In an Abbey Garden
• Mendel studied garden peas because they
– were easy to grow,
– came in many readily distinguishable varieties, had easily visible traits
– are easily manipulated
– can self-fertilize.
Gregor Mendel’s Peas
• Mendel had
– true-breeding pea plants, which were the basis of his experiments.
• True-breeding plants produce offspring that are identical to themselves, if allowed to self-pollinate.
• Mendel wanted to cross-pollinate his pea plants, in order to produce seeds that came from two different parents.
• He did this by cutting off the male parts of one flower and dusting it with the pollen from another flower.
Figure 9.3-3
Removedstamensfrom purpleflower.
Stamens
Transferred pollen fromstamens of white flower
to carpel of purple flower.
Parents
(P)
Carpel
Offspring
(F1)
Pollinated carpel
matured into pod.
Planted seeds
from pod.
4
3
2
1
• A character is a heritable feature that varies among individuals.
• A trait is a variant of a character.
• Each of the characters Mendel studied occurred in two distinct traits.
In an Abbey Garden
Character
WhitePurple
RecessiveDominant
Green Yellow
Terminal Axial
Wrinkled Round
Green Yellow
Seed
shape
Seed
color
Flower
position
Flower
color
Pod
color
RecessiveDominant
Pod
shape
Stem
length
Inflated Constricted
Tall Dwarf
Trait
What Mendel did…..
– created purebred varieties of plants – plants that when they self-fertilized only produced off spring similar to that of the parent.
– then he crossed two different purebred varieties.
In an Abbey Garden
• Hybrids are the offspring of two different purebred varieties.
– The parental plants are the P generation.
– Their hybrid offspring are the F1 generation.
– A cross of the F1
plants forms the F2 generation.
His Results???
Monohybrid Crosses
• A monohybrid cross is a cross between purebred parent plants that differ in only one character.
Figure 9.5-3
Purple flowers
F1 Generation
White flowers
P Generation(purebredparents)
All plants havepurple flowers
F2 Generation
Fertilization
among F1 plants
(F1 F1)
of plants
have purple flowers
of plants
have white flowers
34
14
Mendel’s Law of Segregation
• Lets examine his experiments and see how his results led him to develop 4 hypotheses.
• Mendel developed four hypotheses from the monohybrid cross, listed here using modern terminology (including “gene” instead of “heritable factor”).
1. The alternative versions of genes are called alleles.
Mendel’s 4 Hypothesis
Figure 9.7
Homologous
chromosomes
P
Genotype:
Gene loci
P
a
aa
b
B
Dominant
allele
Recessive
alleleBbPP
Homozygous
for the
dominant allele
Homozygous
for the
recessive allele
Heterozygous
with one dominant
and one recessive
allele
a
2. For each inherited character, an organism inherits two alleles, one from each parent.
– An organism is homozygous for that gene if both alleles are identical.
– An organism is heterozygous for that gene if the alleles are different.
Mendel’s Hypothesis
3. If two alleles of an inherited pair differ,
– then one determines the organism’s appearance and is called the dominant allele
– the other has no noticeable effect on the organism’s appearance and is called the recessive allele.
Mendel’s Hypothesis
4. Gametes (sperm and egg) carry only one allele for each inherited character.
– The two alleles for a character segregate (separate) from each other during the production of gametes (meiosis I = homologous chromosomes separate)
– This statement is called the law of segregation.
Mendel’s Hypothesis
• Do Mendel’s hypotheses account for the 3:1 ratio he observed in the F2 generation?
• A Punnett square highlights
– the four possible combinations of gametes
– the four possible offspring in the F2 generation.
Monohybrid Crosses
Figure 9.11
F1 Genotypes
F2 Genotypes
Bb female Bb male
Formation of eggs Formation of sperm
Male
gametes
Fem
ale
gam
ete
s
2
1
2
1
2
1
2
1
2
1
2
14
1
4
1
4
1
4
1
( )
B
BB B B
B
b
b
b b b b
Figure 9.6P Generation Genetic makeup (alleles)
Alleles carried
by parents
Gametes
Purple flowers
PP
White flowers
pp
All P All p
pP
F1 Generation
(hybrids)
F2 Generation
(hybrids)
Alleles
segregate
Gametes
Purple flowers
All Pp
21
21
Sperm from
F1 plant
Eggs from
F1 plant
Phenotypic ratio
3 purple : 1 white
Genotypic ratio
1 PP : 2 Pp : 1 pp
p
p
P
PPP Pp
Pp pp
Figure 9.6a
Purple flowers White flowers
P Generation Genetic makeup (alleles)
Gametes
Alleles carried
by parentsPP
P
pp
pAll All
Figure 9.6c
F2 Generation
(hybrids)Sperm from
F1 plant
Eggs from
F1 plant
Phenotypic ratio
3 purple : 1 white
Genotypic ratio
1 PP : 2 Pp : 1 pp
p
p
P
P
PP Pp
Pp pp
Warm Up
Freckles (F) in humans is dominant to no freckles (f).
Using the letters F and f, what are the genotypes for…
1. A person who is homozygous dominant.
2. A person who is heterozygous.
3. A person who is homozygous recessive.
What are the phenotypes for…..
1. Ff
2. ff
3. FF
Question:
What information goes inside of the Punnett Square?
What information goes on the outside?