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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHAPTER 13 LECTURE SLIDES To run the animations you must be.

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Presentation on theme: "Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHAPTER 13 LECTURE SLIDES To run the animations you must be."— Presentation transcript:

1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHAPTER 13 LECTURE SLIDES To run the animations you must be in Slideshow View. Use the buttons on the animation to play, pause, and turn audio/text on or off. Please note: once you have used any of the animation functions (such as Play or Pause), you must first click in the white background before you advance the next slide.

2 Chromosomes, Mapping, and the Meiosis–Inheritance Connection Chapter 13

3 What Carried Mendel’s “Factors”? Carl Correns – 1900 –First suggests central role for chromosomes –One of papers announcing rediscovery of Mendel’s work Walter Sutton – 1902 Chromosomal theory of inheritance –Based on observations that similar chromosomes paired with one another during meiosis 3

4 T.H. Morgan – 1910 –Working with fruit fly, Drosophila melanogaster –Discovered a mutant male fly with white eyes instead of red –Crossed the mutant male to a normal red-eyed female All F 1 progeny red eyed = dominant trait 4

5 5 Morgan crossed F 1 females x F 1 males F 2 generation contained red and white- eyed flies –But all white-eyed flies were male Testcross of a F 1 female with a white-eyed male showed the viability of white-eyed females Morgan concluded that the eye color gene resides on the X chromosome FIRST DIRECT EVIDENCE FOR CHROMOSOMAL THEORY OF INHERITANCE! – showed that genes lie on chromosomes

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8 8 Sex Chromosomes Sex determination in Drosophila is based on the number of X chromosomes –2 X chromosomes = female –1 X and 1 Y chromosome = male Sex determination in humans is based on the presence of a Y chromosome –2 X chromosomes = female –Having a Y chromosome (XY) = male

9 Humans have 46 total chromosomes –22 pairs are autosomes –1 pair of sex chromosomes –Y chromosome highly condensed Recessive alleles on male’s X have no active counterpart on Y –“Default” for humans is female Requires SRY gene on Y for “maleness” 9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. X chromosome Y chromosome 2.8 µm © BioPhoto Associates/Photo Researchers, Inc.

10 Hemophilia Disease that affects a single protein in a cascade of proteins involved in the formation of blood clots Form of hemophilia is caused by an X-linked recessive allele –Heterozygous females are asymptomatic carriers Allele for hemophilia was introduced into a number of different European royal families by Queen Victoria of England 10

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12 12 Dosage compensation Ensures an equal expression of genes from the sex chromosomes even though females have 2 X chromosomes and males have only 1 In each female cell, 1 X chromosome is inactivated and is highly condensed into a Barr body Females heterozygous for genes on the X chromosome are genetic mosaics

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14 14 Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at

15 15 Chromosome theory exceptions Mitochondria and chloroplasts contain genes Traits controlled by these genes do not follow the chromosomal theory of inheritance Genes from mitochondria and chloroplasts are often passed to the offspring by only one parent (mother) –Maternal inheritance In plants, the chloroplasts are often inherited from the mother, although this is species dependent

16 16 Genetic Mapping Early geneticists realized that they could obtain information about the distance between genes on a chromosome Based on genetic recombination (crossing over) between genes If crossover occurs, parental alleles are recombined producing recombinant gametes

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19 Review of Expectations – 2 genes on same chromosome 19 R - Round, r-ovalW-White, w-black P1RRWW x rrww F1RrWw F2 - testcrossRWrw RrWwrrww F2 Expected if Same11 ChromosomeRound_Whiteoval_black

20 Review of Expectations- 2 genes on diff. chromosomes 20 R - Round, r-ovalW-White, w-black P1RRWW x rrww F1RrWw F2 - testcrossRWRwrWrw RrWwRrwwrrWwrrww F2 Expected if1111 Indep. Asst.Round_WhiteRound_blackoval_Whiteoval_black

21 Review of Expectations- 2 genes on same chromosome – but now with crossover events 21 R - Round, r-ovalW-White, w-black P1RRWW x rrww F1RrWw F2 - testcrossRWRwrWrw RrWwRrwwrrWwrrww F2 Expected if<1<<1 <1 Crossover between genesRound_WhiteRound_blackoval_Whiteoval_black RECOMBINANTS

22 22 Alfred Sturtevant –Undergraduate in T.H. Morgan’s lab –Put Morgan’s observation that recombinant progeny reflected relevant location of genes in quantitative terms –As physical distance on a chromosome increases, so does the probability of recombination (crossover) occurring between the gene loci

23 Probability of crossover depends on distance 23 C S c s C S c s

24 Recombination Frequency 24 Genotype of gametes formed by double heterozygous parent (Cc Ss) C Sc sC sc S CcSsccssCcssccSs Appearance (phenotype) Colored, Smooth colorless, wrinkled Colored, wrinkled colorless, Smooth % total recombinants if Independent Assortment25% 50% if closely linked (no crossover)50% -- 0% if there's some distance between them45% 5% 10% if there's more distance between them40% 10% 20% if there's even more distance between them35% 15% 30% if there's even more distance between them30% 20% 40% Now they're so far apart they almost sort independently due to the high probability of crossover events25% 50% C S c s S C c s S C c s S s C c Ss c C Ss c C S s c C

25 25 Multiple crossovers If homologues undergo two crossovers between loci, then the parental combination is restored Leads to an underestimate of the true genetic distance Relationship between true distance on a chromosome and the recombination frequency is not linear

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27 Working examples- note the cute way to cross to test for linkage 1. In tomatoes tall growth habit is the result of a dominant gene, D, dwarf growth to its recessive allele, d. Smooth epidermis is due to a dominant gene, P, pubescent epidermis to its recessive allele, p. A homozygous tall smooth variety was crossed with a dwarf pubescent variety. The F1 were test crossed to dwarf pubescent. The results of the test cross were as follows: tall, smooth - 96 tall, pubescent - 4 dwarf, smooth - 3 dwarf, pubescent - 95 Are these genes linked (on the same chromosome)? If so, what is the percent of crossing over? 27

28 Worked solution 28 P1 == DDPP x ddpp Parental gametesDP and dp F1 == DdPp TestCrossDPDpdPdp DdPpDdppddPpddpp Tall, smoothTall, pubesdwarf, smoothdwarf, pubes Same Chromosome because mostly parental types % Crossover = (4+3) / ( ) "="7recombinants 198total offspring % crossover

29 2. In rabbits color is due to a dominant gene, D, albinism to its recessive allele, d. Black is the result of a dominant gene, B, brown to its recessive allele, b. Brown (homozygous) rabbits are crossed with albinos carrying black in the homozygous state. The F1 are crossed to double recessive. From many such crosses the total results are: black - 68 brown albino Are these genes linked? 29

30 Worked solution 30 P1 == DDbb x ddBB Parental gametesDb and dB F1 == DdBb TestCrossDbDBdbdB dbDdbbDdBbddbbddBb Colored, brnColored, blkAlbino, brownAlbino, black 13268?? 200 albinos= Same Chromosome because mostly parental types % Crossover = (68+68?) / ( (68+132) ) "="136recombinants 400total offspring 34% crossover

31 In corn purple plant color is due to a dominant gene, P, green plant color to recessive allele, p. Normal leaves are due to a dominant gene, N, narrow leaves to its recessive allele, n. Homozygous green plants with homozygous normal leaves were crossed with homozygous purple plants having narrow leaves. The F1 were crossed with green plants with narrow leaves. The results of this test cross were as follows: purple plants with normal leaves purple plants with narrow leaves green plants with normal leaves green plants with narrow leaves Are these genes linked? If so what is the percent cross over? 31

32 Worked solution-note that parental gametes are mixed 32 P1 == ppNN x PPnn Parental gametespN and Pn F1 == pPNn TestCrosspNpnPNPn pnppNnppnnPpNnPpnn green, normGrn, narrowPurple, normPurp, narrow Different Chromosomes because equal probabilities

33 In corn tallness is due to a dominant gene, D, dwarfness to its recessive allele, d. Normal leaves is dominant, N, crinkly leaves is recessive, n. Homozygous tall, crinkly- leaved corn was crossed to dwarf, homozygous normal- leaved corn. The F1 which was tall normal leaved, was crossed to double-recessive dwarf crinkly-leaved corn. The results were as follows: Tall crinkly-leaved - 83 Tall, normal-leaved - 19 dwarf crinkly-leaved - 17 dwarf normal-leaved - 81 Are these two pairs of genes linked? If so, what is the percent crossing over? 33

34 Worked solution-note that parental gametes are mixed 34 P1 == DDnn x ddNN Parental gametesDn and dN F1 == DdnN TestCrossDnDNdndN dnDdnnDdNnddnnddNn Tall, crinklyTall, normdwarf, crinklydwarf, norm Same chromosome because mostly parental types (19+17)/(200) = 36/200=18%

35 35 Constructing maps The distance between genes is proportional to the frequency of recombination events recombination recombinant progeny frequency total progeny 1% recombination = 1 map unit (m.u.) 1 map unit = 1 centimorgan (cM) =

36 36 Three-point testcross Uses 3 loci instead of 2 to construct maps Gene in the middle allows us to see recombination events on either side In any three-point cross, the class of offspring with two crossovers is the least frequent class In practice, geneticists use three-point crosses to determine the order of genes, then use data from the closest two-point crosses to determine distances

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38 38 Human genome maps Data derived from historical pedigrees Difficult analysis –Number of markers was not dense enough for mapping up to 1980s –Disease-causing alleles rare Situation changed with the development of anonymous markers –Detected using molecular techniques –No detectable phenotype

39 SNPs Single-nucleotide polymorphisms Affect a single base of a gene locus Used to increase resolution of mapping Used in forensic analysis –Help eliminate or confirm crime suspects or for paternity testing 39

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41 Sickle cell anemia First human disease shown to be the result of a mutation in a protein Caused by a defect in the oxygen carrier molecule, hemoglobin –Leads to impaired oxygen delivery to tissues 41

42 Homozygotes for sickle cell allele exhibit intermittent illness and reduced life span Heterozygotes appear normal –Do have hemoglobin with reduced ability Sickle cell allele is particularly prevalent in people of African descent –Proportion of heterozygotes higher than expected –Confers resistance to blood-borne parasite that causes malaria 42

43 43 Nondisjunction Failure of homologues or sister chromatids to separate properly during meiosis Aneuploidy – gain or loss of a chromosome –Monosomy – loss –Trisomy – gain –In all but a few cases, do not survive

44 Smallest autosomes can present as 3 copies and allow individual to survive –13, 15, 18, 21 and 22 –13, 15, 18 – severe defects, die within a few months –21 and 22 – can survive to adulthood –Down Syndrome – trisomy 21 May be a full, third 21 st chromosome May be a translocation of a part of chromosome 21 Mother’s age influences risk 44

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46 46 Nondisjunction of sex chromosomes Do not generally experience severe developmental abnormalities Individuals have somewhat abnormal features, but often reach maturity and in some cases may be fertile XXX – triple-X females XXY – males (Klinefelter syndrome) XO – females (Turner syndrome) OY – nonviable zygotes XYY – males (Jacob syndrome)

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48 48 Genomic imprinting Phenotype exhibited by a particular allele depends on which parent contributed the allele to the offspring Specific partial deletion of chromosome 15 results in –Prader-Willi syndrome if the chromosome is from the father –Angelman syndrome if it’s from the mother

49 49 Detection Pedigree analysis used to determine the probability of genetic disorders in the offspring Amniocentesis collects fetal cells from the amniotic fluid for examination Chorionic villi sampling collects cells from the placenta for examination

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