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Chapter 15

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Chapter 15

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  1. Chapter 15 The Chromosomal Basis of Inheritance

  2. Figure 15.1 • Overview: Locating Genes on Chromosomes • Genes • Are located on chromosomes • Can be visualized using certain techniques

  3. 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. P Generation Yellow-round seeds (YYRR) Green-wrinkled seeds (yyrr) 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.) y Y r R R Y r y Meiosis Fertilization y r Y R Gametes All F1 plants produce yellow-round seeds (YyRr) R R y F1 Generation y r r Y Y Meiosis LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT r R r R Two equally probable arrangements of chromosomes at metaphase I 1 3 y Y Y y 1 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 cellson the left and right The R and r alleles segregate at anaphase I, yielding two types of daughter cells for this locus. r R R r Anaphase I y Y y Y r r R R 2 Each gamete gets one long chromosome with either the R or r allele. Metaphase II 2 Each gamete gets a long and a short chromosome in one of four allele combinations. Y y y y Y Y Y y Y Y Y y Gametes r R r R R r r R 1 4 1 4 1 4 1 4 yr yr yR YR F2 Generation Fertilization among the F1 plants Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. 3 Fertilization recombines the R and r alleles at random. Figure 15.2 9 : 3 : 3 : 1 • The chromosomal basis of Mendel’s laws

  5. 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

  6. Scientists build on the discoveries of their predecessors: 1905 E.B Wilson - American biologistidentified 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

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

  8. 22 + XX 22 + X (b) The X–0 system 76 + ZZ 76 + ZW (c) The Z–W system 16 (Haploid) 16 (Diploid) (d) The haplo-diploid system Figure 15.9b–d • Different systems of sex determination • Are found in other organisms

  9. 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. 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. 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. Figure 15.3 • 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

  13. 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

  14. 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 • Morgan determined • That the white-eye mutant allele must be located on the X chromosome CONCLUSION 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.

  15. 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. Some recessive alleles found on the X chromosome in humans cause certain types of disorders • Color blindness • Duchenne muscular dystrophy • Hemophilia

  17. b+ vg+ bvg X Parents in testcross b vg b vg bvg b+ vg+ Most offspring or b vg b vg 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 • To see how the inheritance of two different characters is affected by gene location

  18. Gametes from yellow-round heterozygous parent (YyRr) yR YR Yr yr Gametes from green- wrinkled homozygous recessive parent (yyrr) yr YyRr yyrr Yyrr yyRr Parental- type offspring Recombinant offspring 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 Independent assortment

  19. Some process must occasionally break the physical connection between genes on the same chromosome • Crossing overof homologous chromosomes was the mechanism behind the recombination • Crossover animation, narrated Due to the appearance of recombinant phenotypes,

  20. “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. 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 •

  22. b+ vg+ b vg Black body,vestigial wings(double mutant) Testcross parents Gray body, normal wings (F1 dihybrid)  b vg b vg Replication of chromosomes Replication of chromosomes vg vg b b+ b b+ vg+ vg vg vg b b b Meiosis I: Crossing over between b and vg loci produces new allele combinations. vg b vg Meiosis I and II: Even if crossing over occurs, no new allele combinations are produced. Meiosis II: Segregation of chromatids produces recombinant gametes with the new allele combinations. Recombinant chromosome Gametes Ova Sperm b+vg+ b vg b vg+ b+ vg b vg b+  vg+ b+   vg bvg+ Ova bvg Testcross offspring 944 Black- vestigial 965 Wild type (gray-normal) 206 Gray- vestigial 185 Black- normal Sperm Recombination frequency 391 recombinants  = 100 = 17% b+  vg+ b vg+ b vg+ b+vg+ 2,300 total offspring b vg Figure 15.6 b vg b vg b vg b vg Parental-type offspring Recombinant offspring • Linked genes • Exhibit recombination frequencies less than 50%

  23. 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. 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 twogenetic loci is proportional to the distance separating the loci. The recombination frequencies used to constructa 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% vg b cn 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. Figure 15.7 A linkage map • Is the actual map of a chromosome based on recombination frequencies

  25. 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

  26. I IV X Y III II Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial wings Brown eyes 0 48.5 57.5 67.0 104.5 Long aristae (appendages on head) Red eyes Gray body Normal wings Red eyes Wild-type phenotypes Figure 15.8 Gene Mapping • Many fruit fly genes • Were mapped initially using recombination frequencies Try this online gene mapping tutorial, if you’re feeling ambitious