Why Mendel chose the garden pea

Bio 115 Ecology and Evolution
Mendelian heredity: Ovists and Spermists.
Ovists and Spermists are both
Although people realized that traits were
passed on from parents to children by way of
reproduction, the underlying mechanisms
remained largely unknown. The
preformationists believed that all life was
‘preformed’ at the moment of creation and that
successive generations of individuals were
encased, one inside the other, in increasingly
small versions of their adult selves. Thus,
children started as miniaturized adults whose
development was triggered by the act of
The ovists (Paracelsus, Jan Swammerdam, and
Reinier de Graaf, accepted that the offspring
was contained in a preformed state in the egg
cell and that its development was triggered by
the juices of the sperm cell.
In contrast, the spermists claimed that the
offspring sprung from the sperm cell and that
the female only provided a nurturing womb for
the new life to develop. Spermism had to await
the discovery of sperm cells by the first
microscopists (Anthony van Leeuwenhoek and
Nicolas Hartsoeker) who discovered the
homunculus or ‘little man’ hidden within the
sperm cell.
Jan Swammerdam
Anthony van Leeuwenhoiek
Nicolas Hartsoeker
Bio 161
Lesson 01
Reinier de Graaf
Mendelian heredity: Pangenesis and Blending
Pangenesis states that traits are inherited directly. Elements or information of
each of the parents’ body parts are transmitted to the offspring directly, and
independently of the information of other parts. The child is formed after this
hereditary material from all parts of the parents’ bodies has come together.
Hippocrates called this reproductive material ‘gonos’ or seed. Darwin (1868)
suggested that all cells and tissues excrete this material as microscopic granules
or ‘gemmules’.
The pangenesis theory assumes that both male and female contribute to the
offspring and that the traits blend in the offspring.
The two classical assumptions lead to a paradox. Indeed, if variation and heredity
occur within the boundaries of the species (no variation enters from outside the
species) and the traits of the parents are blended in the offspring, than sooner or
later all members of the same species should have the same appearance. Which is
not the case.
Distinction between soma and
germ line. In pangenesis,
information from the somatic cells
(gemmules) is collected in the
germ cells and passed on to the
Bio 161
Lesson 01
In addition, early experimental work of Josef Kolreuter (1760) and TA Knight
(1790) casted doubt on the classical assumptions.
Mendelian heredity: classic assumptions fail
Observations cast doubt on the classic assumptions:
Josef Kolreuter (1760) crossed different strains of tobacco. The resulting
hybrids differed in appearance from both parent strains. When individuals
of the hybrid generation were crossed, their offspring were highly
variable: some resembling the hybrid generation (their parents) others
resembling the original strains (their grandparents).
T.A. Knight (1790) crossed a strain of peas with purple flowers with one
with white flowers. All hybrid offspring had purple flowers. Among the
offspring of these hybrids, however, were some plants with purple and
some with white flowers.
Josef kolreuter
Bio 161
Lesson 01
Because traits could be masked in one generation to reappear in the next,
inheritance could not occur by direct transmission of traits (How could a
trait that is transmitted directly disappear and reappear?). Similarly,
blending theory can not explain differences between offspring of the same
Mendelian heredity: lamarckism.
Lamarckism: inheritance of acquired traits:
Aristole (384-322 BC) proposed that the physical, intellectual, and personality characteristics acquired by people during
their life time were passed on to their offspring. According to this theory, musical talent, cycling performance, or soccer
aptitute can be passed on from the parents to their children if the parents dedicate their life to the pursuit of these
activities because their genetic material is affected by their fervent dedication.
Jean-Baptiste Lamarck (1744-1829) used the idea to explain the mechanism of heredity and suggested that by simply
using or not using certain organs, organs and body parts may be highly developed or atrophioed and the offspring can
then inherit these acquired traits.
Lamarckism rejected
Bio 161
Lesson 01
Mendelian heredity: lamarckism rejected
Lamarckism rejected;
Lamarckism, or the theory of inheritance of acquired traits, influenced
scientific thinking for over 200 years.
However, there are obvious problems to this idea. Children with normal
limbs are begot from deformed parents. Also, think of the jewish practice
of circumcisuin.
August Weismann (1834-1914) nipped the idea of inheriting acquired
characteristics in the bud so to speak when he cut the tails off of rats and
mated them, showing that the offspring still had tails.
Bio 161
Lesson 01
Who is Mendel?
Gregor Johann Mendel (born on 22 July 1822 in Heizendorf, Austria [Hyncice,
Czech Republic] and died on January 6, 1884 in Brno [Brunn], Austria) was
the only son of a farmer and attended local schools and the Philosophic
Institute at Olomouc. In 1843, he entered the Augustinian Order at st.
Thomas Monastery in Brunn and began his theological studies. He was
ordained in 1847.
From 1851 to 1853 he studied zoology, botany, chemistry, and physics at the
University of Vienna. Two professors were especially influentiual: the
physicist Christian Doppler (who instilled a sense for experimental work and
mathematical modeling) and the botanist Franz Unger (who aroused interest
in the causes of variation in plants).
Mendel returned to the monastery in 1854 to teach part-time at a school
where other teachers shared his enthusiasm for sciences. Moreover, St.
Thomas Monastery was a center of creative interest in the sciences and
culture. There were well-known philosophers, a musicologist,
mathematicians, mineralogists, and botanists who were heavily engaged in
scientific research and teaching. Most important, the monks had a longstanding interest in the breeding of plants.
Mendel completed his experimental work with the garden peas between 1854
and 1865. Mendel tried to determine whether it was possible to obtain new
variants of peas by cross-breeding. He presented his work in two lectures
before the Society for the Study of the Natural Sciences in Brunn in 1865. His
paper, Versuche uber Pflanzen-Hybriden (Experiments in plant hybridization)
was published in the Society’s Proceedings in 1866. His work, however, was
largely ignored.
In the spring of 1900, three botanists (Hugo de Vries, Karl Correns, and E.
von Tschermak) reported independent verifications of Mendel’s work.
Bio 161
Lesson 02
Why Mendel chose the garden pea.
Why Mendel chose the garden pea:
Earlier investigators had produced hybrids by crossing different strains.
A large number of true-breeding varieties were available.
Pea plants are small, easy to grow, and have a relative short generation time.
It is easy to control self fertilization and cross-fertilization.
Both male and female sexual organs are enclosed in
the same flower and the male and female gametes
produced can fuse to form viable offspring. Selffertilization takes place automatically within an
individual flower if it is not disturbed.
Bio 161
Lesson 02
Cross-fertilization is the result of cross-pollination. The
latter occurs when pollen of a different flower is
introduced to the female stigma of a flower after first
removal of the petals and male anthers of the flower.
Mendel’s experimental design.
Mendel’s experimental design:
Mendel selected pea varieties that differed in 7 traits from each other. He conducted his crosses in
three stages:
(1) He allowed pea plants of a given variety to
produce progeny by self-fertilization for
several generations. Mendel confirmed that
the traits of the variants he studied were true
(pure)-breeding (or homozygous). All white
flowered plants produced white-flowered
offspring regardless of the number of
generations. He called these plants the
Parental generation or P generation.
(2) Mendel than performed crosses between
varieties showing alternative forms of the trait
in which he was interested. E.g., cross a
white flowered female with a purple flowered
male. (He also carried out the reciprocal
cross!) He called the offspring of the P
generation the first Filial or F1 generation.
(3) Finally, Mendel permitted the F1 hybrids to
self-fertilize for several generations. He
carefully counted and recorded the numbers
of offspring exhibiting each trait in each
succeeding generation.
Bio 161
Lesson 02
What Mendel found: purple- x whiteflowers.
What Mendel found:
In a cross of white-flowered plants with purpleflowered plants, the F1 offspring all had purple
Of the F2 offspring 25% had the recessive white
flowers and 75% were purple-flowered.
All recessive, white-flowered offspring were truebreeding (homozygous). In contrast only 1/3 of the
dominant, purple-flowered offspring were truebreeding, while 2/3 were not.
In general:
Bio 161
Lesson 02
What Mendel found: in general
What Mendel found:
In a cross between two contrasting varieties of
peas the F1 offspring did not show properties
intermediate of those of the parents, as the
blending theory predicted. In every case, the
offspring resembled one of the parents.
The form of the trait expressed in the F1 is said
to be dominant; the alternative form of the trait
is said to be recessive. Some human traits.
¾ (75%) of the F2 exhibited the dominant trait,
and ¼ (25%) of the F2 offspring displayed the
recessive trait. The dominant: recessive ratio
among the F2 plants was always 3:1.
The 3:1 ratio of the F2 generation was really a
disguised 1:2:1 ratio with ¼ true-breeding
dominant, 2/4 non-true-breeding dominant, and
¼ true-breeding recessive individuals.
Results of Mendel’s other experiments.
Bio 161
Lesson 02
Results of Mendel’s other experiments.
Bio 161
Lesson 02
Mendel’s Model of Heredity (part 1)
To explain his results/
observations, Mendel proposed
a simple model for inheritance
of traits.
Parents do not transmit
physiological traits directly to
their offspring. Rather, they
transmit discrete packages of
information (Mendel’s factors,
or genes) that encode the trait.
Mendel expressed the factors
by a set of symbols. The
dominant purple color is
represented by P (upper case)
and the recessive white color
by p (lower case).
Each individual receives two
factors of the same trait, one
from the father and one from
the mother. Individuals are said
to be diploid.
There are alternative forms or
alleles of a factor. An individual
with thwo identical alleles of a
trait is homozygous; an
individual with two different
alleles of a trait is
The genotype of an individual
that is true-breeding white (pp)
and purple (PP) are
homozygous. Heterozygous
individuals have the genotype
Bio 161
Lesson 02
Mendel’s Model of Heredity (part 1)
Apply to the Purple x White cross.
To explain his results/
observations, Mendel proposed
a simple model for inheritance
of traits. (continued)
Apply to the Yellow x Green cross.
The two alleles, one
contributed by the male gamete
and one by the female gamete,
remain discrete units in the
cells that develop into the new
Thus, when the individual
matures and produces its own
gametes, the alleles for each
trait segregate randomly into
these gametes.
In heterozygous individuals,
only one allele (the dominant)
is expressed, while the other
(the recessive) allele is present
but not expressed.
The presence of a particular
allele does not ensure that the
alternative form of the trait
encoded by it will be expressed
in an individual carrying that
The genotype is the totality of
the alleles of an individual and
the phenotype is the actual
physical appearance of the
Examples of human traits.
Bio 161
Lesson 02
Mendel’s cross of Purple x white
flowered peas.
The cross
between two
individuals each
for a single trait
is called a
Bio 161
Lesson 02
Mendel’s cross of Green x yellow
A cross of true-breeding green and yellow pea pods yields all green offspring. The F2 generation yields 75%
green and 25% yellow pea-pods.
Bio 161
Lesson 02
Human dominant and recessive
Many traits in humans also exhibit simple dominance or recessive
inheritance, similar to the traits Mendel studied in peas.
Bio 161
Lesson 02
Mendel’s interpretation of the crosses: the Punnett square (P cross).
A. Parental cross (P) and F1 generation:
Cross of true-breeding purple-flowered female
and true-breeding white-flowered male.
(female genotype)
(female gametes)
(male gametes)
Place the different possible types of female
gametes along the top of the square (because the
female is homozygous all gametes are P, purple)
and place all possible different types of male
gametes (all are white, p) along the side of the
Bio 161
Lesson 02
Each potential zygote can be represented
by combining a female (column) and male
(row) gamete.
Mendel’s model, analyzed by way of a
Punnett square, clearly predicts that all F1
offspring will be heterozugous and purple.
Mendel’s interpretation of the crosses: the Punnett square (F1 cross)
B. The F1 cross and F2 generation:
Cross (or self-fertilization) of
heterozygous, purple, male and female
F1 plants.
P p p
(female genotype)
(female gametes)
p p
(male gametes)
Place the different possible types of female
gametes (P and p) along the top of the square and
place all possible different types of male gametes
(P and p) along the side of the square.
Bio 161
Lesson 02
Each potential zygote can be represented
by combining a female (column) and male
(row) gamete.
1st Law of Heredity or Law of
Mendel’s 1st l\Law of Heredity or the Law of
Mendel’s model thus accounts perfectly for the
phenotypic ratio of offspring observed in each cross,
and thereby, explains how traits are inherited, that is,
the process of heredity.
His model is based on the fact that alleles of a trait
segregate from each other and are randomly
distributed over the gametes during gametogenesis.
Law of Segregation:
Alleles segregate randomly during gametogenesis.
Bio 161
Lesson 02
As we will study later (Bio185), the
segregation behavior of the alternative
alleles is rooted in the alignment and
separation of homologues chromosomes
during meiosis I.
De Graaf
Van Leeuwenhoek
August Weissmann
Morgan’s first cross.
Cross-over and recombination.
Linkage and
Co-dominant traits.
ABO blood group system.
Co-dominant traits:
Piebald in animals.
Incomplete dominance.
Jimsonweed (Datura stramonium)
Q. A cross of a chicken with a
walnut comb with a chicken with a
single comb yields equal proportions
of offspring with a walnut, rose, pea,
or single comb.
result when
more than
one locus
affects a trait.
O-B- (natural)
ooB- (black)
Corn snake
Oobb (albino)
O-bb (orange)
Flower color in foxglove
M- D- --
Mm -- --
M- dd ww
M- D- ww
Q1. Adherents of the theory
that holds that all life was
preformed at the moment of
creation and that successive
generations of individuals
were encased, one inside the
other, in increasingly small
versions of their adult selves
are called
Q2. Antoon van Leeuwenhoek was a(n)
______________ , whereas Reinier de
Graaf was a(n) __________ .
Q3. Kolreuter (1760) and Knight (1790)
independently observed grandchildren of
the same color as one of the grandparents.
These observations could not be reconciled
with the __________ theory.
Q4. The fact that a
man who lost both legs
to amputation does not
have leg-less children
pleads against the
________________ .
Q5. The AABbcc individual is haploid,
diploid, triploid, or hexaploid.
Q1. How many traits are defined by
the genotype AaBbCc?
Q2. How many traits are
involved in a cross that
yields 27 red, longhaired and 9 black,
straight-haired kittens?
Q3. The eye color of fruit flies can be
black, violet, brown, sepia, red,
orange, peach, yellow, or white. Any
given fruitfly carries at most _______
different eye color alleles.
Q4. If the genotype of an individual is
MMOOpp what is(are) the genotype(s)
of the gametes?
Q5. In peas purple is dominant over
white. What is the probability that a
test cross yields a white-flowered
offspring? (= What is the proportion of
white-flowered offspring among the
Q1. How many different gametes are
produced by an individual with genotype
Q2. How many different phenotypes
and genotypes are among the offspring
of the test cross of a parent with
genotype YyWwSs?
Q3. If yellow > green, smooth >
wrinkled, and large>small, what is the
probability that an offspring of a
trihybrid cross will be large, yellow, and
Q4. If ‘blue’
and ‘white’ are
alternatives of
the same trait,
what is the most
likely genotype
of the parents
that produced
865 blue and
735 white
Q5. In peas, yellow>green and
smooth>wrinkled. What cross (genotype
of parents) yields the following
phenotypic ratio of offspring?
yellow, smooth
yellow, wrinkled
green, smooth
green, wrinkled
Q1. What is the probability of offspring
of a trihybrid cross being heterozygous
for each trait ?
Q2. The ABO-bloodgroup trait has
three alternative forms: A, B, and O.
O is recessive to A and B but A and
B are co-dominant.
What cross yields all possible ABO
Q3. If all traits show complete
dominance, what are the degrees of
freedom when you attempt to
explain the results of a mating with
a tri-hybrid cross?
Q4. The bloodgroup trait M has two
co-dominant alleles, M and m. A
cross yields 8 MM, 21 Mm, and 11
mm individuals. The coresponding
model predicts 10 MM, 20 Mm, and
10 mm offspring. Write out the
terms of C2?
Q5. The C2-test of a cross yields P>0.05.
What is P? P is the probability that
A. I am correct by chance alone when
accepting the model.
B. the model explains the cross by chance
C. Chi-square is calculated correctly by
chance alone.
D. the offspring are obtained in such a
cross by chance alone.
E. the differences between the
observations and expectations are due
to chance alone.
Q2. What is the probability that the
offspring of a trihybrid cross will
have the genotype aaB-cc?
Q1. (a) What is co- dominance?
Q1. (b) What is epistasis?
Q2. (a) If both traits show incomplete
dominance what is the number of
phenotypes among the offspring of a
dihybrid cross?
Q2. (b) If both traits show incomplete
dominance, what is the most abundant
phenotype among the offspring of a
dihybrid cross?
Q3. If the shape of radishes can be
round, oval, and long (incomplete
dominance) what is the genotype of
the parents that yields ¼ round, ½
oval, and ¼ long offspring?
Q4. Plum color is determined by
two alleles with incomplete
dominance and can be red, yellow,
or green. What cross of two
differently colored parents will
never yield red fruits?
Q5. In rabbits short-hair > long-hair
and black > blue. What cross yields
1 blue and 3 black, long-haired
Q6. Jimson weed can be round,
oval, long and smooth, fuzzy, or
spiked. A cross of which 25% of the
offspring are fuzzy, oval plants is a
Q. A cross of a chicken with a
walnut comb with a chicken with a
single comb yields equal proportions
of offspring with a walnut, rose, pea,
or single comb.
Q3. To explain the results of a
cross by way of a test cross C2
test yields P=0.03. What do you
Q4. What is the formula of C2?
Q5. What are the degrees of
freedom for a C2 test using a
dihybrid cross of one trait with
incomplete dominance and one
trait with codominance?
Q5. Use the ‘pink’ sheet. For six
comparisons you calculate a Csquare = 11.50. What is P?
Q1. Alcohol dehydrogenase of
honey bees protects against
alcohol poisoning. Adhfast moves
rapidly and far during
electrophoresis; Adhslow moves
slowly. In a monohybrid cross, half
of the offspring has both Adhfast
and Adhslow. The Adh is a
________ trait.
Q3. To explain the results of a
cross by way of a test cross the C2
test yields P=0.1. What do you
Q4. What are the degrees of
freedom for a C2 test using a test
cross involving one incomplete,
one co-, and one complete
dominant trait?
Q1. A cross yields 9 chicks with
leg feathers and 3 chicks without
leg feathers.
(a)What one-trait cross explains
this result?
(b)What two-trait cross explains
this result?
Q2. The length of cat whiskers is a
polygenic trait, according to the
length of their whiskers cats can be
grouped into 7 classes. How many
traits contribute to the length of
Q3. The eye color of fruit flies can
be black, sepia, violet, burgundy,
red, vermillion, orange, peach, or
white. Eye color is a ___________
a. Polygenic.
b. Polymorphic.
c. Pleiotropic.
d. Epistatic.
e. Co-dominant.
Q4. A cross yields 865 red and 735
white flowers.
(a)What one-trait model best
explains the cross?
(b)What two-trait model best
explains the cross?
Q5. Why do you use a C-square

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