1. Chapter 08 *Lecture Outline Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. *See separate FlexArt PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes.

2. INTRODUCTION Genetic variation refers to differences between members of the same species or those of different species –Allelic variations are due to mutations in particular genes –Chromosomal aberrations are substantial changes in chromosome structure or number These typically affect more than one gene They are quite common, which is surprising 8-2 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

3. 8.1 Variation in Chromosome Structure Cytogenetics -The field of genetics that involves the microscopic examination of chromosomes A cytogeneticist typically examines the chromosomal composition of a particular cell or organism –This allows the detection of individuals with abnormal chromosome number or structure –This also provides a way to distinguish between species Refer to Figure 8.1a 8-3 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

4. © Scott Camazine /Photo Researchers Human Fruit flyCorn © Michael Abbey/Photo Researchers © Carlos R Carvalho/Universidade Federal de Viçosa. Figure 8.1 8-4 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display (a) Micrographs of metaphase chromosomes

5. 8-5 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Cytogeneticists use three main features to identify and classify chromosomes 1. Location of the centromere 2. Size 3. Banding patterns These features are all seen in a Karyotype A micrograph in which all of the chromosomes within a single cell are arranged in a standard fashion The procedure for making a karyotype was discussed in Chapter 3 (See Figure 3.2) Cytogenetics

6. A karyotype of a diploid human cell Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8-6 Figure 3.2 (c) For a diploid human cell, two complete sets of chromosomes from a single cell constitute a karyotype of that cell. 11  m © Leonard Lessin/Peter Arnold 12345 67891011 12 131415161718 19202122 XY

7. MetacentricSubmetacentricAcrocentricTelocentric P q P q P q P q (b) A comparison of centromeric locations 8-7 Figure 8.1 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Short arm; For the French, petite Long arm Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

8. 8-8 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Since different chromosomes can be the same size and have the same centromere position, chromosomes are treated with stains to produce characteristic banding patterns Example: G-banding Chromosomes are exposed to the dye Giemsa Some regions bind the dye heavily Dark bands Some regions do not bind the dye well Light bands In humans 300 G bands are seen in metaphase 800 G bands in prometaphase Cytogenetics

9. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8-9 Figure 8.1 Banding pattern during metaphase Banding pattern during prometaphase P q P q (d) Conventional numbering system of G bands in human chromosomes XY22212019181716151413 1 1 2 3 3 2 1 1 2 3 4 121110987 6 54321 6 5 4 3 2 1 2 1 3 2 1 1 2 1 2 1 2 3 4 1 2 3 4 5 5 4 3 2 1 6 5 4 3 2 1 1 2 3 4 1 2 3 4 5 6 7 1 2 3 4 7 6 5 4 3 2 1 4 3 2 1 1 2 3 1 2 3 4 5 6 7 8 9 6 5 4 3 2 1 1 2 3 4 5 6 7 8 1 2 3 1 2 3 4 5 5 4 3 2 1 1 2 3 4 5 1 2 3 1 2 3 4 5 5 4 3 2 1 1 2 3 4 5 6 1 2 3 4 5 6 7 2 1 2 1 5 4 3 2 1 1 1 2 1 2 3 4 5 6 3 2 1 2 1 1 2 3 1 2 3 4 4 3 2 1 3 2 1 1 2 3 1 2 1 2 3 4 5 4 3 2 1 1 1 2 3 4 5 6 5 4 3 2 1 1 2 3 4 1 2 3 4 5 3 2 1 1 2 3 4 5 1 2 3 4 3 2 1 1 2 3 4 1 2 3 4 1 2 2 1 1 2 3 1 2 3 4 5 6 7 8 1 3 2 1 1 2 3 1 2 1 2 3 4 3 2 1 1 2 3 1 2 3 4 3 2 1 1 2 1 2 3 4 5 3 2 1 1 2 3 3 2 1 1 2 3 3 2 1 1 2 3 1 2 1 3 2 1 1 1 2 1 2 2 1 1 2 3 3 2 1 1 2 3 4 5 1 2 3 4 5 6 1 1 2 3 1 1 2 1 1 2 1 1 2 1 1 2 1 1 1 1 1 1 2 1 1 1 1 2 1 1 2 2 1 1 2 3 2 1 1 2 1 1 2 3 1 1 2 3 2 1 1 2 2 1 1 2 3 2 1 1 2 2 1 1 2 3 1 1 2 1 1 2 1 1 2

10. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The banding pattern is useful in several ways: 1. It distinguishes Individual chromosomes from each other 2. It detects changes in chromosome structure 3. It reveals evolutionary relationships among the chromosomes of closely related species Cytogenetics 8-10

11. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display There are two primary ways in which the structure of chromosomes can be altered 1. The total amount of genetic material in the chromosome can change Deficiencies/Deletions Duplications 2. The total amount of genetic material remains the same, but is rearranged Inversions Translocations 8-11 Mutations Can Alter Chromosome Structure

12. 8-12 Deficiency (or deletion) –The loss of a chromosomal segment Duplication –The repetition of a chromosomal segment compared to the normal parent chromosome Inversion –A change in the direction of part of the genetic material along a single chromosome Translocation –A segment of one chromosome becomes attached to a different chromosome –Simple translocations One way transfer A piece of a chromosome is attached to another chromosome –Reciprocal translocations Two way transfer Two different types of chromosomes exchange pieces, producing two abnormal chromosomes with translocations Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

13. (a) (b) (c) (d) (e) qp Deletion Duplication Inversion Simple Reciprocal 4 3 21123431123 4 321123432321123 43211234321123 1123 111232 4321123 432211 121 1 1 4321 43211 123 translocation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2 Figure 8.2 8-13 Human chromosome 1 Human chromosome 21

14. (a) Terminal deletion(b) Interstitial deletion Single break Two breaks and reattachment of outer pieces (Lost and degraded) + + 4321123 43 43 21123 2 32 1123 41123 8-14 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display A chromosomal deficiency occurs when a chromosome breaks and a fragment is lost Deficiencies Figure 8.3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

15. 8-15 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The phenotypic consequences of deficiencies depends on the 1. Size of the deletion 2. Chromosomal material deleted Are the lost genes vital to the organism? When deletions have a phenotypic effect, they are usually detrimental For example, the disease cri-du-chat syndrome in humans Caused by a deletion in the short arm of chromosome 5 Refer to Figure 8.4 Deficiencies

16. (a) Chromosome 5(b) A child with cri-du-chat syndrome Deleted region Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © Biophoto Assocates/Science Source/Photo Researchers © Jeff Noneley 8-16 Figure 8.4

17. 8-17 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display A chromosomal duplication is usually caused by abnormal events during recombination Repetitive sequences can cause misalignment between homologous chromosomes. If a crossover occurs, nonallelic homologous recombination results Duplications Figure 8.5 Repetitive sequences Misaligned crossover A A B B C C D D A A B B C C D D Duplication Deletion ABCD ABCD ABCC ABD D Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

18. 8-18 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Like deletions, the phenotypic consequences of duplications tend to be correlated to size Duplications are more likely to have phenotypic effects if they involve a large piece of the chromosome However, duplications tend to have less harmful effects than deletions of comparable size In humans, relatively few well-defined syndromes are caused by small chromosomal duplications Duplications

19. 8-19 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The majority of small chromosomal duplications have no phenotypic effect However, they are vital because they provide the raw material for the addition of genes to a species This can ultimately lead to the formation of gene families A gene family consists of two or more genes that are derived from the same ancestral gene Duplications and Gene Families

20. Duplications can provide additional genes, forming gene families Over time, duplicated genes may accumulate mutations which alter their function –As a result, they may have similar but distinct functions –They are now members of a gene family –Two or more genes derived from a common ancestor are homologous –Homologous genes within a single species are paralogs –Refer to figure 8.6 8-20 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

21. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Abnormal genetic event that causes a gene duplication Gene Over the course of many generations, the 2 genes may differ due to the gradual accumulation of DNA mutations. Paralogs (homologous genes) Gene Mutation Gene 8-21 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Figure 8.6 Genes derived from a single ancestral gene

22. 8-22 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The globin genes all encode subunits of proteins that bind oxygen Over 500 million years, the ancestral globin gene has been duplicated and altered so there are now 14 paralogs in this gene family on three different chromosomes Different paralogs carry out similar but distinct functions All bind oxygen Myoglobin stores oxygen in muscle cells Hemoglobins bind and transport oxygen via red blood cells Different globins are expressed in the red blood cells during different developmental stages provide different characteristics corresponding to the oxygen needs of the embryo, fetus and adult Refer to figure 8.7

23. Myoglobins  chains  chains Hemoglobins Millions of years ago 1,000 800 600 400 200 0 Mb Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ζ ψζψζ ψα2ψα2 ψα1ψα1 α2α2 α1α1  ε GG AA ψβψβ δ β Ancestral globin 8-23 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Figure 8.7 Duplication Better at binding and storing oxygen in muscle cells Better at binding and transporting oxygen via red blood cells Expressed very early in embryonic life Expressed maximally during the second and third trimesters Expressed after birth

24. 8-24 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Copy Number Variation (CNV) A segment of DNA that varies in copy number among members of same species May be missing a particular gene May be a duplication Surprisingly common in animals and plants 1-10% of a genome may show CNV Associated with some human diseases schizophrenia autism susceptibility to infectious disease cancer Copy Number Variation is Relatively Common

25. 8-25 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Chromosomal deletions and duplications have been associated with human cancers May be difficult to detect with karyotype analysis Comparative genomic hybridization can be used Developed by Anne Kallioniemi and Daniel Pinkel in 1992 Largely used to detect changes in cancer cell chromosomes Experiment 8A-Comparitive Genomic Hybridization to detect deletions and duplications

26. 8-26 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Experimental level Conceptual level Isolate DNA from human breast cancer cells and normal cells. This involved breaking open the cells and isolating the DNA by chromatography. (See Appendix for description of chromatography.) 1. Label the breast cancer DNA with a green fluorescent molecule and the normal DNA with a red fluorescent molecule. This was done by using the DNA from step 1 as a template, and incorporating fluorescently labeled nucleotides into newly made DNA strands. 2. The DNA strands were then denatured by heat treatment. Mix together equal amounts of fluorescently labeled DNA and add it to a preparation of metaphase chromosomes from white blood cells. The procedure for preparing metaphase chromosomes is described in Figure 3.2. The metaphase chromosomes were also denatured. 3. Allow the fluorescently labeled DNA to hybridize to the metaphase chromosomes. 4. Visualize the chromosomes with a fluorescence microscope. Analyze the amount of green and red fluorescence along each chromosome with a computer. 5. DNA From breast cancer cells From normal cells Metaphase chromosomes Slide Metaphase chromosome Deletions in the chromosomes of cancer cells show a green to red ratio of less than 1, whereas chromosome duplications show a ratio greater than 1. Ratio of green and red fluorescence intensities 0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5 2.0 2.5 Deletion Duplication Chr. 1 – 20 Mb Chr. 9 Chr. 11Chr. 17 Chr. 16 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. THE DATA

27. 8-27 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Interpreting the Data The data shows the ratio of green fluorescence (cancer DNA) to red (normal DNA). Chromosome 1 shows a Duplication (ratio of 2) Chromosome 9, 11, 16, 17 show Deletions (ratio of 0.5) Allows the detection of large chromosomal changes Newer techniques use microarrays (see Chapter 20)

28. 8-28 A chromosomal with an inversion has a segment that has been flipped to the opposite orientation Inversions Figure 8.10 Inverted region A (a) Normal chromosome BCDEFGHI A (b) Pericentric inversion BCGFEDHIA (c) Paracentric inversion EDCBFGHI Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Centromere lies within inverted region Centromere lies outside inverted region

29. 8-29 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In an inversion, the total amount of genetic information stays the same Therefore, the great majority of inversions have no phenotypic consequences In rare cases, inversions can alter the phenotype of an individual Break point effect An inversion break point occurs in a vital gene Position effect A gene is repositioned in a way that alters its gene expression About 2% of the human population carries inversions that are detectable with a light microscope Most of these individuals are phenotypically normal However, some individuals with inversions may produce offspring with phenotypic abnormalities

30. 8-30 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Individuals with one copy of a normal chromosome and one copy of an inverted chromosome Inversion Heterozygotes Such individuals may be phenotypically normal They also may have a high probability of producing gametes that are abnormal in their total genetic content The abnormality is due to crossing over within the inverted segment During meiosis I, pairs of homologous sister chromatids synapse with each other For the normal and inversion chromosome to synapse properly, an inversion loop must form If a crossover occurs within the inversion loop, highly abnormal chromosomes are produced Refer to figure 8.11

31. Replicated chromosomes ABCDEFGHI A A B B C C D D E E A B C D E F F G G H H I ABCDEFGHI FedhiGHI ABCDEfg c ba I F G H I abcgfedhi a a b b c c g g g f f f e e e d d d h h i abcgfedhi i a b c g f e d hi With inversion: Homologous pairing during prophase Crossover site Products after crossing over Normal: With inversion: Acentric fragment Duplicated/ deleted Dicentric chromosome Dicentric bridge Normal: (a) Pericentric inversion (b) Paracentric inversion Crossover site ABCDEFGHI ABCDEF ABCdea GHI IHGFEDcbfghi adecbfghi ABCDEFGHI aedcbfghi aedcbfghi Homologous pairing during prophase Products after crossing over Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 8.11 8-31 No centromere, chromosome is lost Chromosome will break if centromeres move to opposite poles

32. 8-32 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display A chromosomal translocation occurs when a segment of one chromosome becomes attached to another In reciprocal translocations two non-homologous chromosomes exchange genetic material Reciprocal translocations arise from two different mechanisms 1. Chromosomal breakage and DNA repair 2. Abnormal crossovers Refer to Figure 8.12 Translocations

33. 8-33 Figure 8.12 22 Environmental agent causes 2 chromosomes to break. Reactive ends Nonhomologous chromosomes Reciprocal translocation 1177 22 2 2 DNA repair enzymes recognize broken ends and incorrectly connect them. (a) Chromosomal breakage and DNA repair (b) Nonhomologous crossover 71 Reciprocal translocation Crossover between nonhomologous chromosomes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Telomeres prevent chromosomal DNA from sticking to each other

34. 8-34 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Reciprocal translocations lead to a rearrangement of the genetic material, not a change in the total amount Thus, they are also called balanced translocations Reciprocal translocations, like inversions, are usually without phenotypic consequences In a few cases, they can result in position effects Translocations

35. 8-35 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In simple translocations the transfer of genetic material occurs in only one direction These are also called unbalanced translocations Unbalanced translocations are associated with phenotypic abnormalities or even lethality Example: Familial Down Syndrome In this condition, the majority of chromosome 21 is attached to chromosome 14 (Figure 8.13a) The individual would have three copies of genes found on a large segment of chromosome 21 Therefore, they exhibit the characteristics of Down syndrome Refer to Figure 8.13b

36. Person with a normal phenotype who carries a translocated chromosome Translocated chromosome containing long arms of chromosome 14 and 21 Fertilization with a normal gamete Gamete formation 14 21 Possible gametes: Possible offspring: NormalBalanced carrier Familial Down syndrome (unbalanced) Unbalanced, lethal (a) Possible transmission patterns (b) Karyotype of a male with familial Down syndrome (c) Child with Down syndrome Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © Will Hart/PhotoEdit © Paul Benke/University of Miami School of Medicine 123 45 6789101112 131415161718 19202122XY 46, XY,214,1t(14q21q) Figure 8.13 8-36

37. 8-37 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Familial Down Syndrome is an example of Robertsonian translocation This translocation occurs as such Breaks occur near the centromeres of two non- homologous acrocentric chromosomes The small acentric fragments are lost The larger fragments fuse at their centromeric regions to form a single chromosome which is metacentric or submetacentric This type of translocation is the most common chromosomal rearrangement in humans Approximately one in 900 births

38. 8-38 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Individuals carrying balanced translocations have a greater risk of producing gametes with unbalanced combinations of chromosomes This depends on the segregation pattern during meiosis I During meiosis I, homologous chromosomes synapse with each other For the translocated chromosomes to synapse properly, a translocation cross must form Refer to Figure 8.14 Balanced Translocations and Gamete Production

39. Figure 8.14 8-39 Translocation cross Two normal haploid cells + 2 cells with balanced translocations Possible segregation during anaphase of meiosis I Normal chromosome 1 Chromosome 1 plus a piece of chromosome 2 Normal chromosome 2 Chromosome 2 plus a piece of chromosome 1 All 4 haploid cells unbalanced All 4 haploid cells unbalanced 1 1 1 1 2 2 2 1 1 2 2 1 1 2 2 1 1 2 2 1 2 1 11 2 22 2 11 2 11 222 2 2 1 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (a) Alternate segregation(c) Adjacent-2 segregation (very rare)(b) Adjacent-1 segregation

40. 8-40 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Meiotic segregation can occur in one of three ways 1. Alternate segregation Chromosomes diagonal to each other within the translocation cross segregate into the same cell following meiosis I One cell receives 2 normal chromosomes and the other receives 2 translocated chromosomes Leads to viable gametes 2. Adjacent-1 segregation Adjacent non-homologous chromosomes segregate into the same cell after meiosis I Both cells have one normal and one translocated chromosome Leads to 4 genetically unbalanced gametes 3. Adjacent-2 segregation Centromeres do not segregate properly during meiosis I One cell receives both copies of the centromere on chromosome 1 and the other both copies of the centromere on chromosome 2 Leads to 4 genetically unbalanced gametes

41. 8-41 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Alternate and adjacent-1 segregations are the likely outcomes when an individual carries a reciprocal translocation Indeed, these occur at about the same frequency Moreover, adjacent-2 segregation is very rare Therefore, an individual with a reciprocal translocation usually produces four types of gametes Two of which are viable and the other two, nonviable This condition is termed semisterility

42. 8.2 VARIATION IN CHROMOSOME NUMBER Chromosome numbers can vary in two main ways –Euploidy Variation in the number of complete sets of chromosome –Aneuploidy Variation in the number of particular chromosomes within a set –Euploid variations occur occasionally in animals and frequently in plants –Aneuploid variations, on the other hand, are regarded as abnormal conditions Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8-42

43. 1(X) Normal female fruit fly: (a) Diploid; 2n (2 sets) Triploid; 3n (3 sets) Tetraploid; 4n (4 sets) Chromosome composition Polyploid fruit flies: (b) Variations in euploidy Trisomy 2 (2n + 1) Monosomy 1 (2n – 1) Aneuploid fruit flies: (c) Variations in aneuploidy 234 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 8.15 8-43 Polyploid organisms have three or more sets of chromosomes Individual is said to be trisomic Individual is said to be monosomic

44. 8-44 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The phenotype of every eukaryotic species is influenced by thousands of different genes The expression of these genes has to be intricately coordinated to produce a phenotypically normal individual Aneuploidy commonly causes an abnormal phenotype It leads to an imbalance in the amount of gene products Three copies of a gene will lead to 150% production A single chromosome can have hundreds or even thousands of genes Refer to Figure 8.16 Aneuploidy

45. 100% 1 Normal individual Trisomy 2 individual Monosomy 2 individual 23 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 100% 150% 100% 50% 100% 8-45 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Figure 8.16 In most cases, these effects are detrimental They produce individuals that are less likely to survive than a euploid individual

46. 8-46 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Alterations in chromosome number occur frequently during gamete formation About 5-10% of embryos have an abnormal chromosome number Indeed, ~ 50% of spontaneous abortions are due to such abnormalities In some cases, an abnormality in chromosome number produces an offspring that can survive Refer to Table 8.1 Aneuploidy

47. 8-47

48. 8-48 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The autosomal aneuploidies compatible with survival are trisomies 13, 18 and 21 These involve chromosomes that are relatively small Carrie fewer genes than larger chromosomes Aneuploidies involving sex chromosomes generally have less severe effects than those of autosomes This is explained by X inactivation In an individual with more than one X chromosome, all additional X chromosomes are converted into Barr bodies The phenotypic effects listed in Table 8.1 may be due to 1. The expression of X-linked genes prior to embryonic X- inactivation 2. An imbalance in the expression of pseudoautosomal genes

49. 8-49 Some human aneuploidies are influenced by the age of the parents Older parents more likely to produce abnormal offspring Example: Down syndrome (Trisomy 21) Incidence rises with the age of either parent, especially mothers Figure 8.17 Infants with Down syndrome (per 1000 births) Age of mother 80 50 10 0 20255045403530 60 70 90 20 30 40 1 / 1925 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1 / 1205 1 / 885 1 / 365 1 / 110 1 / 32 1 / 12

50. 8-50 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Down syndrome is caused by the failure of chromosome 21 to segregate properly This nondisjunction most commonly occurs during meiosis I in the oocyte The correlation between maternal age and Down syndrome could be due to the age of oocytes Human primary oocytes are produced in the ovary of the female fetus prior to birth They are arrested in prophase of meiosis I until the time of ovulation As a woman ages, her primary oocytes have been arrested in prophase I for a progressively longer period of time This added length of time may contribute to an increased frequency of nondisjunction Paternal non-disjunction causes Down syndrome 5% of the time

51. 8-51 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Most species of animals are diploid In many cases, changes in euploidy are not tolerated Polyploidy in animals is generally a lethal condition Some euploidy variations are naturally occurring Female bees are diploid Male bees (drones) are monoploid Contain a single set of chromosomes A few examples of vertebrate polyploid animals have been discovered Refer to Figure 8.18 Euploidy

52. (b) Hyla versicolor(a) Hyla chrysoscelis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © A.B. Sheldon 8-52 Figure 8.18 Diploid Tetraploid

53. 8-53 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In many animals, certain body tissues display normal variations in the number of sets of chromosomes Diploid animals sometimes produce tissues that are polyploid This phenomenon is termed endopolyploidy Liver cells, for example, can be triploid, tetraploid or even octaploid (8n) May enhance ability of cell to produce specific gene products Polytene chromosomes of insects provide an unusual example of natural variation in ploidy Euploidy

54. 8-54 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Occur in the salivary glands of Drosophila and a few other insects Chromosomes undergo repeated rounds of chromosome replication without cellular division In Drosophila, pairs of chromosomes double approximately nine times (2 9 = 512) These doublings produce a bundle of chromosomes that lie together in a parallel fashion This bundle is termed a polytene chromosome Polytene Chromosomes

55. 8-55 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Figure 8.19 (a) Repeated chromosome replication produces polytene chromosome. (c) Relationship between a polytene chromosome and regular Drosophila chromosomes L R Chromocenter Each polytene arm is composed of hundreds of chromosomes aligned side by side. 4 32 x L R Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Each chromosome attaches to the chromocenter near its centromere Central point where chromosomes aggregate

56. 8-56 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Because of their size, polytene chromosomes lend themselves to an easy microscopic examination They are so large, they can even be seen in interphase Polytene chromosomes exhibit a characteristic banding pattern (Figure 8.19b) Each dark band is known as a chromomere The DNA within the dark band is more compact than that in the interband region Cytogeneticists have identified about 5,000 bands Polytene chromosomes have facilitated the study of the organization and functioning of interphase chromosomes

57. (b) A polytene chromosome Figure 8.19 8-57

58. 8-58 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In contrast to animals, plants commonly exhibit polyploidy 30-35% of ferns and flowering plants are polyploid Many of the fruits and grain we eat come from polyploid plants Refer to Figure 8.20a In many instances, polyploid strains of plants display outstanding agricultural characteristics They are often larger in size and more robust Euploidy

59. (a) Cultivated wheat, a hexaploid species Figure 8.20 8-59 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

60. 8-60 Polyploids having an odd number of chromosome sets are usually sterile These plants produce highly aneuploid gametes Example: In a triploid organism there is an unequal separation of homologous chromosomes (three each) during anaphase I Figure 8.21 Each cell receives one copy of some chromosomes and two copies of other chromosomes

61. 8-61 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Sterility is generally a detrimental trait However, it can be agriculturally desirable because it may result in 1. Seedless fruit Seedless watermelons and bananas Triploid varieties Asexually propagated by human via cuttings 2. Seedless flowers Marigold flowering plants Triploid varieties Developed by Burpee (Seed producers) Energy goes into flower production instead of making seeds (competitors can’t sell seeds grown from their plants)

62. 8.3 NATURAL AND EXPERIMENTAL WAYS TO PRODUCE VARIATIONS IN CHROMOSOME NUMBER There are three natural mechanisms by which the chromosome number of a species can vary –1. Meiotic nondisjunction –2. Mitotic abnormalities –3. Interspecies crosses Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8-62

63. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Meiotic Nondisjunction Nondisjunction refers to the failure of chromosomes to segregate properly during anaphase Meiotic nondisjunction can produce haploid cells that have too many or too few chromosomes If such a gamete participates in fertilization The resulting individual will have an abnormal chromosomal composition in all of its cells Refer to Figure 8.22 8-56

64. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8-64 Figure 8.22 All four gametes are abnormal During fertilization, these gametes produce an individual that is trisomic During fertilization, these gametes produce an individual that is monosomic for the missing chromosome n + 1 (a) Nondisjunction in meiosis I Nondisjunction in meiosis I Normal meiosis II n – 1 n + 1n – 1 Nondisjunction in Meiosis I

65. Nondisjunction in meiosis II Normal meiosis I (b) Nondisjunction in meiosis II nn n + 1 n – 1 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8-65 Figure 8.22 50% Abnormal gametes 50% Normal gametes Nondisjunction in Meiosis II

66. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Meiotic Nondisjunction In rare cases, all the chromosomes can undergo nondisjunction and migrate to one daughter cell This is termed complete nondisjunction It results in one diploid cell and one without chromosomes The chromosome-less cell is nonviable The diploid cell can participate in fertilization with a normal gamete This yields a triploid individual 8-66

67. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Mitotic Abnormalities Abnormalities in chromosome number often occur after fertilization In this case, the abnormality occurs during mitosis not meiosis 1. Mitotic disjunction (Figure 8.23a) Sister chromatids separate improperly This leads to trisomic and monosomic daughter cells 2. Chromosome loss (Figure 8.23b) One of the sister chromatids does not migrate to a pole This leads to normal and monosomic daughter cells 8-67

68. 8-68 Figure 8.23 This cell will be monosomic This cell will be trisomic Will be degraded if left outside of the nucleus when nuclear envelope reforms This cell will be monosomic This cell will be normal (a) Mitotic nondisjunction (b) Chromosome loss Not attached to spindle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

69. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Mitotic Abnormalities Genetic abnormalities that occur after fertilization lead to mosaicism The organism contains a subset of cells that are genetically different from the rest f the organism The size and location of the mosaic region depends on the timing and location of the original abnormality In the most extreme case, an abnormality could take place during the first mitotic division Refer to Figure 8.24 for a bizarre example 8-69

70. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Consider a fertilized Drosophila egg that is XX One of the X’s is lost during the first mitotic division This produces an XX cell and an X0 cell 8-70 The XX cell is the precursor for this side of the fly, which developed as a female The X0 cell is the precursor for this side of the fly, which developed as a male This peculiar and rare individual is termed a bilateral gynandromorph Figure 8.24

71. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Complete nondisjunction can produce an individual with one or more sets of chromosomes This condition is termed autopolyploidy 8-71 Figure 8.25 Diploid species (a) Autopolyploidy (tetraploid) Polyploid species (tetraploid) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

72. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Interspecies Crosses A much more common mechanism for changes in the number of sets of chromosomes is alloploidy It is the result of interspecies crosses Most likely occurs between closely related species 8-72 Figure 8.25 Species 1Species 2 (b) Alloploidy (allodiploid) Alloploid Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

73. (c) Allopolyploidy (allotetraploid) Allopolyploid Species 1Species 2 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display An allodiploid has one set of chromosomes from two different species An allopolyploid contains a combination of both autopolyploidy and alloploidy 8-73 Figure 8.25 An allotetraploid: Contains two complete sets of chromosomes from two different species Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

74. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In two very closely related species, the number and types of chromosomes might be very similar 8-74 Figure 8.26 shows the karyotype of an interspecies hybrid between the roan antelope (Hippotragus equinus) and the sable antelope (Hippotragus niger) These two closely related species have the same number of chromosomes that are similar in size have similar banding patterns Evolutionary related chromosomes from two diferrent species are termed homeologous chromosomes The allodiploid is fertile because the homeologous chromosomes can properly synapse during meiosis

75. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Robinson, T.J. & Harley, E.H. "Absence of geographic chromosomal variation in the roan and sable antelope and the cytogenetics of a naturally occurring hybrid." Cytogenet Cell Genet. 1995; 71(4): 363-9. Permission granted by S. Karger AG, Basel. Reprinted with permission. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1718 1920 21 2223 24 25 26 27 28 29 XX 8-75 Figure 8.26

76. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In 1928, the Russian cytogeneticist G. Karpechenko conducted an interspecies cross between raddish (Raphanus) and cabbage (Brassica) Both are diploid and contain 18 chromosomes Therefore the interspecies hybrid contains 18 chromosomes too However, the radish and cabbage are not closely related species Their chromosomes are distinctly different from one another and cannot synapse Thus, the radish/cabbage hybrid is sterile However, an allotetraploid would be fertile It contains 36 chromosomes which undergo proper synapsis Refer to Figure 8.27 8-76

77. (b) Allotetraploid with a diploid set from each species Metaphase I (a) Allodiploid with a monoploid set from each species Metaphase I Radish chromosome Cabbage chromosome 8-77 Figure 8.27 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display No synapsis between the 9 radish and 9 cabbage chromosomes Proper synapsis between the 18 radish chromosomes and the 18 cabbage chromosomes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

78. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Experimental Treatments Can Promote Polyploidy Polyploid and allopolyploid plants often exhibit desirable traits Thus, the development of polyploids is of considerable interest among plant breeders Can be induced by abrupt temperature changes and drugs The drug colchicine is commonly used to promote polyploidy It binds to tubulin (a protein found in the spindle apparatus) Thus, it promotes nondisjunction 8-78

79. Diploid plant A tetraploid plant Treat with colchicine. Allow to grow. Take a cutting of the tetraploid portion. Root the cutting in soil. Tetraploid portion of plant (note the larger leaves) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8-79 Figure 8.28 Caused by complete nondisjunction

80. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Cell Fusion Techniques Can Be Used to Make Hybrid Plants Researchers have recently developed techniques to produce hybrids with altered chromosome composition In cell fusion, individual cells are mixed together and made to fuse It can create new strains of plants It allows the crossing of two species that cannot interbreed naturally Refer to Figure 8.29 8-80

81. 8-81 Festuca arundinacea Lolium multiflorum Cells without cell walls Cells with two separate nuclei Figure 8.29 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Tall fescue grass Italian ryegrass Protoplasts Cell wall Heterokaryon Hybrid cell Allotetraploid Add agent that digests cell walls. Treat protoplasts with agents to promote cellular fusion. Grow on laboratory media to generate a hybrid plant. Spontaneous nuclear fusion produces hybrid cell with single nucleus. Phenotypic characteristics are intermediate between the “parents”

82. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Experimental Production of Monoploids The production of monoploids can be used to develop homozygous diploid strains of plants In 1964, Sipra Guha-Mukherjee and Satish Maheswari developed a method to produce monoploid plants from pollen grains This experimental technique is called anther culture It is described in Figure 8.30 8-82

83. Treat section with colchicine. Colchicine treated Diploid plant Propagate treated section. Anthers Plantlets Transplant and grow. Grow several weeks. Cold shock 8-83 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Parental plant is diploid but not homozygous for all its genes Figure 8.30 Is homozygous for all its genes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Induces pollen grains to begin development

84. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Experimental Production of Monoploids In certain animal species, monoploids can be produced by treatments that induce the eggs to develop without sperm fertilization This is know as parthenogenesis In many cases, the haploid zygote is short-lived Example: Zebrafish (Danio rerio) Haploid egg is induced to begin development by exposure to UV-irradiated (inactivated) sperm 8-84