3. At the time, most breeders believed parents of
contrasting appearance always produce offspring
of intermediate appearance.
Mendel’s experiments helped him formulate the
particulate theory of inheritance.
Inheritance involves reshuffling of genes from
generation to generation.
5. One-Trait Inheritance
Mendel performed cross-breeding experiments
between true-breeding plants.
Chose varieties that differed in only one trait
(monohybrid cross)
Performed reciprocal crosses
Parental generation = P
First generation offspring = F
1
Second
generation offspring = F2
6. Figure 13-1
Self-pollination
SELFPOLLINATION
Female organ
(receives pollen)
Eggs
Male organs
(produce pollen
grains, which
produce male
gametes)
Cross-pollination
CROSSPOLLINATION
1. Remove male organs
2. Collect pollen from a
3. Transfer pollen to the
from one individual.
different individual.
female organs of the
individual whose male
organs have been
removed.
7. Mendel Studied a Single Trait
Mendel cross-fertilized two plants, one with
white flowers with one with purple flowers. The
hybrids, F1 generation, all had purple flowers.
Studying one trait through cross-fertilization is
termed a monohybrid cross.
9. Mendel Studied a Single Trait
Mendel’s experiments cont’d
Mendel allowed F1 generation plants to self-
fertilize.
Their offspring, the F2 generation, expressed
(demonstrated) both purple and white flowers.
The ratio of plants with purple to white flowers
was always 3:1.
Where did these white flowered plants come
from?
11. Mendel Studied a Single Trait
Mendel cont’d
The F1 generation plants all resembled only
parent plant; i.e. one variation of the trait is
dominant.
The F2 generation showed plants with both
variations of the character, purple and white.
The variation of the trait that was only seen in
the F2 generation (white flowers) is recessive.
12. Mendel Studied a Single Trait
Mendel cont’d
The F2 generations were allowed to self-
fertilize. Looking at the F3 generation, Mendel
discovered that the F2 generation actually
consisted of 3 different types of plants:
Pure breeding purple
Not pure breeding purple (produced both
purple and white flowered plants.
Pure breeding white.
The ratio was actually 1:2:1.
16. Genes and Mendel’s Findings
Traits are carried by genes.
An individual has 2 genes or alleles for each trait,
1 on each homologous chromosome.
Meiosis results in separation of the homologous
chromosomes and the alleles so that each is
carried by a different gamete.
17. Genes and Mendel’s Findings
An individual with 2 identical alleles is said to be
homozygous, while an individual with 2 different
alleles is said to be heterozygous.
The genetic make-up of an individual is its
genotype. The appearance or expression of the
genotype is called its phenotype.
18. Genes and Mendel’s Findings
Mendel’s results can be predicted using Punnett
squares.
Dominant genes are represented by uppercase
letters, ex. round peas (R) . Expressed when
there is 1 or 2 dominant alleles present.
Recessive genes are represented by lowercase
letters, ex. wrinkled peas (r). Only expressed
when there are 2 recessive alleles present.
19. A cross between two homozygotes
Homozygous
mother
Meiosis
Homozygous
father
Meiosis
Male gametes
Female gametes
Offspring genotypes: All Rr (heterozygous)
Offspring phenotypes: All round seeds
A cross between two heterozygotes
Heterozygous
mother
Female gametes
Heterozygous
father
Male gametes
Figure 13-4
Offspring genotypes: 1/4 RR : 1/2 Rr : 1/4 rr
Offspring phenotypes: 3/4 round : 1/4 wrinkled
22. Genes and Mendel’s Findings
Mendels’ Principle of Segregation
Each individual has two factors for each trait.
The factors segregate during gamete formation.
Each gamete contains only one factor from each pair of
factors.
Fertilization gives each new individual two factors for
each trait.
23. Figure 13-7
Rr parent
Dominant allele
for seed shape
Recessive allele
for seed shape
Chromosomes replicate
Meiosis I
Alleles segregate
Gametes
Meiosis II
Principle of segregation: Each gamete carries only
one allele for seed shape, because the alleles have
segregated during meiosis.
24. Two-Trait Inheritance
Mendel performed cross using true-
breeding plants differing in two traits.
Dihybrid Cross
Observed phenotypes among F plants.
2
Formulated
assortment
law of independent
25. Mendel Studied 2 Traits
Mendel then looked at two traits simultaneously –
dihybrid cross. Ex. plants that produced round (R),
yellow (Y) peas and plants that produced wrinkled
(r), green (y) peas.
The pure breeding parents’ genotypes were RRYY
and rryy, fig 13.5.
What is the genotype and phenotype of the F1
generation? The F2 generation?
29. Genes and Mendel’s Findings
Mendel’s Principle of Independent Assortment
Each pair of factors segregates independently of the
other pairs.
All possible combinations of factors can occur in the
gametes.
30. Figure 13-8
R
y
y R
r
r
Y
R
Rr
Replicated chromosomes
prior to meiosis
Y
r
R R r
Alleles for seed shape
Alleles for seed color
r
Chromosomes can line up in
two ways during meiosis I
Y Y y y
R
Meiosis I
R
Y
y yY Y
r
r
R
y y
Y
Meiosis I
R
Gametes
Y
Y
1/4 RY
Y
Meiosis II
r
R
r
Y
y y
Meiosis II
R
r
r
y
y
1/4 ry
r
R
R
y
y
1/4 Ry
r
Y
1/4 rY
Principle of independent assortment: The genes for seed shape and seed color
assort independently, because they are located on different chromosomes.
Y
