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Chapter 7 Darwin, Mendel and Theories of Inheritance

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1 Chapter 7 Darwin, Mendel and Theories of Inheritance
Figure CO: Finches

2 Overview How do species transform into other species?
How do variations arise? The problem of blending inheritance Solved by Mendel’s principles Other modes of inheritance Further history of genetics as a discipline Sex determination and sexual reproduction

3 Seeking a Mechanism of Heredity
Continuous versus Discontinuous Variation Resolving the important issue of small versus large phenotypic differences was not possible because the mechanisms of inheritance were not understood Lamarckian Inheritance Blending Inheritance Pangenesis

4 Lamarckian Inheritance
To explain why some features persisted while others disappeared, Lamarck invoked use and disuse and the inheritance of acquired characters

5 Blending Inheritance New adaptations would be successively diluted with each generation of interbreeding Baldwins It is obvious to anyone that children resemble a mixture of their parent’s features Mountbattens

6 Mackeral tabby is dominant
Blending Inheritance If correct, natural selection could not maintain a favorable trait for more than a few generations Darwin countered: Isolation – the adaptive character trait could be maintained if those individuals expressing it were isolated from other members of the species Breeders recognized that some traits were “Prepotent” (dominant) and did not blend or dilute through the generations Mackeral tabby is dominant to blotched tabby

7 Blending Inheritance If correct, natural selection could not maintain a favorable trait for more than a few generations Darwin countered: Variation is common Natural selection favors certain variants

8 Blending Inheritance If correct, natural selection could not maintain a favorable trait for more than a few generations Darwin countered: Environments change – different variants become superior

9 Blending Inheritance Darwin accepted the theory of blending inheritance because, despite evidence to the contrary, there was no competing hypothesis to explain heredity Sadly, Darwin and his contemporaries missed Mendel’s insights on inheritance

10 Intraspecific Variation
Darwin noted that individuals within populations were variable for many traits But Darwin never knew the origin of this variation

11 Pangenesis Nine years after publishing The Origin, Darwin conceived of a hypothesis for inheritance, Pangenesis, which is modeled on a concept proposed by Hippocrates ( BC) in writings from ~410 BC Interestingly, Aristotle considered and rejected Hippocrates’s Pangenesis as a mechanism of inheritance

12 Pangenesis Darwin’s hypothesis for inheritance, Pangenesis, synthesized some earlier concepts from Buffon, Bonnet, Owen and Herbert Spencer Darwin proposed that gemmules or pangenes were produced (in varying frequencies) by all the tissues of a parent and incorporated into the developing eggs or sperm

13 Pangenesis The presence of the gemmules and their migration seemed to explain inherited change from use and disuse and their discrete identity helped counter the problem of blending inheritance

14 Pangensis Questioned August Weismann ( ) was the most respected evolutionary biologist of his generation His series of experiments cutting off the tails of mice disproved inheritance of acquired characteristics, though not really disproving pangenesis as he claimed He provided the Germ Plasm Theory as an alternative in which all hereditary material is housed within and passed by gametes

15 Pangensis Questioned Francis Galton ( ), one of Darwin’s cousins, disproved pangenesis by transfusing blood between rabbit strains and demonstrating that the offspring did not acquire traits from the strains that donated the blood Darwin responded that gemmules might not be transported in the blood though he had done some rabbit breeding experiments trying to demonstrate pangenesis

16 Constancy and Variation
The search for the mechanism of heredity continued That mechanism had to explain phenotypic constancy and variation Constancy has the evolutionary significance that all life processes depend on the transmission of information from previous generations like produces like Variations are needed for natural selection in the face of changing environments

17 Gregor Mendel ( ) published (1865)

18 Mendel’s Laws & Experiments
Mendel developed three fundamental principles of heredity: Principle of Dominance Principle of Segregation Principle of Independent Assortment

19 The Principle of Segregation
Factors (genes) are neither changed nor blended in the heterozygote during reproduction, but segregate from each other to be transmitted as discrete particles Figure 02: Mendel’s results for the inheritance of seed shape (smooth or wrinkled) in pea plants

20 Monohybrid Crosses

21 The Nature of Mendelian Genes
Discontinuous Variation Dominant allele – the presence of a single copy of the allele will determine the phenotype (heterozygous or homozygous state) Recessive allele – the presence of two copies of the allele is necessary to determine the phenotype (homozygous state) R D

22 The Nature of Mendelian Genes
Discontinuous Variation Incomplete dominance – the heterozygous phenotype is intermediate (red [RR] – pink [Rr] – white flowers [rr]) Splash (BlBl), Blue (Blbl) and Black (blbl) chickens chestnut white palomino

23 The Nature of Mendelian Genes
Co-dominance – both phenotypes expressed equally (roan cattle produce some all red hairs; others all white); ABO blood groups; sickle cell and normal Hgb

24 The Nature of Mendelian Genes
Multiple alleles (> 2) may be present at the locus ABO blood groups brown hair color

25 The Nature of Mendelian Genes
Discontinuous Variation In general, alleles represent specific DNA sequences, and are passed unchanged from one generation to the next, so long as no mutations occur within the sequence However, there can be variation in the phenotype, even when the genotype is constant Other genes (alleles at different loci) may influence the trait The alleles may exhibit degrees of “penetrance” Environmental factors may alter the expression of the alleles Most alleles are dominant, probably because they code for advantageous traits

26 Gene Penetrance When the phenotype is not expressed, despite the determining genotype being present; the genotype doesn’t “penetrate”

27 Mendel Studied Seven Different Traits in Pea Plants
Pea (Seed) Shape: smooth/wrinkled Pea Pod Shape: inflated/constricted Pea (Seed) Color: yellow/green Pea Pod Color: green/yellow Plant Height: tall/dwarf Pea Flower Color: purple/white Leaf Position: axial/terminal

28 Mendel Studied Seven Different Traits in Pea Plants
Mendel’s data indicated that characters were not diluted out by blending inheritance Mendel hypothesized that there was some sort of indivisible unit of inheritance he termed “elementen” In modern terms, Mendel was identifying the different alleles present at a gene at a locus on a chromosome, though he knew nothing of chromosomes

29 Mendel Studied Seven Different Traits in Pea Plants
Mendel studied 7 characters that appeared to assort independently The pea has seven chromosomes Mendel’s pea’s seven characters behaved as if each gene happened to be on a separate chromosome We now know that this is not the case

30 Mendel Studied Seven Different Traits in Pea Plants
Flower color and seed color are located on chromosome 1 but are so far apart that they do not appear to be linked Pod shape, flower position on the stem and plant height are linked on chromosome 4 Crossing-over occurs so frequently between these loci, that the genes assort independently

31 Mendel Studied Seven Different Traits in Pea Plants
Mendel observed segregation in monohybrid crosses for all seven characters, but did not report dihybrid crosses for the linked characters Either Mendel did not do the crosses, or did them and found the results unexplainable and did not report them This allows him to discern the third relationship: Independent Assortment

32 The Principle of Independent Assortment
Alleles for different phenotypic characters (genes at different loci) are transmitted within gametes to offspring independently of one another Figure 03: Segregation and independent assortment of seed texture and seed color Adapted from Strickberger, M. W. Genetics, Third edition. Macmillan, 1985.

33 Dihybrid Crosses If the genes are on separate chromosomes they will assort independently When two doubly heterozygous parents are crossed, the offspring phenotypic ratio will be 9:3:3:1 9 green wrinkled; 3 green smooth; 3 yellow wrinkled; 1 yellow smooth

34 Dihybrid Crosses 9:3:3:1

35 Dihybrid Crosses Even if the two loci are on the same chromosome, i.e., linked, the traits will assort independently if the loci are far enough apart on the chromosome so that many crossovers occur during meiosis

36 Crossing Over and Recombination
During meiosis, chromosomes duplicate and homologous pairs synapse Chromatids exchange homologous sections carrying alleles, producing recombinant daughter chromosomes with a different combination of alleles

37 What Was the Source of Variation?
Darwin and his contemporaries knew nothing of mutations, or even that chromosomes contained genes as physical entities Therefore Darwin’s critics questioned whether or not population variations could be exhausted so that natural selection would come to a halt

38 How Were Variations Passed to Offspring?
Darwin and many of his contemporaries assumed that the heritable traits of two individuals would be blended by some unknown mechanism when they reproduced Phenotypic expression may be blended, but alleles are preserved and pass unaltered through gametes The experimental geneticists, the “mutationists,” of the early 20th century, who rediscovered Mendel’s work resolved this problem

39 Chromosomes and Genetics
Walter Flemming discovers chromosomes and mitosis (1880) Francis Galton coined the term "eugenics" (1883)

40 Chromosomes and Genetics
Edouard-Joseph-Louis-Marie van Beneden ( ) discovered that each species has a fixed number of chromosomes; he also discovered the formation of haploid cells during cell division of sperm and ova (meiosis) in 1887

41 Three Botanists – Hugo DeVries, Carl Correns, and Erich von Tschermak – Independently Rediscovered Mendel’s Work* in 1900 [*from the Proceedings of the Natural History Society of Brünn in 1866]

42 Hugo de Vries ( ) Hugo de Vries was the Dutch botanist who continued Darwin’s idea of pangenes as the particulate units of inheritance which de Vries described in his Intracellular Pangenesis (1889)

43 Hugo de Vries De Vries proposed the Mutation Theory of Evolution, a form of saltationism [saltus = leap] circa 1903 This was the idea that sudden large or dramatic changes in phenotype, “discontinuous variations,” due to single mutations, were the driving force behind evolution, especially the origin of new species

44 Hugo de Vries De Vries studied plant hybrids, with particular emphasis on the evening primrose, Oenothera lamarckiana de Vries noted distinct traits which bred true which he believed indicated that species arose through sudden spontaneous mutations causing significant morphological changes He was wrong! Not in the data, but in the mechanism. Oenothera ring chromosomes

45 Mendelian Inheritance Has Its Physical Basis in the Behavior of Chromosomes During Sexual Life Cycles Around 1900, cytologists and geneticists began to see parallels between the behavior of chromosomes and the behavior of Mendel’s factors Chromosomes and genes are both present in pairs in diploid cells Homologous chromosomes separate and alleles segregate during meiosis Fertilization restores the paired condition for both chromosomes and genes

46 Chromosome Theory of Inheritance
Around 1902, Walter Sutton, Theodor Boveri, and others noted these parallels and a chromosome theory of inheritance began to take form.

47 Other Early 20th Century Mutationists
R. Punnett Thomas Hunt Morgan William Bateson, who coined the term genetics Wilhelm Johannsen, a Dane, who coined the terms gene, genotype and phenotype

48 Morgan Traced a Gene to a Specific Chromosome
Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome in the early 20th century Like Mendel, Morgan made an insightful choice for an experimental organism, Drosophila melanogaster, a fruit fly species that eats fungi on fruit Fruit flies are prolific breeders and have a generation time of two weeks Fruit flies have three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males)

49 Thomas Hunt Morgan Morgan spent a year looking for variant individuals among the flies he was breeding He discovered a single male fly with white eyes instead of the usual red eyes Discovering the first sex-linked trait The normal character phenotype is the wild type. Alternative traits are mutant phenotypes

50 Mutation occurs but it is NOT passed on to the next generation
MUTATION: any process by which the base pair sequence of a DNA molecule is altered Somatic mutation Germ-line mutation Mutation occurs in gametes and is passed to the next generation, now mutation occurs in both its somatic and germ-line cells Mutation occurs but it is NOT passed on to the next generation Mutation rate: the number of mutations occurring or estimated to occur per generation or per nucleotide pair Mutation frequency: expressed as the proportion of individuals in a population with the mutation

51 Drosophila Mutants

52 Founders of Mathematical Genetics
R. Punnett G.H. Hardy W. Weinberg W.E. Castle R. Punnett took the problem of establishing the mathematical relationship to a mathematician colleague, G.H. Hardy (1908). W. Weinberg (1908) and W.E. Castle (1903) were contemporaries who independently worked out the details in a similar fashion.

53 Some Experiments Did Appear To Support Lamarckian Selection
Austrian Herpetologist Paul Kammerer (1920s) experimented with amphibians whose phenotype seemed to change heritably after exposure to different environments The Case of the Midwife Toad, by Arthur Koestler (1971) champions Kammerer Science will always have some conflicts over the interpretation of data

54 Modern “Saltationists”
Richard Goldschmidt ( ), the German (American immigrant) geneticist who advocated a non-Darwinian origin of species and higher taxa He proposed macromutations and “hopeful monsters” as the source of speciation and macroevolution The Material Basis of Evolution (1940) a plant mutation called fasciation

55 Modern “Saltationists”
Carl Woese ( ) American microbiologist who: Defined Archae Proposed the Three Domain (6 Kingdoms) classification of Life Proposed an “RNA World” intermediate in the process of Abiogenesis, the original formation of life on earth

56 Mutations Are the Raw Material of Evolution
Without mutations, there would be no: new alleles new genes evolution

57 Laboratory Studies of Genetics
Spanning the 20th century and beyond Escherischia coli Drosophila melanogaster Saccharomyces cerevisiae Mus musculus

58 Fig

59 Deviations from Mendelian Genetics
Extranuclear Inheritance Some traits do not follow a nuclear pattern of inheritance but rather transmit through the cytoplasm of the egg. Maternal inheritance = cytoplasmic inheritance Mitochondria and Chloroplasts have their own DNA genomes and cell organelles are provided from the female’s egg cytoplasm; not from the male’s sperm cell or pollen grain cytoplasm

60 Maternal Inheritance = Cytoplasmic Inheritance
Maternally transmitted mitochondrial DNA mutations can reduce lifespan by 1/3 in mice but the mechanism has not been identified (Scientific Reports, 4:6569, 2014)

61 Maternal Inheritance = Cytoplasmic Inheritance
Transmission of chloroplasts is similar in plants Variegated leaves and fruits can also be caused by defects or mutations in chloroplast DNA Different populations of cells in these fruits received more or less of the defective chloroplasts

62 Sex Determination Sex chromosomes Autosomes and sex determination
For many organisms, especially mammals, sex determination is associated with chromosomal differences between the two sexes, typically XX females and XY males. Autosomes and sex determination The sex of an individual is determined by the ratio of X chromosomes to sets of autosomes (A). Environmentally induced sex determination Wide variety of mechanisms E.g. green spoon worm, Bonellia viridis

63 Sex Determination Some system of sex chromosomes in most animals

64 Sex Determination There are a variety of chromosomal systems in animals In angiosperms, the majority do not have separate sexes; those that do generally have an XY system

65 XY sex determination In some plants

66 ZW sex determination

67 XO sex determination XX = female and X0 = male
Grasshooppers, cockroaches, etc., Caenorhabditis elegans

68 Sex Determination In Drosophila, sex is determined by the ratio of X chromosomes to sets of autosomes However, fruit flies do also carry a Y chromosome with some genes related to maleness Males: 1 X: 1 set of autosomes; ratio = 1.0 Females: 1 X: 2 set of autosomes ; ratio = 0.5

69 Sex Determination In many fungi, specific sex genes, located on an autosome, are involved Protistans also have a wide variety of methods of sex determination and reproduction

70 Haplo-Diploid Sex Determination in All Hymenoptera
32 (Diploid) 16 (Haploid) Figure 15.6 Some chromosomal systems of sex determination Workers (sisters) are more closely related on average (75%) to each other than to their mother, the queen (50%)

71 Haplo-Diploid Sex Determination in All Hymenoptera
Haploid male bee copulates with a diploid female → haploid sperm is stored in the female bee’s spermatheca When a female “wants” to produce son, she lays an unfertilised (haploid) egg → male offspring To produce female offspring, the mother needs to add sperm to her egg as it passes down her oviduct Only 2 chromosomes are shown here

72 Environmental Sex Determination
Bonellia viridis, the green spoon worm, generates free-swimming larvae Those larvae that reach sea bottom develop into females Those larvae that land on a female’s proboscis develop into parasitic males who live in the female’s reproductive tract

73 Environmental Sex Determination
Osedax sp., tube worms, which feed on whale carcass bone also exhibit dwarf males who live in the female’s external capsule Figure B03A: Tubeworms Figure B03B: Tubeworms Figure B03C: Dwarf males Reprinted from Deep Sea Research Part II: Topical Studies in Oceanography, vol. 56, Robert C. Vrijenhoek, Cryptic species, phenotypic plasticity..., pp Copyright 2009, with permission from Elsevier. [ Courtesy of Greg Rouse

74 Environmental Sex Determination
All crocodilians, most turtles, a few lizards and rare birds have temperature dependent sex determination (TSD) Those with TSD do not have sex chromosomes Global warming may interfere with sex ratios in these species! Australian Bush Turkey (Megapode)

75 Environmental Sex Determination

76 Environmental Sex Determination
The majority of reef fish change sex at some point during their lives In fact,  reef fish that remain as the same sex for their entire life span (gonochoristic) are in the  minority Some species will begin life as  males and switch to females (protandry), and others switch from female to male  (protogyny) Some will change sex in both directions, and others will be both sexes at the same time

77 Social Control protogynous grouper Epinephelus, females first
protandrous clownfish Amphiprion percula, males first protogynous blue-headed wrasse Thalassoma bifasciatum, females first the female is the largest individual, the male the second largest; the rest of the group are smaller non-breeders without functioning gonads

78 Environmental Sex Determination
Lariophagus parasitoid wasps lay eggs into granary weevil larvae Larger larvae receive eggs which will become female Smaller larvae receive eggs which will become male This allows young females to acquire more nutrients

79 Parthenogenesis Parthenogenesis is a form of asexual reproduction in which females produce eggs that develop without fertilization Parthenogenesis is seen to occur naturally in some invertebrates, along with several fish, amphibians, and reptiles as well as in many plants There are no known cases of parthenogenesis in mammals

80 Sexual Reproduction Two sources of variation
Recombination can produce different combinations of genes along a chromosome Individuals incorporate different beneficial mutations from other population members through their parents mating

81 Sexual Reproduction Meiosis always shuffles the chromosomes, and crossing over further increases the genetic variability of the gametes produced

82 Deviations from Mendelian Genetics
Sex-Linked Genes and Sexual Reproduction Genes do not necessarily assort independently of each other if they are linked together on the same chromosome While true of linkage on both autosomes and sex chromosomes, the patterns of inheritance are more dramatic when genes are linked on a sex chromosome, since recessive alleles will be expressed in the homogametic sex

83 Sex-Linked Genes

84 Table T01: Major Discoveries Leading to Our Current Concepts of the Nature of the Gene

85 Bacterial Transformation (1928)
Frederick Griffith ( ) studied Streptococcus pneumoniae, hoping to find a vaccine The rough strain was not virulent while the encapsulated smooth strain was virulent, i.e., able to cause pneumonia in mice He could transform rough bacteria into smooth bacteria by exposing them to dead smooth bacteria Griffith did not know it was DNA uptake and recombination by the living cells

86 Bacterial Transformation (1944)
Oswald Avery, Colin MacLeod, and Maclyn McCarty used enzymes to eliminate the various classes of biological polymers, one at a time They demonstrated that DNA was the “transforming factor” first identified by Griffith

87 The Double Helix (1953) DNA x-ray diffraction
Frances Crick, James Watson Rosalind Franklin

88 Arthur Kornberg ( ) In 1956 Kornberg isolated the first DNA polymerizing enzyme, now known as DNA polymerase I This won him the Nobel prize in 1959 First in vitro synthesis of E. coli DNA in 1968!

89 Kornberg: DNA Replication 1958

90 Genetic Code

91 First Genome Sequenced 1995
Haemophilus influenza – 1995 Yeast – 1996 First human chromosome and entire Drosophila melanogaster genome – 1999 First draft of human genome – 2000 Complete sequence of human genome – 2006 The first cell with a synthetic genome – 2010 What’s next? J. Craig Venter ( )

92 Chapter 7 End

93 The Nature of Mendelian Genes
Discontinuous Variation

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