1. 1 Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Chapter 07 Lecture Outline

2. 2

3. 7.1 Overview of Linkage  Definition of genetic linkage  How linkage affects the outcome of crosses 3 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

4. Each species of organism contains thousands of genes o Yet most species have at most a few dozen chromosomes Therefore, each chromosome carries hundreds or even thousands of different genes o When genes are physically associated they do not independently assort and will not necessarily follow Mendel’s Law of Independent Assortment 4 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

5. Bateson and Punnett Discovered Two Traits That Did Not Assort Independently Conducted a two- factor cross in the sweet pea involving flower color and pollen shape Expected to yield a 9:3:3:1 phenotypic ratio in the F 2 generation 5 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 296 19 27 85 Observed number 15.6 1.0 1.4 4.5 Ratio 9 3 3 1 240 80 27 Expected number Purple flowers, long pollen (PPLL) Purple flowers, long pollen Purple flowers, round pollen Red flowers, long pollen Red flowers, round pollen F 2 offspring P generation F 1 offspring Purple flowers, long pollen (PpLl ) Self-fertilization x Red flowers, round pollen (ppll ) A much greater proportion of the two types found in the parental generation

6. These genes appear to be coupled Synteny – physical linkage of genes into the same chromosome The term linked or linkage means that two or more genes do not independently assort and tend to be transmitted together Chromosomes are called linkage groups 6 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

7. The number of linkage groups is the number of types of chromosomes of the species o For example, in humans 22 autosomal linkage groups One X chromosome linkage group One Y chromosome linkage group 7 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

8. 7.2 Relationship Between Linkage and Crossing Over  How crossing over can change the arrangements of alleles along a chromosome  How the distance between linked genes affects the proportions of recombinant and nonrecombinant offspring  Chi square analysis to distinguish between linkage and independent assortment  How Creighton and McClintock’s data indicated that recombinant offspring carry chromosomes resulting from crossing over 8 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

9. Genes that are far apart on the same chromosome may independently assort from each other o They may separate by crossing over Recombination between homologous chromosomes in meiosis exchanges pieces of the chromosomes o How often they independently assort depends on how far apart they are Greater distance allows greater potential for crossing over events between the genes 9 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

10. Crossing Over Produces Recombinant Phenotypes Without crossing over o Gametes carry parental combinations of chromosomes o Nonrecombinant cells With crossing over o Non-sister chromatids of homologous chromosomes exchange DNA segments o Results in some of the gametes carrying new combinations of alleles – genetic recombination o Recombinant cells 10

11. Morgan Showed Linkage of X-linked Genes First direct evidence of linkage Investigated several traits that followed an X-linked pattern of inheritance o Body color o Eye color o Wing length 11 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display F 1 generation contains wild-type females and yellow-bodied, white-eyed, miniature-winged males. x x X ywm X ywm Y X y + w + m + X ywm X y + w + m + Y P generation

12. Gray body, red eyes, long wings Gray body, red eyes, miniature wings Gray body, white eyes, long wings Gray body, white eyes, miniature wings Yellow body, red eyes, long wings Yellow body, red eyes, miniature wings Yellow body, white eyes, long wings Yellow body, white eyes, miniature wings 439 208 1 5 7 0 178 365 319 193 0 11 5 0 139 335 758 401 1 16 12 0 317 700 FemalesMalesTotal F 2 generation Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Mostly combinations of traits like the parents o Meaning that All three genes are located on the X chromosome Therefore, they tend to be transmitted together as a unit 12

13. Flies with the parental traits were the most common o But... some flies had recombined traits o And some combinations were more common than others 13 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Gray body, red eyes1,15 9 Yellow body, white eyes1,01 7 Gray body, white eyes 17 Yellow body, red eyes 12 Total2,20 5 Analysis of the data by pairs of genes Red eyes, normal wings 770 White eyes, miniature wings 716 Red eyes, miniature wings 401 White eyes, normal wings 318 Total2,20 5 These recombinants are rare These recombinants are more common

14. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Gray body, red eyes1,159 Yellow body, white eyes1,017 Gray body, white eyes 17 Yellow body, red eyes 12 Total2,205 Analysis of the data by pairs of genes Red eyes, normal wings 770 White eyes, miniature wings 716 Red eyes, miniature wings 401 White eyes, normal wings 318 Total2,205 These recombinants are rare These recombinants are more common 14

15. How could Morgan explain these data? He knew that Janssens had observed chiasmata microscopically in 1909 o He had proposed that crossing over involves a physical exchange between homologous chromosomes Morgan realized that crossing over between homologous X chromosomes was consistent with his data 15 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

16. Morgan’s explanation: o The genes are all located on the X chromosome o Homologous X chromosomes (in the female) can exchange pieces of chromosomes to create new combinations of alleles o Likelihood of crossing over depends on the distance between the two genes Crossing over is more likely to occur between two genes that are far apart 16 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

17. 17

18. Creighton and McClintock Demonstrated Crossing Over Harriet Creighton and Barbara McClintock worked with corn Crosses following two linked genes and a chromosome with mutations that were visible by light microscopy o Chromosome number 9 that had two visible abnormalities o Correlated the occurrence of recombinant offspring with the exchange of segments of homologous chromosomes 18 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Normal chromosome 9 Abnormal chromosome 9 Knob (a) Normal and abnormal chromosome 9 Translocated piece from chromosome 8

19. They also knew of two relevant genes: o A gene for kernel color located near the knobbed end C = Colored c = colorless o A gene for kernel endosperm texture located near the translocated end Wx = Starchy endosperm wx = waxy endosperm 19 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Original chromosomes Recombinant chromosomes CWx cwx C cWx Crossing over

20. They could follow the inheritance of the two genes with the transfer of the abnormalities using a microscope o Recombination of the alleles for the genes resulted in a physical exchange of the DNA from one chromosome to the other 20 CWx Cwx cWx cwx Nonrecombinant chromosomes Allele arrangements: Colorless, starchy Colored, waxy Colorless, waxy Colored, starchy or Recombinant chromosomes (c) A comparison of allele arrangements on nonrecombinant and recombinant chromosomes

21. 7.3 Genetic Mapping in Plants and Animals  Why genetic mapping is useful  Calculating the map distance between linked genes using data from a testcross 21 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

22. Genetic Mapping in Plants And Animals Genetic mapping is also known as gene mapping or chromosome mapping Purpose is to determine the linear order of linked genes along the same chromosome 22 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

23. Genetic maps are useful in many ways including o Understanding genome of a species o Cloning genes o Inferring evolutionary relationships o Understanding human disease o Agriculture and selective breeding Genetic maps can be made by o Recombination analysis o Molecular approaches (Chapter 22) 23 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

24. Maps created by recombination analysis estimate the relative distances between linked genes o Based on the number of times that a crossover event occurs between genes o Makes a linkage map Experimentally, the percentage of recombinant offspring is correlated with the distance between the two genes o If the genes are far apart  many recombinant offspring o If the genes are close  very few recombinant offspring 24 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

25. Genetic mapping experiments are typically accomplished by carrying out a testcross o Heterozygote X Homozygote recessive 25 s e s e s+s+ se+e+ e sse+e+ es+s+ see ssee s e s + e s e s e+e+ Parent s e s e Short bristles Ebony body Nonrecombinant Total: 542 x Parent Long bristles Ebony body Recombinant Short bristles Gray body Recombinant s e s+s+ e+e+ Long bristles Gray body Nonrecombinant 5377675 s+s+ e+e+ s e s+s+ se+e+ e ssee Long bristles Gray body Short bristles Ebony body Chromosomes are the product of a crossover during meiosis in the heterozygous parent Recombinant offspring are fewer in number than non-recombinant offspring

26. The number of recombinants are used to calculate a relative distance between the genes Map distance= = = 12.3 map units 26 Number of recombinant offspring Total number of offspring X 100 76 + 75 542 + 537 + 76 + 75 X 100

27. The units of distance are called map units (mu) o They are also referred to as centiMorgans (cM) Therefore, the s and e genes are 12.3 map units apart from each other along the same chromosome One map unit is equivalent to 1% recombination 27 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

28. As genes get farther apart the chance of getting two (or more even numbered) crossover events between the genes increases With an even number of crossover events the DNA between the genes is exchanged but the genes remain on the same chromosome o These events are not counted as recombinations even though crossing over occurred Observed recombinations tend to underestimate the actual measure of map distance 28 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

29. Percentage of recombinant offspring in a testcross Actual map distance along the chromosome (computed from the analysis of many closely linked genes) 50 25 10 0 050100150 Map units Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Multiple crossovers set a quantitative limit on measurable recombination frequencies as the physical distance increases A testcross is expected to yield a maximum of only 50% recombinant offspring – same as if genes were on different chromosomes 29

30. Three Factor Crosses Data from three-factor crosses can also yield information about map distance and gene order Example: the following experiment outlines a common strategy for using three-factor crosses to map genes o Consider fruit flies that differ in body color, eye color and wing shape 30 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display –b = black body color –b + = gray body color –pr = purple eye color –pr + = red eye color –vg = vestigial wings –vg + = normal wings

31. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Step 1: Cross two true-breeding strains that differ with regard to three alleles. –To get F 1 individuals that are heterozygous for all three genes b+b+ pr + vg + b+b+ pr + vg + b pr vg b pr vg Parental flies x bb prpr vgvgb + b + pr + pr + vg + vg + 31

32. Step 2: Perform a testcross by mating F 1 female heterozygotes to male flies that are homozygous recessive for all three alleles x bb prpr vgvg Homozygous recessive bprvg b+b+ pr + vg + b pr vg b pr vg b + b pr + pr vg + vg F 1 heterozygote 32

33. Step 3: Collect data for the F 2 generation – total 1005 flies 33

34. The three genes exist as two alleles each o Therefore, there are 2 3 = 8 possible combinations of offspring o If the genes assorted independently, all eight combinations would occur in equal proportions In the offspring of crosses involving linked genes o Parental phenotypes occur most frequently o Double crossover phenotypes occur least frequently o Single crossover phenotypes occur with “intermediate” frequency 34

35. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display A double crossover separates the gene in the middle from the other two genes at either end Requires two crossover events The combination of traits in the double crossover tells us which gene is in the middle Conclude that the eye color gene is between the other two 35

36. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Step 4: Calculate the map distance between pairs of genes –Separate data according to pairs of genes Sum number of flies for each pair that do not have the parental combinations For example; Parental offspringTotalNonparental OffspringTotal Gray body, red eyes (411 + 61) 472 Gray body, purple eyes (30 + 2) 32 Black body, purple eyes (412 + 60) 472 Black body, red eyes (28 + 1) 29 944 61 36

37. The map distance between body color and eye color is calculated by dividing the number of recombinants for the pair by the total number of flies Map distance = 61 944 + 61 X 100= 6.1 map units Repeat for the other pairs 37

38. The map distance between body color and wing shape is Map distance = 179 826 + 179 X 100 = 17.8 map units The map distance between eye color and wing shape is Map distance = = 12.3 map units 124 881 + 124 X 100 38

39. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Step 5: Construct the map –The data is also consistent with the map being drawn as vg – pr – b (from left to right) –In detailed genetic maps, the locations of genes are mapped relative to the centromere 6.1 12.3 bprvg Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 39

40. Note that o The sum of distances between body color/eye color and eye color/wing shape is greater than the calculated distance between body color and wing shape 6.1 + 12.3 = 18.4 but the calculated distance based on recombination frequency is 17.8 o This discrepancy arises because of the inability to account for double crossovers – crossovers that switched the eye color gene alleles only 40

41. o Can correct for this by multiplying the number of flies where double crossovers occurred by 2 Map distance = 41 179 +2(2+1) 826+179 X 100 = 18.4 mu

42. 7.4 Mitotic Recombination  The process of mitotic recombination and how it can produce a twin spot 42 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

43. Crossing Over Occasionally Occurs During Mitosis Mitosis does not involve the homologous pairing of chromosomes to form bivalents o Therefore crossing over in mitosis is expected to be much less likely than during meiosis Nevertheless, crossing over does occur on rare occasions o In these cases, it may produce a pair of recombinant chromosomes that have a new combination of alleles o This is known as mitotic recombination 43 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

44. If mitotic recombination occurs during an early stage of embryonic development o The daughter cells containing the recombinant chromosomes continue to divide o This may ultimately result in a patch of tissue with characteristics that are different from those of the rest of the organism Curt Stern proposed that unusual patches on the bodies of certain Drosophila strains were due to mitotic recombination 44 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

45. Stern was working with strains carrying X-linked genes affecting body color and bristle morphology 45 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display –y = yellow body color –y + = gray body color –sn = short body bristles (“singed”) –sn + = normal body bristles Females that are y + y sn + sn are expected to have gray body and normal bristles

46. However, when he microscopically observed these flies, he noticed places in which two adjacent regions were different from the rest of the body and from each other o This is called a twin spot Stern proposed that twin spots are due to a single mitotic recombination within one cell during embryonic development Refer to Figure 7.10 46

47. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Normal mitotic divisions to produce embryo Rare mitotic recombination in one embryonic cell Subsequent separation of sister chromatids Cytokinesis produces 2 adjacent cells Continued normal mitotic divisions to produce adult fly with a twin spot Sister chromatids Mitotic crossover Embryonic cell y + sn y + sn y sn + y sn + y + sn y sn + y + sn y sn + y + sn y sn + y sn + y + sn y + sn y sn + y sn + y sn + X chromosome composition of fertilized egg y + sn y + sn + Figure 7.10 These cells are y+y sn+sn Twin spot is surrounded by cells with gray color and normal bristles Patch with gray color and singed bristles Patch with yellow color and normal bristles