1. Genetics: From Genes to Genomes
Powerpoint to accompany
Genetics: From Genes to Genomes
Third Edition
Hartwell ● Hood ● Goldberg ● Reynolds ● Silver ● Veres
Chapter 4
Prepared by Malcolm Schug
University of North Carolina Greensboro
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2. The Chromosome Theory of Inheritance
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3. Outline of Chromosome Theory of Inheritance
Observations and experiments that placed the hereditary material in the nucleus on the chromosomes
Mitosis ensures that every cell in an organism carries same set of chromosomes.
Meiosis distributes one member of each chromosome pair to gamete cells.
Gametogenesis, the process by which germ cells differentiate into gametes
Validation of the chromosome theory of inheritance
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4. Evidence that Genes Reside in the Nucleus
1667 – Anton van Leeuwenhoek
Microscopist
Semen contains spermatozoa (sperm animals).
Hypothesized that sperm enter egg to achieve fertilization
– confirmation of fertilization through union of eggs and sperm
Recorded frog and sea urchin fertilization using microscopy and time-lapse drawings and micrographs
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5. Evidence that Genes Reside in Chromosomes
1880s – innovations in microscopy and staining techniques identified thread-like structures
Provided a means to follow movement of chromosomes during cell division
Mitosis – two daughter cells contained same number of chromosomes as parent cell (somatic cells)
Meiosis – daughter cells contained half the number of chromosomes as the parents (sperm and eggs)
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6. One Chromosome Pair Determines an Individual’s Sex.
Walter Sutton – Studied great lubber grasshopper
Parent cells contained 22 chromosomes plus an X and a Y chromosome.
Daughter cells contained 11 chromosomes and X or Y in equal numbers.
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7. Great lubber grasshopper
After fertilization
Cells with XX were females.
Cells with XY were males.
Figure 4.2
Great lubber grasshopper
(Brachystola magna)
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Fig. 4.5

8. Photomicrograph of human
Sex chromosome
Provide basis for sex determination
One sex has matching pair.
Other sex has one of each type of chromosome.
Figure 4.3a
Photomicrograph of human
X and Y chromosome
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Fig. 4.6a

9. Sex determination in humans
Children receive only an X chromosome from mother but X or Y from father.
Figure 4.3b
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Fig. 4.6b

10. At Fertilization, Haploid Gametes Produce Diploid Zygotes.
Gamete contains one-half the number of chromosomes as the zygote.
Haploid – cells that carry only a single chromosome set
Diploid – cells that carry two matching chromosome sets
n – the number of chromosomes in a haploid cell
2n – the number of chromosomes in a diploid cell
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11. diploid vs haploid cell in
Drosophila
melanogaster
Figure 4.4
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Fig. 4.2

12. The number and shape of chromosomes vary from species to species.
Organism n 2n
Drosophila melanogaster 4 8
Drosophila obscura 5 10
Drosophila virilus 6 12
Peas 7 14
Macaroni wheat 28
Giant sequoia trees 11 22
Goldfish 47 94
Dogs 39 78
Humans 23 46
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13. Anatomy of a chromosome
Figure 4.5
Metaphase chromosomes are classified by the position of the centromere
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Fig. 4.3

14. Homologous chromosomes match in size, shape, and banding patterns.
Homologous chromosomes (homologs) contain the same set of genes.
Genes may carry different alleles.
Nonhomologous chromosomes carry completely unrelated sets of genes.
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15. Karyotypes can be produced by cutting micrograph images of stained chromosomes and arranging them in matched pairs
Figure 4.6
Human male karyotype
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Fig 4.4

16. Autosomes – pairs of nonsex chromosomes
Sex chromosomes and autosomes are arranged in homologous pairs
Note 22 pairs of autosomes and 1 pair of sex chromosomes
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17. There is variation between species in how chromosomes determine an individual’s sex.
__________________________________________________
Chromosome Females Males Organism
XX-XY XX XY Mammals, Drosophila
XX-XO XX XO Grasshoppers
ZZ-ZW ZW ZZ Fish, Birds, Moths
Table 4.1
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18. Drosophila Humans Complement of sex chromosomes
Humans – presence of Y determines sex
Drosophila – ratio of autosomes to X chromosomes determines sex
XXX XX
XXY XO XY
XYY OY
Drosophila
Dies
Normal female
Sterile male
Normal male
Humans
Nearly normal
female
Kleinfelter male (sterile); tall, thin
Turner female (sterile); webbed neck
Normal or nearly normal male
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19. Mitosis ensures that every cell in an organism carries the same chromosomes.
Cell cycle – repeating pattern of cell growth and division
Alternates between interphase and mitosis
Interphase – period of cell cycle between divisions/cells grow and replicate chromosomes
G1 – gap phase – birth of cell to onset of chromosome replication/cell growth
S – synthesis phase – duplication of DNA
G2 – gap phase – end of chromosome replication to onset of mitosis
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20. The cell cycle Fig. 4.7a Figure 4.7a
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Fig. 4.7a

21. Chromosome replication during S phase of cell cycle
Synthesis of chromosomes
Note the formation of sister chromatids
Figure 4.7 b
Fig. 4.7 b
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22. Interphase Within nucleus Outside of nucleus
G1, S, and G2 phase – cell growth, protein synthesis, chromosome replication
Outside of nucleus
Formation of microtubules radiating out into cytoplasm crucial for interphase processes
Centrosome – organizing center for microtubules located near nuclear envelope
Centrioles – pair of small darkly stained bodies at center of centrosome in animals (not found in plants)
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23. Mitosis – Sister chromatids separate
Prophase – chromosomes condense
Inside nucleus
Chromosomes condense into structures suitable for replication.
Nucleoli begin to break down and disappear.
Outside nucleus
Centrosomes which replicated during interphase move apart and migrate to opposite ends of the nucleus.
Interphase microtubules disappear and are replaced by microtubules that rapidly grow from and contract back to centrosomal organizing centers.
Fig. 4.8 a
Figure 4.8 a
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24. Mitosis - continued Prometaphase Nuclear envelope breaks down
Fig. 4.8b
Prometaphase
Nuclear envelope breaks down
Microtubules invade nucleus
Chromosomes attach to microtubules through kinetochore
Mitotic spindle – composed of three types of microtubules
Kinetochore microtubules – centrosome to kinetochore
Polar microtubules – centrosome to middle of cell
Astral microtubules – centrosome to cell’s periphery
Figure 4.8 b
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25. Mitosis - continued Metaphase – middle stage
Chromosomes move towards imaginary equator called metaphase plate
Figure 4.8 c
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Fig. 4.8 c

26. Mitosis - continued Anaphase
Separation of sister chromatids allows each chromatid to be pulled towards spindle pole connected to by kinetochore microtubule.
Figure 4.8 d
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Fig. 4.8 d

27. Mitosis – continued Telophase Spindle fibers disperse
Nuclear envelope forms around group of chromosomes at each pole
One or more nucleoli reappear
Chromosomes decondense
Mitosis complete
Figure 4.8 e
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Fig. 4.8 e

28. Mitosis - continued Cytokinesis - cytoplasm divides
Starts during anaphase and ends in telophase
Animal cells – contractile ring pinches cells into two halves
Plant cells – cell plate forms dividing cell into two halves
Figure 4.8 f
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Fig. 4.8 f

29. Checkpoints help regulate cell cycle
Figure 4.11
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Fig. 4.11

30. Meiosis produces haploid germ cells.
Somatic cells – divide mitotically and make up vast majority of organism’s tissues
Germ cells – specialized role in the production of gametes
Arise during embryonic development in animals and floral development in plants
Undergo meiosis to produce haploid gametes
Gametes unite with gamete from opposite sex to produce diploid offspring.
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31. Meiosis Chromosomes replicate once. Nuclei divide twice.
Figure 4.12
Fig. 4.12
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32. Meiosis – Prophase I Feature Figure 4.13 Feature Figure 4.13
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33. Meiosis – Prophase I continued
Feature Figure 4.13
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34. Crossing over during prophase produces recombined chromosomes.
Figure 4.14 a-c
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Fig a-c

35. Figure 4.14 d, e
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Fig d, e

36. How crossing over produces recombined gametes
Figure 4.15
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Fig. 4.15

37. Meiosis I – Metaphase and Anaphase
Figure 4.13
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38. Meiosis – Telophase I and Interkinesis
Figure 4.13
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39. Meiosis – Prophase II and Metaphase II
Figure 4.13
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40. Meiosis – Anaphase II and Telophase II
Figure 4.13
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41. Meiosis - Cytokenesis Fig. 4.13 Figure 4.13
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Fig. 4.13

42. Meiosis contributes to genetic diversity in two ways.
Independent assortment of nonhomologous chromosomes creates different combinations of alleles among chromosomes.
Crossing-over between homologous chromosomes creates different combinations of alleles within each chromosome.
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43. Figure 4.17
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Fig. 4.17

44. Gametogenesis involved mitosis and meiosis.
Oogenesis – egg formation in humans
Diploid germ cells called oogonia multiply by mitosis to produce primary oocytes.
Primary oocytes undergo meiosis I to produce one secondary oocyte and one small polar body (which arrests development).
Secondary oocyte undergoes meiosis II to produce one ovum and one small polar body.
Polar bodies disintegrate leaving one large functional gamete
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45. Oogenesis in humans Fig 4.18 Figure 4.18
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Fig 4.18

46. Gametogenesis Spermatogenesis in humans
Symmetrical meiotic divisions produce four functional sperm.
Begins in male testis in germ cells called spermatogonia
Mitosis produces diploid primary spermatocytes.
Meiosis I produces two secondary spermatocytes per cell.
Meiosis II produces four equivalent spermatids.
Spematids mature into functional sperm.
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47. Spermatogenesis in humans
Figure 4.19
Fig. 4.19
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48. The chromosome theory correlates Mendel’s laws with chromosome behavior during meiosis.
Each cell contains two copies of each chromosome
Chromosome complements appear unchanged during transmission from parent to offspring.
Homologous chromosomes pair and then separate to different gametes.
Maternal and paternal copies of chromosome pairs separate without regard to the assortment of other homologous chromosome pairs.
At fertilization an egg’s set of chromosomes unite with randomly encountered sperm’s chromosomes.
In all cells derived from a fertilized egg, one half of chromosomes are of maternal origin, and half are paternal.
Behavior of genes
Each cell contains two copies of each gene.
Genes appear unchanged during transmission from parent to offspring.
Alternative alleles segregate to different gametes.
Alternative alleles of unrelated genes assort independently.
Alleles obtained from one parent unite at random with those from another parent.
In all cells derived from a fertilized gamete, one half of genes are of maternal origin, and half are paternal.
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49. Specific traits are transmitted with specific chromosomes.
A test of the chromosome theory
If genes are on specific chromosomes, then traits determined by the gene should be transmitted with the chromosome.
T.H. Morgan’s experiments demonstrating sex-linked inheritance of a gene determining eye-color demonstrate the transmission of traits with chromosomes.
1910 – T.H. Morgan discovered a white – eyed male, Drosophila melanogaster, among his stocks.
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50. Nomenclature for Drosophila genetics
Wild-type allele - allele that is found in high frequency in a population
Denoted with a “+”
Mutant allele - allele found in low frequency
Denoted with no symbol
Recessive mutation - gene symbol is in lower case
Dominant mutation - gene symbol is in upper case
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51. Examples of notations for Drosophila
Gene symbol is chosen arbitrarily
e.g., Cy is curly winged, v is vermilion eyed, etc.
Cy, Sb, D are dominant (upper case letter).
vg, y, e, are recessive (lower case letter).
vg+ - wild-type recessive allele for vestigial gene locus
Cy+ - wild-type dominant allele for curly gene locus
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52. Criss-cross inheritance of the white gene demonstrates X-linkage.
Figure 4.20
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Fig. 4.20

53. Rare events of nondisjunction in XX female produce XX and O eggs.
Figure 4.21 a
Segregation in an XX female
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Fig a

54. Segregation in an XXY female Fig. 4.21 b Figure 4.21 b
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Fig b

55. X and Y linked traits in humans are identified by pedigree analysis.
X-linked traits exhibit five characteristics seen in pedigrees.
Trait appears in more males than females.
Mutation and trait never pass from father to son.
Affected male does pass X-linked mutation to all daughters, who are heterozygous carriers.
Trait often skips a generation.
Trait only appears in successive generations if sister of an affected male is a carrier. If so, one half of her sons will show trait.
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56. Example of sex-linked recessive trait in human pedigree – hemophilia
Figure 4.23 a
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Fig a

57. Example of sex-linked dominant trait in human pedigree – hypophosphatemia
Figure 4.23 b
Fig b
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