitious oblem.html


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