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

Chapter 7

Darwin, Mendel andDarwin, Mendel andTheories of InheritanceTheories of InheritanceFigure CO:

Finches

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

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

Lamarckian Inheritance• To explain why some features

persisted while others disappeared, Lamarck invoked use and disuse and the inheritance of acquired characters

Blending Inheritance

• New adaptations would be successively diluted with each generation of interbreeding

It is obvious to anyone that children resemble a mixture of their parent’s features

Baldwins

Mountbattens

Blending Inheritance• If correct, natural selection could not maintain a

favorable trait for more than a few generations• Darwin countered:

1. Isolation – the adaptive character trait could be maintained if those individuals expressing it were isolated from other members of the species

2. Breeders recognized that some traits were “Prepotent” (dominant) and did not blend or dilute through the generations

Mackeral tabby is dominantto blotched tabby



3. Variation is common

4. Natural selection favors certain variants



5. Environments change – different variants become superior

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

Intraspecific Variation

• Darwin noted that individuals within populations were variable for many traits

• But Darwin never knew the origin of this variation

Pangenesis• Nine years after publishing The Origin,

Darwin conceived of a hypothesis for inheritance, Pangenesis, which is modeled on a concept proposed by Hippocrates (460-370 BC) in writings from ~410 BC

• Interestingly, Aristotle considered and rejected Hippocrates’s Pangenesis as a mechanism of inheritance

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

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

Pangensis Questioned• August Weismann (1834-1914) 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

Pangensis Questioned• Francis Galton (1822-1911), 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

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

Gregor Mendel (1822-1884)

published(1865)

Mendel’s Laws & Experiments

• Mendel developed three fundamental principles of heredity:–Principle of Dominance–Principle of Segregation–Principle of Independent

Assortment

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

Monohybrid Crosses

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


– 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

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

The Nature of Mendelian Genes– Multiple alleles (> 2) may

be present at the locus– ABO blood groups– brown hair color


– 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

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

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


• 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


• 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


• 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


• 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

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 .

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

Dihybrid Crosses

9:3:3:1

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

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

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

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

• Walter Flemming discovers chromosomes and mitosis (1880)

• Francis Galton coined the term "eugenics" (1883)

Chromosomes and Genetics

Chromosomes and Genetics

• Edouard-Joseph-Louis-Marie van Beneden (1846-1910) 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

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]

Hugo de Vries (1848-1935)

• 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)

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

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

• 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

Mendelian Inheritance Has Its Physical Basis in the Behavior of Chromosomes

During Sexual Life Cycles

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.

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

• 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)

Morgan Traced a Gene to a Specific Chromosome

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

MUTATION: any process by which the base pair sequence of a DNA molecule is altered

Somatic mutation

Germ-line mutation

Mutation occurs but it is NOT passed on

to the next generation

Mutation occurs in gametes and is

passed to the next generation, now

mutation occurs in both its somatic and

germ-line cells

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

Drosophila Mutants

Founders of Mathematical Genetics

R. Punnett G.H. Hardy W.E. CastleW. Weinberg

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.

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

• http://home.nycap.rr.com/useless/kammerer/• http://www.mbl.edu/publications/Ciona/Kammerer/

• Science will always have some conflicts over the interpretation of data

http://home.nycap.rr.com/useless/kammerer/

http://www.mbl.edu/publications/Ciona/Kammerer/

Modern “Saltationists”

• Richard Goldschmidt (1878-1958), 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

Modern “Saltationists”

• Carl Woese (1928 - 2012) 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

Mutations Are theRaw Material of Evolution

• Without mutations, there would be no:– new alleles

– new genes

– evolution

Laboratory Studies of Genetics

Saccharomyces cerevisiae

Escherischia coli Drosophila melanogaster

Mus musculus

Spanning the 20th century and beyond

Fig. 15.12

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

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)

http://www.the-scientist.com/?articles.view/articleNo/41177/title/Mom-s-Mitochondria-Affect-Pup-Longevity/

http://www.the-scientist.com/?articles.view/articleNo/41177/title/Mom-s-Mitochondria-Affect-Pup-Longevity/

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

Sex Determination• Sex chromosomes

– 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

Sex Determination

• Some system of sex chromosomes in most animals

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

XY sexdetermination

In some plants

ZW sex determination

XO sex determinationXX = female and X0 = male

Grasshooppers, cockroaches, etc., Caenorhabditis elegans

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 = 1.0Females: 1 X: 2 set of autosomes ; ratio = 0.5

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

32(Diploid)

16(Haploid)

Haplo-Diploid Sex Determination in All Hymenoptera

Workers (sisters) are more closely related on average (75%) to each other than to their mother, the queen (50%)

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

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

Figure B03A: Tubeworms

Reprinted from Deep Sea Research Part II: Topical Studies in Oceanography, vol. 56, Robert C. Vrijenhoek, Cryptic species, phenotypic plasticity..., pp. 1713-1723. Copyright 2009, with permission from Elsevier. [http://www.sciencedirect.com/science/journa


Figure B03B: Tubeworms

Figure B03C: Dwarf males Courtesy of Greg Rouse

• Osedax sp., tube worms, which feed on whale carcass bone also exhibit dwarf males who live in the female’s external capsule


• 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)


Environmental Sex Determination• The majority of reef fish change sex at some The majority of reef fish change sex at some

point during their livespoint during their lives• In fact, reef fish that remain as the same sex In fact, reef fish that remain as the same sex

for their entire life span (for their entire life span (gonochoristicgonochoristic) are in ) are in the minoritythe minority

• Some species will begin life as males and Some species will begin life as males and switch to females (switch to females (protandryprotandry), and others ), and others switch from female to male (switch from female to male (protogynyprotogyny))

• Some will change sex in both directions, and Some will change sex in both directions, and others will be both sexes at the same time others will be both sexes at the same time

protogynous grouper Epinephelus, females first

Social Control

protogynous blue-headed wrasse Thalassoma bifasciatum, females first

protandrous clownfish Amphiprion percula, males first

the female is the largest individual, the male the second largest; the rest of the group are smaller non-breeders without functioning gonads

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

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

Parthenogenesis

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

Sexual Reproduction

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

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

Sex-Linked Genes

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

Bacterial Transformation (1928)

• Frederick Griffith (1879-1941) 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

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

The Double Helix (1953)

Frances Crick, James Watson Rosalind Franklin

DNA x-ray diffraction

Arthur Kornberg (1918-2007)

• 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!

Kornberg: DNA Replication 1958

Genetic Code 1961-1963

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(1946 - )

Chapter 7

End


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