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Chapter 14
Mendel and the Gene Idea
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: Drawing from the Deck of Genes
• What genetic principles account for the passing of
traits from parents to offspring?
•
The “blending” hypothesis is the idea that genetic
material from the two parents blends together (like
blue and yellow paint blend to make green)
•
The “particulate” hypothesis is the idea that parents
pass on discrete heritable units (genes)
• Mendel documented a particulate mechanism
through his experiments with garden peas
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Concept 14.1: Mendel Pea Experiments
• Mendel discovered the basic principles of
heredity by breeding garden peas in carefully
planned experiments.
• Advantages of pea plants for genetic study:
–
There are many varieties with distinct heritable features, or
characters (such as flower color); character variants (such as
purple or white flowers) are called traits
–
Mating of plants can be controlled
–
Each pea plant has sperm-producing organs (stamens) and eggproducing organs (carpels)
–
Cross-pollination (fertilization between different plants) can be
achieved by dusting one plant with pollen from another
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
For his experiments, Mendel chose to
CROSS POLLINATE (mate different
plants to each other) plants that were
TRUE BREEDING (meaning if the
plants were allowed to self-pollinate, all
their offspring would be of the same
variety).
P generation – parentals; true-breeding
parents that were cross-pollinated
F1 generation – (first filial) - hybrid
offspring of parentals that were allowed
to self-pollinate
F2 generation – (second filial) offspring of F1’s
Key Terminology
• Mendel chose to track only those characters that varied in
an either-or manner
• He also used varieties that were true-breeding (plants that
produce offspring of the same variety when they selfpollinate)
•
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 © 2008 Pearson Education Inc., publishing as Pearson 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 crossed 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
•
So where did the trait for the
white flower go?
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Mendel’s Reasoning
• Mendel reasoned that only the purple flower
factor was affecting flower color in the F1 hybrids
• Mendel called the purple flower color a dominant
trait and the white flower color a recessive trait
• Mendel observed the same pattern of inheritance
in six other pea plant characters, each
represented by two traits
• What Mendel called a “heritable factor” is what
we now call a gene
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Table 14-1
Mendel’s Model – 4 Big Ideas
1.
Alternative versions (different alleles) of genes account
for variations in inherited characters.
2.
For each character, an organism inherits two alleles, one
from each parent.
3.
If the two alleles differ, the dominant allele is expressed
in the organism’s appearance, and the other, a recessive
allele is masked.
–
4.
(Law of Dominance)
Allele pairs separate during gamete formation. This
separation corresponds to the distribution of homologous
chromosomes to different games in meiosis.
–
(Law of Segregation)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-4
Figure 14.4 Mendel’s law of segregation (Layer 2)
The LAW OF SEGREGATION states that during
the formation of gametes, the two traits carried
by each parent separate.
Parent cell with full
gene and Tt alleles.
Traits have separated
during gamete
formation from meiosis.
Mendel’s Law of Independent Assortment
• States that each allele pairs of different genes
segregates independently during gamete
formation;
– applies when genes for two characteristics are
located on different pairs of homologous
chromosomes
– this means that, for example, a person having
BROWN hair does NOT influence the
likelihood of them getting BROWN eyes.
•
USEFUL ANIMATION:
http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Mendel’s Law of Independent Assortment
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Punnett Squares
• The possible combinations
of sperm and egg can be
shown using a Punnett
square, a diagram for
predicting the results of a
genetic cross between
individuals of known genetic
makeup
• A capital letter represents a
dominant allele, and a
lowercase letter represents
a recessive allele
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Useful Genetics Vocabulary
• An organism with two identical alleles for a
character is said to be homozygous for the gene
controlling that character
•
Example: TT or tt for tall trait
• An organism that has two different alleles for a
gene is said to be heterozygous for the gene
controlling that character
•
Example: Tt for tall trait
• Unlike homozygotes, heterozygotes are not truebreeding…they are HYBRIDS!
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 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 Testcross
• How can we tell the genotype of an individual
with the dominant phenotype?
• Such an individual must have one dominant
allele, but the individual could be either
homozygous dominant or heterozygous
• The answer is to carry out a testcross: breeding
the mystery individual with a homozygous
recessive individual
• If any offspring display the recessive phenotype,
the mystery parent must be heterozygous
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-7
TECHNIQUE

Dominant phenotype, Recessive phenotype,
unknown genotype:
known genotype:
PP or Pp?
pp
Predictions
If PP
Sperm
p
p
P
Pp
Eggs
If Pp
Sperm
p
p
or
P
Pp
Eggs
P
Pp
Pp
pp
pp
p
Pp
Pp
RESULTS
or
All offspring purple
1/2
offspring purple and
1/2 offspring white
Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote
Sickle cell is a disease
caused by the
substitution of a single
amino acid in the
hemoglobin protein of
red blood cells. When
oxygen concentration
of affected individual is
low, the hemoglobin
crystallizes into long
rods.
Heterozygotes for
sickle cell have
increased resistance to
malaria because the rod
shape of blood
interrupts the parasites
life cycle. So, sickle
cell is prevalent among
African Americans.
Probabilities Practice
• What is the probability that the genotype Aa will
be produced by the parents Aa x Aa?
– ½
• What is the probability that the genotype ccdd
will be produced by the parents CcDd x CcDd?
– 1/16
• What is the probability that the genotype Rr will
be produced by the parents Rr x rr?
– ½
• What is the probability that the genotypes TTSs
will be produced by the parents TTSs x TtSS?
– 1/4
Genetics Practice Problems
• How many unique gametes could be produced
through independent assortment by an individual
with the genotype AaBbCCDdEE?
– 8
• What is the expected genotype ratio for a dihybrid
heterozygous cross?
– 9:3:3:1
You should now be able to:
1. Define the following terms: true breeding,
hybridization, monohybrid cross, P
generation, F1 generation, F2 generation
2. Distinguish between the following pairs of
terms: dominant and recessive; heterozygous
and homozygous; genotype and phenotype
3. Use a Punnett square to predict the results of
a cross and to state the phenotypic and
genotypic ratios of the F2 generation
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings