Presentation is loading. Please wait.

Presentation is loading. Please wait.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (2) Exam : 3-5 pm on Oct. 4th, 2010 (1 st exam)  Problem types: C hoice 70% +

Similar presentations


Presentation on theme: "Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (2) Exam : 3-5 pm on Oct. 4th, 2010 (1 st exam)  Problem types: C hoice 70% +"— Presentation transcript:

1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (2) Exam : 3-5 pm on Oct. 4th, 2010 (1 st exam)  Problem types: C hoice 70% + Short or long answer 30%  Posting of score & expected GPA: on the board at room SB132, New Science Building ( 과학원 )  Problem types: C hoice 70% + Short or long answer 30%  Posting of score & expected GPA: on the board at room SB132, New Science Building ( 과학원 ) (1) How to get lecture slides  www.ykjanglab.pe.kr/korean/ 연구성과 / 세미나 및 강의 www.ykjanglab.pe.kr/korean/ 연구성과 /  File name: UIC_Chapter00_lecture_Biology II_20100000  www.ykjanglab.pe.kr/korean/ 연구성과 / 세미나 및 강의 www.ykjanglab.pe.kr/korean/ 연구성과 /  File name: UIC_Chapter00_lecture_Biology II_20100000 Announcement This Biology II course is for UIC students!!!

2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Announcement (continued) (3) Grading:  1st exam (25%) + 2 nd exam (25%) + 3 rd exam (25%) + lab work (20%) + attendance & assignments (5%)  1st exam (25%) + 2 nd exam (25%) + 3 rd exam (25%) + lab work (20%) + attendance & assignments (5%) (4) Homework  Summarize each chapter (2 pages) and then submit your handwritten summary before at next class

3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Brief Contents in This Course Chapter 15 : The chromosomal basis of inheritance (Prof. Jang) Chapter 16 : The molecular basis of inheritance (Prof. Jang) Chapter 17 : From gene to protein (Prof. Jang) Chapter 18 : Regulation of gene expression (Prof. Jang) Part I: Genetics Chapter 13 : Meiosis and sexual life cycles Chapter 14 : Mendel and the gene idea What you studied already

4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Brief Contents in This Course Chapter 19 : Viruses Chapter 20: Biotechnology Chapter 21: Genomes and their evolution Chapter 39: Plant responses to internal and external signals Chapter 40 : Basic principles of animal form and function Chapter 41 : Animal nutrition Chapter 43 : The Immune system Chapter 45 : Hormones and the endocrine system Part II: Oct. 06 - Nov. 16 by H.S. Pai Part III: Nov. 17 - Dec. 21 by K.M. Choe

5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 15 On 2010-09-06 by Yeun-Kyu Jang The Chromosomal Basis of Inheritance What you learned already Chapter 13 : Meiosis and sexual life cycles Chapter 14 : Mendel and the gene idea

6 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ☞ Genes (  passengers) and chromosomes (  wagon) Overview: Locating Genes on Chromosomes Mendel’s “hereditary factors” were genes, though this wasn’t known at the time Today we can show that genes are located on chromosomes Q: How to visualize that? The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene

7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fig. 15-1 FISH (fluorescent in situ hybridization): Chromosomes tagged to reveal a specific gee (yellow color)

8 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Chromosomes: a physical basis for the principles of heredity (=Mendel’s laws of segregation and independent assortment)  This chapter’s aim: Is to study the chromosomal basis for the transmission of genes from parents to offspring

9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes Mitosis and meiosis were first described in the late 1800s The chromosome theory of inheritance states : – Mendelian genes have specific loci (positions) on chromosomes – Chromosomes undergo segregation and independent assortment The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment

10 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The law of segregation – The two alleles (Y & y) for a single heritable character (ex: seed color) separate (segregate) during gamete formation and end up in different gametes Using the information from a dihybrid cross, Mendel developed the law of independent assortment – Each pair of alleles for two different traits segregates independently during gamete formation (Segregation of one gene’ alleles does not interfere with that of other gene’s alleles) – Ex: The alleles for seed color and seed shape sort into gametes independently of each other. What we learned: Mendel’s laws

11 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 : 1 1414 1414 1414 1414 YR Yr yryR Gametes Y R R Y y r r y R Y y r 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 The chromosomal basis of Mendel’s laws yrYr

12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Summary for the chromosomal basis of Mendel’s laws Law of segregation: Two alleles for each gene segregate during gamete formation and end up in different gametes  Each gamete gets only one of the two alleles  In terms of chromosomes, this segregation corresponds to the distribution of homologous chromosomes to different gamete (R or r allele; Y or y allele)

13 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Summary for the chromosomal basis of Mendel’s laws Law of independent assortment: Alleles of genes on nonhomologous chromosomes assort independently during gamete formation  Each gamete gets one of four allele combinations (YR:Yr:yR:yr = 1:1:1:1) depending on the alternative arrangements of homologous chromosome pairs in metaphase I

14 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan’s Experimental Evidence for chromosomal theory: Scientific Inquiry Thomas Hunt Morgan (Embryologist at Columbia Univ. early in the 20 th century) – Provided the first solid evidence associating a specific gene with a specific chromosome – Namely, provided convincing evidence that chromosomes are the location of Mendel’s heritable factors

15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan’s Choice of Experimental Organism Several characteristics make fruit flies a convenient organism for genetic studies: – They breed at a high rate – A generation can be bred every two weeks – They have only four pairs of chromosomes

16 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 Morgan’s first mutant: wild- type flies have red eyes (left) and one mutant male showed white eyes (right) This variation made it possible for Morgan to trace a gene for eye color to a specific chromosome

17 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan first invented a notation for symbolizing alleles in Drosophila (still widely used) – The allele for white eyes in flies is symbolized by w (indicated as a mutant allele) – A superscript + identifies the allele for the wild- type trait: w + for the allele for red eyes Figure 15.3 w+w+w+w+  Genotype of wild-type: ww  Genotype of mutant:

18 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How to Correlate 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 – The white-eye mutant allele must be located on the X chromosome

19 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 Inquiry: In a cross between a WT female and a mutant white-eyed male, what color eyes will the F 1 and F 2 offspring have? Morgan determined – That the white- eye mutant allele must be located on the X chromosome

20 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CONCLUSION 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, as diagrammed here. P Generation F 1 Generation F 2 Generation Ova (eggs) Ova (eggs) Sperm X X X X Y W W+W+ W+W+ W W+W+ W+W+ W+W+ W+W+ W+W+ W+W+ W+W+ W+W+ W W+W+ W W W all red-eyes The recessive trait of white eyes was appeared only in males

21 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Conclusion (1): first experimental evidence for chromosome theory of inheritance 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

22 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept Check 15.1 (see p289) Which one of Mendel’s laws relates to the inheritance of alleles for a single character? Which law relates to the inheritance of alleles for two characters in a dihybrid cross? What is the physical basis of these laws?

23 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 15.2: Sex-linked genes exhibit unique patterns of inheritance  We consider the role of sex chromosome in heritance  First step: reviewing the chromosomal basis of sex determination in humans and some other animals What will you examine here

24 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 (sex chromosomes) How to determine sex in an organism?

25 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In humans and other mammals, there are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome Only the ends of the Y chromosome have regions that are homologous with the X chromosome The SRY gene on the Y chromosome codes for the development of testes Fig. 15.5: Human sex chromosomes

26 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some chromosomal systems of sex determination In humans and other mammals – There are two varieties of sex chromosomes, X and Y Figure 15.6a (a) The X-Y system 44 + XY 44 + XX Parents 22 + X 22 + Y 22 + X Sperm 44 + XX 44 + XY Zygotes (offspring) Ova (eggs) In mammals, the sex of an offspring depends on whether the sperm cell contains an X chromosome or a Y

27 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Different systems of sex determination – Are found in other organisms Figure 15.6b–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 In grasshoppers, cockroaches, there is only one type of sex chromosome (the X) In birds, some fishes, and some insects, the sex chromosome present in the egg (not the sperm) determines the sex of offspring (Z and W) In most species of bees and ants, there are no sex chromosomes. Female develop from fertilized eggs and are thus diploid. Males develop from unfertilized eggs and haploid (no fathers)

28 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance of Sex-Linked Genes Key points to keep in mind The sex chromosomes have genes for many characters unrelated to sex A gene located on either sex chromosome is called a sex-linked gene In humans, sex-linked usually refers to a gene on the larger X chromosome

29 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sex-linked trait by recessive allele on X chromosome Female: has two X chromosomes – Homozygous recessive alleles : expression of phenotype – Heterozygous: no expression of phenotype (called carrier) Male: has only one X chromosome – You don’t use the terms homozygous and heterozygous: use “hemizygous” – Any male receiving the recessive allele from his mother will express the trait  far more males than females have sex-linked recessive disorders (The freq. of sex-linked recessive disorders in male is higher than that in female)

30 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 : 1/3500 males born in US; progressive weakening of the muscles; the absence of a key muscle protein called dystrophin – Hemophilia: shows delayed blood clotting due to the absence of one or more of the proteins required for blood clotting; treated with intravenous injections of the missing protein What kinds of X-lined disorders are found?

31 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 15.7a–c A father with the disorder will transmit the mutant allele to all daughters but to no sons. When the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers of the mutation. (a) How do sex-lined recessive traits transmit to offspring? XAXaXAXa

32 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The transmission of sex-linked recessive traits Figure 15.7a–c If a carrier female 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. (b) XaXAXaXA

33 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The transmission of sex-linked recessive traits Figure 15.7a–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. (c) XaXaXaXa

34 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings X inactivation in Female Mammals In mammalian females – One of the two X chromosomes in each cell is randomly inactivated during embryonic development – As a result, the cells of females and males have the same effective dose (one copy) of genes on the X chromosome (Got even!! = Dosage compensation) – The inactive X condenses into a compact object called a Barr body (discovered by a cytologist Murray Barr) – Most of genes are not expressed but Barr-body chromosomes are reactivated during oogenesis to form female gamete ovum – Random selection of X inactivation results in females with mosaic trait (Mosaicism)

35 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mystery of the mottled coloration in cat coat On random inactivation of an X chromosome in a particular cell, all mitotic descendants of that cell have the same inactive X. If a female is heterozygous for a particular gene located on the X chromosome  She will be a mosaic for that character: about half will express one allele while the others will express the alternate allele Two cell populations in adult cat: Active X Orange fur Inactive X Early embryo: X chromosomes Allele for black fur Cell division and X chromosome inactivation Active X Black fur Inactive X Figure 15.8 Allele for orange fur X-inactivation and the tortoiseshell cat

36 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mystery of the presence or absence of sweat glands in female skin In humans, mosaicism can be observed in a recessive X-linked mutation that prevents the development of sweat glands A woman who is heterozygous for this trait has patches of normal skin and patches of skin lacking sweat glands

37 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept Check 15.2 (see p292) During early embryonic development of female carriers for color blindness, the normal allele is inactivated by chance in about half the cells. Why, then, aren’t 50% of female carriers color- blind?

38 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Answer: The cells in the eye must come from multiple cells in the early embryo The descendants of half of these cells express normal allele Thus having half the number of mature eye cells expressing the normal allele must be sufficient for normal color vision

39 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 15.3: Linked genes tend to be inherited together because they are located near each other on the same chromosome Each chromosome has hundreds or thousands of genes Location of some genes on the same chromosome is expected The genes tend to be inherited together in genetic crosses: are said to be linked genes The linked genes do not always follow Mendel’s law of independent assortment

40 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How Linkage Affects Inheritance Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters Morgan crossed flies that differed in traits of body color and wing size

41 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fig. 15-UN1 b + vg + Parents in testcross Most offspring b + vg + b vg  or

42 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes) He noted that these genes do not assort independently, and reasoned that they were on the same chromosome

43 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Double mutant (black body, vestigial wings) Double mutant (black body, vestigial wings) Wild type (gray body, normal wings) P Generation (homozygous) b + b + vg + vg + x b b vg vg F 1 dihybrid (wild type) (gray body, normal wings) b + b vg + vg b b vg vg TESTCROSS x b + vg + b vg b + vg b vg + b vg b + b vg + vg bb vgvg b + b vgvg bb vg + vg 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal Sperm Parental-type offspring Recombinant (nonparental-type) offspring RESULTS EXPERIMENT Morgan first mated true-breeding wild-type flies with black, vestigial-winged flies to produce heterozygous F 1 dihybrids, all of which are wild-type in appearance. He then mated wild-type F 1 dihybrid females with black, vestigial-winged males, producing 2,300 F 2 offspring, which he “scored” (classified according to phenotype). CONCLUSION If these two genes were on different chromosomes, the alleles from the F 1 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 (Genetic recombination) Figure 15.9 Double mutant (black body, vestigial wings) Double mutant (black body, vestigial wings) Ova (eggs) Inquiry: Are the genes for body color and wing size in fruit flies located on the same chromosome or different chromosomes?

44 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fig. 15-9-4 EXPERIMENT P Generation (homozygous) RESULTS Wild type (gray body, normal wings ) Double mutant (black body, vestigial wings)  b b vg vg Double mutant TESTCROSS  b + b + vg + vg + F 1 dihybrid (wild type) b + b vg + vg Testcross offspring Eggs b + vg + b vg b + vg b vg + Black- normal Gray- vestigial Black- vestigial Wild type (gray-normal) b vg Sperm b + b vg + vg b b vg vg b + b vg vg b b vg + vg PREDICTED RATIOS If genes are located on different chromosomes: If genes are located on the same chromosome and parental alleles are always inherited together: 1 1 1 1 1 1 0 0 965 944206 185 : : : : : : : : :

45 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings However, nonparental phenotypes were also produced Genetic recombination might be involved in breaking the linkage between genes on the same chromosome

46 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan concluded that – Genes that are close together on the same chromosome are linked and do not assort independently – Unlinked genes are either on separate chromosomes or are far apart on the same chromosome and so assort independently – Appearance of non-parental phenotypes suggest that the body-color and wing size genes are only partially linked genetically To understand this unexpected result, we need to further explore genetic recombination

47 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Recombination and Linkage Meiosis and random fertilization generate genetic variation among offspring of sexually reproducing organisms What you learned in the Chapter 13 What is the chromosomal basis of recombination in relation to the genetic findings of Mendel and Morgan? What will you examine next

48 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recombination of Unlinked Genes can occur via Independent Assortment of Chromosomes When Mendel followed the inheritance of two characters unlinked – He observed that some offspring have combinations of traits that do not match either parent in the P generation (called recombinant types or recombinants or nonparental types) 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

49 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel observed that combinations of traits in some offspring differ from either parent Offspring with a phenotype matching one of the parental phenotypes are called parental types Offspring with nonparental phenotypes (new combinations of traits) are called recombinant types, or recombinants (Half the number of offspring would be recombinant types) A 50% frequency of recombination is observed for any two genes on different chromosomes

50 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recombination of Linked Genes: Crossing Over Morgan discovered that genes can be linked, but the linkage was incomplete, as evident from recombinant phenotypes Morgan proposed that some process must sometimes break the physical connection between genes on the same chromosome That mechanism was the crossing over of homologous chromosomes

51 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fig. 15-10 Testcross parents Replication of chromo- somes Gray body, normal wings (F 1 dihybrid) Black body, vestigial wings (double mutant) Replication of chromo- somes b + vg + b vg b + vg + b + vg b vg + b vg Recombinant chromosomes Meiosis I and II Meiosis I Meiosis II b vg + b + vg b vg b + vg + Eggs Testcross offspring 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal b + vg + b vg b + vg b vg b vg + Sperm b vg Parental-type offspringRecombinant offspring Recombination frequency = 391 recombinants 2,300 total offspring  100 = 17% 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. Meiosis I and II: Even if crossing over occurs, no new allele combinations are produced. Gametes Chromosomal basis for recombination of linked genes: Linked genes exhibit RF less than 50% RF: the percentage of recombinants in the total offspring

52 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Relation of RF with Gene-to-Gene distances The percentage of recombinant offspring, the recombination frequency (RF), is related to the distance between linked genes

53 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A genetic map (  Gene Address Book) – Is an ordered list of the genetic loci along a particular chromosome – Can be developed using recombination frequencies The discovery of linked genes and recombination via crossing over – Led one of Morgan’s students (Alfred H. Sturtevant) to develop a method for constructing a genetic map Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry

54 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sturtevant’s work for constructing a genetic map He hypothesized that RFs depend on the distances between genes on a chromosome -He assumed that crossing over is a random event and thus the chance is approximately equal at all points along a chromosome  He predicted that the farther apart two genes are, the higher the probability of a crossover between them and therefore the higher the RF  Using recombination data, he assigned relative positions to genes on the same chromosomes (Constructing linkage map: genetic map based on RF )

55 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A linkage map How to construct a linkage map  Is the actual map of a chromosome based on recombination frequencies  shows the relative locations of genes along a chromosome  The distances between genes are expressed as map units (centimorgans): one map unit (1 m.u.) equivalent to 1% RF

56 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Example  Consider three genes: the body-color (b), wing-size (vg), and cinnabar eyes (cn) [brighter red than wt eye color]  The observed recombination frequencies between three Drosophila gene pairs : b–cn 9%, cn–vg 9.5%, and b–vg 17%  Make a linear order : cn is positioned about halfway between the other two genes: Figure 15.11 How to construct a linkage map (continued) Recombination frequencies 9%9.5% 17% bcnvg Chromosome 1 st cross-over 2 nd CO b, black body color vg, vestigial wing cn, cinnabar eyes

57 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  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 effect of the first crossover and thus reduce the observed b–vg RF. Example How to construct a linkage map (continued) Recombination frequencies 9%9.5% 17% bcnvg Chromosome 1 st cross-over 2 nd CO Inquiry: The b–vg recombination frequency (17%) is slightly less than the sum (18.5%) of the b–cn and cn– vg frequencies. Why?

58 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Many fruit fly genes – Were mapped initially using recombination frequencies Figure 15.12 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 0 48.557.567.0 104.5 A partial genetic (linkage) map of Drosophila chromosome

59 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders  Gene mutations, chromosome structure, chromosome number  Many causes can damage chromosomes or alter their number in a cell  Large-scale chromosomal alterations: Often lead to spontaneous abortions or cause a variety of developmental disorders What kind of changes to the genome affecting phenotype?

60 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Abnormal Chromosome Number When nondisjunction occurs – Pairs of homologous chromosomes do not separate properly during either meiosis I or meiosis II – Gametes receive two copies (n+1) or no copies (n-1) of a particular chromosome Figure 15.13a, b Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n + 1 n  1 n – 1 n + 1n –1 n n Number of chromosomes Nondisjunction of homologous chromosomes in meiosis I Nondisjunction of sister chromatids in meiosis II (a) (b) Meiotic nondisjunction: gametes with an abnormal chromosome number can arise by nondisjunction in either meiosis I or meiosis II

61 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The consequences of nondisjunction after fertilization? Aneuploidy – Results from the fertilization of aberrant gametes (nondisjunction occurred) with a normal one – Is a condition in which offspring have an abnormal number of a particular chromosome

62 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chromosome number of haploid (gamete): n Chromosome number of normal zygote: 2n If a zygote is trisomic (2n+1 chromosomes) – It has three copies of a particular chromosome If a zygote is monosomic (2n-1 chromosomes) – It has only one copy of a particular chromosome

63 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Polyploidy – Is a condition in which there are more than two complete sets of chromosomes in an organism – Triploidy (3n): can be produced by fertilization of an abnormal diploid egg produced by nondisjunction – Tetraploidy (4n): the failure of 2n zygote to divide after replication of its chromosomes and then subsequent mitosis may produce a 4n embryo Figure 15.14 A tetraploid mammal : a burrowing rodent ( 굴쥐 ) This species may have arisen when an ancestor doubled its chromosome number, presumably by errors in mitosis or meiosis within the animal’s reproductive organs

64 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Alterations of Chromosome Structure Breakage of a chromosome can lead to four types of changes in chromosome structure:  Called “Chromosomal rearrangement” – Deletion – Duplication – Inversion – Translocation

65 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Alterations of chromosome structure Figure 15.15a–d A B CD E FG H Deletion A B C E G H F A B CD E FG H Duplication A B C B D E C F G H A A MN OPQR B CD EFGH B CDEFGH Inversion Reciprocal translocation A BPQ R M NOCDEF G H A D CBEFH G (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. (Give & Take mode: exchange their fragments each other) Breakage points

66 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Human Disorders Due to Chromosomal Alterations Alterations of chromosome number and structure – Are associated with a number of serious human disorders

67 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings egg Down Syndrome Down syndrome – Is usually the result of an extra chromosome 21, trisomy 21 (45+XX) – 5% of persons with this syndrome contain a combined 14-21 chromosome via translocation of ch21 to ch14 – Shows characteristic facial features, short stature, heart defects, respiratory infection, mental retardation prone to developing leukemia and Alzheimer’s disease Figure 15.16 The child exhibits the facial features characteristic of Down syndrome. The karyotype shows trisomy 21, the most common cause of this disorder Ch21 Ch14-21 sperm zygote

68 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The consequences of aneuploidy of Sex Chromosomes? Nondisjunction of sex chromosomes – Produces a variety of aneuploid conditions Klinefelter syndrome (XXY male) – Is the result of an extra chromosome in a male, producing XXY individuals – Occurs once in approximately 2,000 live births; small testes and sterile Turner syndrome (XO female) – Is the result of monosomy X, producing an X0 karyotype (only known viable monosomy in humans) – 1/5,000 live births; immature sex organs and sterile

69 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Disorders Caused by Structurally Altered Chromosomes Cri du chat (“cry of the cat”) – Is a disorder caused by a specific deletion in chromosome 5 – Shows mental retardation, small head with unusual facial features, cry that sounds like the mewing of a cat, die in infancy or early childhood

70 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Certain cancers – Are caused by translocations of chromosomes Figure 15.17 Normal chromosome 9 Reciprocal translocation Translocated chromosome 9 Philadelphia chromosome Normal chromosome 22 Translocated chromosome 22 Chromosomal translocation associated with chronic myelogenous leukemia (CML): the reciprocal translocation between ch9 and ch22 results in an abnormally short ch22 (called Philadelphia chr) and abnormally long ch9

71 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 15.5: Some inheritance patterns are exceptions to the standard chromosome theory Two normal exceptions to Mendelian genetics include – Genes located in the nucleus: genomic imprinting at Igf2 gene – Genes located outside the nucleus: extranuclear genes contained in mitochondria, and plastids including chloroplast

72 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genomic Imprinting What you learned in Mendelian genetics Some traits in mammals depend on which parents passed along the alleles What exceptions will you consider by now We assumed that a specific allele will have the same effect regardless of whether it was inherited from the mother or the father The phenotypic effects of certain genes depend on which allele is inherited from the mother and which is inherited from the father: such variation in phenotype is called genomic imprinting

73 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genomic imprinting Involves the silencing of certain genes that are “stamped” with an imprint during gamete production A zygote expresses only one allele of an imprinted gene (either from the mother or the father)  The imprints are transmitted to all the body cells during development A gene imprinted for maternal allele expression is always imprinted for maternal allele expression, generation to generation

74 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genomic imprinting (Example) Figure 15.18a, b ( a) A wild-type mouse is homozygous for the normal igf2 allele. Normal Igf2 allele (expressed) Normal Igf2 allele with imprint (not expressed) Paternal chromosome Maternal chromosome Wild-type mouse (normal size) Normal Igf2 allele Paternal Maternal Mutant lgf2 allele Paternal Maternal Dwarf mouse Normal Igf2 allele with imprint Normal size mouse (b) When a normal Igf2 allele is inherited from the father, heterozygous mice grow to normal size. But when a mutant allele is inherited from the father, heterozygous mice have the dwarf phenotype. Genomic imprinting of the mouse Igf2 gene: (a) The paternal Igf2 allele is only expressed because methylation of a certain DNA sequence on the paternal chromosome leads to expression of the paternal Igf2 allele (b) Matings between wild-type and those homozygous for the recessive mutant Igf2 allele produce heterozygous offspring that can be either normal or dwarf, depending on which parent passes on the mutant allele Heterozygous mice with normal Igf2 allele inherited from the father Heterozygous mice with mutant Igf2 allele inherited from the father

75 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance of Organelle Genes Extranuclear genes – Are found in organelles in the cytoplasm – Mitochondria, chloroplast, plastids – They do not display Mendelian inheritance

76 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The inheritance of traits controlled by genes present in the chloroplasts or mitochondria – Depends solely on the maternal parent because the zygote’s cytoplasm comes from the egg Figure 15.19 Discovery of organelle genes by Karl Correns in 1909  Variegated leaves from Croton diocus result from mutations in pigment genes located in plastids, which is generally inherited from the maternal parent

77 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some diseases affecting the muscular and nervous systems – Are caused by defects in mitochondrial genes that prevent cells from making enough ATP Mitochondrial mutations and human diseases Some mitochondrial mutations inherited from the mother may contribute to some cases of diabetes, heart disease and Alzheimer’s disease

78 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept Check 15.5 (see p302) Gene dosage, the number of active copies of a gene, is important to proper development. Identify and describe two processes that help establish the proper dosage of certain genes.


Download ppt "Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (2) Exam : 3-5 pm on Oct. 4th, 2010 (1 st exam)  Problem types: C hoice 70% +"

Similar presentations


Ads by Google