Chapter 7
Figure CO:
Finches
Darwin, Mendel and
Theories of Inheritance
Overview
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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
Baldwins
It is obvious to anyone
that children resemble a
mixture of their parent’s
features
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 dominant
to blotched tabby
Blending Inheritance
• If correct, natural selection could not maintain a
favorable trait for more than a few generations
• Darwin countered:
3. Variation is common
4. Natural selection favors certain variants
Blending Inheritance
• If correct, natural selection could not maintain a
favorable trait for more than a few generations
• Darwin countered:
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 after
Darwin
• 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
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
palomino
white
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
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
Gene Penetrance
When the phenotype is
not expressed, despite
the determining
genotype being
present; the genotype
doesn’t “penetrate”
Mendel Studied Seven Different
Traits in Pea Plants
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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 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
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
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
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
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
9 green wrinkled; 3 green smooth;
3 yellow wrinkled; 1 yellow smooth
• 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
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
Chromosomes and Genetics
• Walther Flemming discovers
chromosomes and mitosis (1880)
• Francis Galton coined the term
"eugenics" (1883)
Chromosomes and Genetics
• Edouard-Joseph-LouisMarie van Beneden (18461910) 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
Oenothera ring chromosomes
• 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.
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
“elementen”
– 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
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
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)
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. 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.
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
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 the
Raw Material of Evolution
• Without mutations,
there would be no:
– new alleles
– new genes
– evolution
Laboratory Studies of Genetics
Spanning the 20th century and beyond
Escherischia coli
Saccharomyces cerevisiae
Drosophila melanogaster
Mus musculus
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)
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 sex
determination
In some plants
ZW sex determination
XO sex determination
XX = 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
Males: 1 X: 1 set of autosomes; ratio = 1.0
some genes related to
Females: 1 X: 2 set of autosomes ; ratio = 0.5
maleness
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
Haplo-Diploid Sex Determination
in All Hymenoptera
32
(Diploid)
16
(Haploid)
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 freeswimming 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
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 B03B: 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 B03A: Tubeworms
Figure B03C: Dwarf males
Courtesy of Greg Rouse
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)
Environmental Sex Determination
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
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
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
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
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)
DNA x-ray diffraction
Frances Crick, James Watson
Rosalind Franklin
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
The Nature of Mendelian Genes
• Discontinuous Variation