1. Why do we look like family members
or not?
By: Peter T. Andres
Reporter

2. Gregor Mendel

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.

4. Figure 13-2
Trait
Phenotypes
Seed shape
Round
Wrinkled
Yellow
Green
Inflated
Constricted
Green
Yellow
Seed color
Pod shape
Pod color
Flower color
Purple
White
Flower and
pod position
Axial (on stem)
Terminal (at tip)
Stem length
Tall
Dwarf
short

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.

8. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Pollen transferred from
white flower to stigma
of purple flower
Anthers
removed
All purple flowers as a result

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?

10. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
P (parental)
generation
Crossfertilize
Purple
F1 generation
White
Self-fertilize
F2 generation
Purple
Purple
3
Purple
White
:
1

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.

13. Monohybrid Cross

14. Monohybrid Cross

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

20. Monohybrid Cross

21. Monohybrid Cross

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?

26. Hypothesis of independent assortment:
Alleles of different genes don’t stay together when gametes form.
Female parent
F1 PUNNET SQUARE
Male gametes
Male parent
Female gametes
F1 offspring all RrYy
F2 female parent
Alleles at R gene and Y gene
go to gametes independently
of each other
F2 PUNNET SQUARE
Female gametes
F2 male
parent
Male gametes
Figure 13-5a
F2 offspring genotypes: 9/16 R–Y– : 3/16 R–yy : 3/16 rrY– : 1/16 rryy
F2 offspring phenotypes: 9/16 : 3/16 : 3/16 : 1/16

27. Dihybrid Cross

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