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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Mendel’s Experimental, Quantitative Approach
• Advantages of pea plants for genetic study:
– They are available in many varieties. For
example, one variety has purple flowers, while
a contrasting variety has white flowers.
– The use of peas also gave Mendel strict
control over which plants mated with which.
So he could always be sure of the parentage
of the new seeds.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Pea Flowers
– Each pea flower has pollen-producing organs
(stamens) and egg-producing organs (carpels).
– Cross-pollination (The transfer of pollen from an
anther of the flower of one plant to a stigma of the
flower of another plant) can be achieved by
dusting one plant with pollen from another.
Cross-pollination
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-2
Removed stamens
from purple flower
Transferred spermbearing pollen from
stamens of white
flower to eggbearing carpel of
purple flower
Parental
generation
(P)
Carpel
Stamens
Pollinated carpel
matured into pod
Planted seeds
from pod
First
generation
offspring
(F1)
Examined
offspring:
all purple
flowers
Some genetic vocabulary
• Some genetic vocabulary
– Character: a heritable feature, such as flower
color
– Trait: a variant of a character, such as purple
or white flowers
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Mendel also made sure he started his experiments
with varieties that were true-breeding.
• A true-breeding plant is one that, when selffertilized, only produces offspring with the same
traits.
→
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
→
→
• In a typical experiment, Mendel mated two
contrasting, true-breeding varieties, a process
called hybridization
• The true-breeding parents are the P generation
• The hybrid offspring of the P generation are called
the F1 generation
• When F1 individuals
self-pollinate, the F2
generation is produced
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Law of Segregation
• When Mendel crossed
contrasting, true-breeding
white and purple flowered pea
plants, all of the F1 hybrids
were purple
• When Mendel self-pollinated
the F1 hybrids, many of the F2
plants had purple flowers, but
some had white
• Mendel discovered a ratio of
about three to one, purple to
white flowers, in the F2
generation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Mendel reasoned that only the purple flower gene
was affecting flower color in the F1 hybrids
• Mendel called the purple flower color a dominant
trait and white flower color a recessive trait
• Mendel observed the same pattern of inheritance
in other six pea plant characters.
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Mendel’s Model
• Mendel developed a hypothesis to explain the 3:1
inheritance pattern he observed in F2 offspring.
• Four related concepts make up this model
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The first concept:
• The gene for a particular character
such as flower color resides at a
specific position on a certain
chromosome known as locus.
• The gene for flower color in pea
plants as an example exists in two
versions, one for purple flowers and
the other for white flowers.
• These alternative versions of a
gene (gene for purple and gene for
white) are now called alleles.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-4
Allele for purple flowers
Locus for flower-color gene
Allele for white flowers
Homologous
pair of
chromosomes
The second concept:
• for each character, an
organism inherits two
alleles, one from each
parent
• These two alleles may
be identical or different.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The third concept:
•If the two alleles at a locus differ
– Then one, the dominant allele, is fully
expressed in the organism’s appearance
– The other allele, the recessive allele, has no
noticeable effect on the organism’s
appearance in the presence of the dominant
allele.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The fourth concept:
• Now known as the law of
segregation, states that the
two alleles for a heritable
character separate
(segregate) during gamete
formation and end up in
different gametes
• Thus, an egg or a sperm gets
only one of the two alleles that
are present in the somatic cells
of an organism
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Mendel derived the law of
segregation by performing
monohybrid crosses i.e.
breeding experiments using
parental varieties that differ in
a single character such as
flower color.
• http://www.youtube.com/watch
?v=F3AKldl6JZg
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Punnett Square
• The combination resulting from a
genetic cross may be predicted by
using a Punnett Square.
• A capital letter represents a
dominant allele, and a lowercase
letter represents a recessive allele
http://www.youtube.com/watch?v=d4izVAkhM
PQ&feature=related
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-5_2
P Generation
Appearance:
Genetic makeup:
Purple
flowers
PP
White
flowers
pp
P
p
Gametes
F1 Generation
Appearance:
Genetic makeup:
Purple flowers
Pp
1
Gametes:
2
1
P
p
2
F1 sperm
P
p
PP
Pp
Pp
pp
F2 Generation
P
F1 eggs
p
3
:1
Useful Genetic Vocabulary
• An organism that is homozygous for a
particular gene, also called a homozygote
– Has a pair of identical alleles for that
gene, such as homozygous dominant
(PP) and homozygous recessive (pp)
– Exhibits true-breeding
• An organism that is heterozygous for a
particular gene, also called a heterozygote
– Has a pair of alleles that are different for
that gene such as (Pp).
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Useful Genetic Vocabulary
• An organism’s phenotype
– Is its physical appearance
• An organism’s genotype
– Is its genetic makeup
• In the example of flower color in
pea plants, PP and Pp plants
have the same phenotype
(purple) but different genotypes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PP
or
Pp
LE 14-6
3
Phenotype
Genotype
Purple
PP
(homozygous
Purple
Pp
(heterozygous
1
2
1
Purple
Pp
(heterozygous
White
pp
(homozygous
Ratio 3:1
Ratio 1:2:1
1
The Law of Independent Assortment
• Mendel identified his second law of
inheritance by following the inheritance
of two characters at the same time
• A dihybrid cross is a mating of
parental varieties differing in two
characters such as seed color and
seed shape in peas.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Law of Independent Assortment
• Using a dihybrid cross, Mendel developed the law
of independent assortment
• The law of independent assortment states that
each pair of alleles segregates independently of
other pairs of alleles during gamete formation.
The
genotype
GgYy will
give four
classes of
gametes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-8
P Generation
YYRR
yyrr
Gametes YR
yr
YyRr
F1 Generation
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
Sperm
1
Sperm
1
2
YR
1
2
yr
1
1
2
2
1
4
Yr
1
4
yR
1
4
yr
YR
4
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
YYRR
1
YR
Eggs
Eggs
F2 Generation
(predicted
offspring)
4
YyRr
1
Yr
4
yr
YyRr
3
4
yyrr
1
1
yR
4
4
1
Phenotypic ratio 3:1
yr
4
9
16
3
16
3
16
3
16
Phenotypic ratio 9:3:3:1
The Law of Independent Assortment
• Dihybrid Cross:
http://www.youtube.com/watch?v=9kAFEbhzz94
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Spectrum of Dominance
• Complete dominance occurs when phenotypes
of the heterozygote and dominant homozygote are
identical
• In incomplete dominance, one allele is not
completely dominant over the other, so the
heterozygote has a phenotype that is intermediate
between the phenotypes of the two homozygotes.
• In codominance, two dominant alleles affect the
phenotype in separate, distinguishable ways
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Multiple Alleles
• Most genes exist in populations in more than two
allelic forms
• For example, the four phenotypes of the ABO
blood group in humans are determined by three
alleles for the enzyme (I) that attaches A or B
carbohydrates to red blood cells: IA, IB, and i.
• The enzyme encoded by the IA allele adds the A
carbohydrate (antigen), whereas the enzyme
encoded by the IB allele adds the B carbohydrate
(antigen); the enzyme encoded by the i allele adds
neither
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Multiple Alleles
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Pleiotropy
• Most genes have multiple phenotypic effects, a
property called pleiotropy
• For example, pleiotropic alleles are responsible for
the multiple symptoms of certain hereditary
diseases, such as cystic fibrosis and sickle-cell
disease.
• Symptoms of sickle-cell disease include physical
weakness, pain, organ damage, and even
paralysis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Extending Mendelian Genetics for Two or
More Genes
• Some traits may be
determined by two or
more genes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Polygenic Inheritance
• polygenic inheritance: two or more genes affect a
single phenotype that is expressed as a result of
an additive effect of these genes.
• Skin color in humans is an example of polygenic
inheritance
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-12
AaBbCc
aabbcc
20/64
Fraction of progeny
15/64
6/64
1/64
Aabbcc
AaBbCc
AaBbcc AaBbCc AABbCc AABBCc AABBCC
Nature and Nurture:
The Environmental Impact on Phenotype
• Another departure from Mendelian genetics arises when
the phenotype for a character depends on environment as
well as genotype
• The norm of reaction is the phenotypic range of a
genotype influenced by the environment
• For example, hydrangea flowers of the same genotype
range from blue-violet to pink, depending on soil acidity
• Norms of reaction are generally broadest for polygenic
characters
• Such characters are called multifactorial because genetic
and environmental factors collectively influence phenotype
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 14.4: Many human traits follow
Mendelian patterns of inheritance
• Humans are not good subjects for genetic
research because:
– generation time is too long;
– parents produce relatively few offspring;
– and breeding experiments are
unacceptable
• However, basic Mendelian genetics endures
as the foundation of human genetics
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-14a
Ww
ww
ww
Ww ww ww Ww
WW
or
Ww
Ww
Ww
ww
Dominant trait (widow’s peak)
Second generation
(parents plus aunts
and uncles)
Third
generation
(two sisters)
ww
Widow’s peak
First generation
(grandparents)
No widow’s peak
Recessively Inherited Disorders
• Many genetic disorders are inherited in
a recessive manner
• Recessively inherited disorders
show up only in individuals
homozygous for the allele
• Carriers are heterozygous individuals
who carry the recessive allele but are
phenotypically normal
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sickle-Cell Disease
• Sickle-cell disease affects one out of
400 African-Americans
• The disease is caused by the
substitution of a single amino acid in
the hemoglobin protein in red blood
cells
• Symptoms include physical weakness,
pain, organ damage, and even
paralysis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Dominantly Inherited Disorders
• Some human disorders
are due to dominant
alleles
• One example is
achondroplasia, a form
of dwarfism that is
lethal when
homozygous for the
dominant allele
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings