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Biology
A Guide to the Natural World
Chapter 11 • Lecture Outline
The First Geneticist: Mendel and His Discoveries
Fifth Edition
David Krogh
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11.1 Mendel and the Black Box
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Mendel and the Black Box
• Gregor Mendel was the first person to
comprehend some of the most basic
principles of genetics.
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Gregor Mendel
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Figure 11.1
Gregor Mendel
• Mendel reached these understandings in the
mid-nineteenth century working in what is
now the Czech Republic and using as his
experimental subjects, a species of garden
pea, Pisum sativum.
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11.2 The Experimental Subjects:
Pisum sativum
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The Experimental Subjects
• Mendel looked at seven characters in his
plants—attributes such as seed color and
texture.
• He observed which of those traits showed up in
succeeding generations.
• In his plants, each of these characters came in
two varieties or traits, one of them dominant,
the other recessive.
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Cross Pollination
1. Before fertilization occurs, peel back the closed
petals of a pea plant (in this case, one that came
from a line that yielded yellow peas). Then pull
out the pollen-bearing stamens with tweezers so
that self-fertilization is no longer possible.
flower grown from
a yellow seed
2. Next, gather pollen from a green-seed
plant by dabbing its anthers with a
paintbrush.
flower grown from
a green seed
crosspollination
offspring
(yellow seeds)
3. Finally, rub these pollen grains onto the stigma of
the first plant. The results of the cross-pollination
can be observed when the fertilized eggs mature
into seeds in the ovary, meaning peas in a pod.
The resulting seeds are yellow in this case
because yellow is dominant over green.
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Figure 11.3
Phenotypes and Genotypes
• A phenotype is any physiological feature,
bodily characteristic, or behavior of an
organism.
• In Mendel’s plants, purple flowers were one
phenotype, and white flowers were another.
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Phenotypes and Genotypes
• Phenotypes in any organism are in
significant part determined by that
organism’s genotype, meaning its genetic
makeup.
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Table 11.1
Pea-Plant Characters Studied by Mendel
Character Studied Dominant Trait
Recessive Trait
Seed shape
smooth
wrinkled
Seed color
yellow
green
Pod shape
inflated
wrinkled
Pod color
green
yellow
Flower color
purple
white
on stem
at tip
Flower position
Stem length
tall
dwarf
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Table 11.1
Three Genotypes Yield Two
Phenotypes
YY
Yy
Yy
yy
Three genotypes yield . . .
two phenotypes.
yellow
green
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Figure 11.7
11.3 Starting the Experiments:
Yellow and Green Peas
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Starting the Experiments
• Mendel realized that it was possible for
organisms to have identical phenotypes—
for all his pea plants to have yellow seeds,
for example—and yet to have differing
underlying genotypes.
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Starting the Experiments
• One of Mendel’s central insights was that
the basic units of genetics are material
elements that, in his pea plants, came in
pairs.
• These elements, today called genes, come in
alternative forms called alleles.
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Starting the Experiments
• One allele for a gene resides on one
chromosome.
• The other allele for the same gene resides
on a second chromosome that is
homologous to the first.
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possible pairing of
homologous chromosomes
dominant
allele
recessive
allele
location of
gene for
seed color
maternal paternal
maternal paternal
maternal
paternal
homozygous
dominant
heterozygous
homozygous
recessive
yellow
seeds
yellow
seeds
green
seeds
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Figure 11.8
Genes Retain Their Character
• Another of Mendel’s insights was that
genes retain their character through many
generations rather than being “blended”
together.
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(a) P generation crosses
female
male
1. Female gametes are being provided by a plant that has the dominant,
yellow alleles (YY); male gametes are being provided by a plant that
has the recessive, green alleles (yy).
2. The cells of the pea plants that give rise to gametes start to go
through meiosis.
P generation
female
gametes
male
gametes
possible
outcomes in
fertilization
F1 generation
3. The two alleles for pea color, which lie on separate homologous
chromosomes, separate in meiosis, yielding gametes that each bear
a single allele for seed color. In the female, each gamete bears a Y
allele; in the male, each bears a y allele.
4. The Punnett square shows the possible combinations that can result
when the male and female gametes come together in the moment of
fertilization. (If you have trouble reading the Punnett square, see
Figure 11.5b). The single possible outcome in this fertilization is a
mixed genotype, Yy.
5. Because Y (yellow) is dominant over y (green), the result is that all the
offspring in the F1 generation are yellow because they all contain a
Y allele.
1. A p gamete from the
male combines with
a p gamete from the
female to produce
an offspring of pp
genotype (and white
color).
male gametes
male gametes
(b) How to read a Punnett square
female gametes
female gametes
2. A p gamete from the
male combines with a
P gamete from the female
to produce an offspring of
Pp genotype
(and purple color).
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Figure 11.5
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11.4 Another Generation
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Law of Segregation
• A third insight of Mendel’s was that alleles
separate prior to the formation of gametes.
• The alleles Mendel was observing resided
on homologous chromosomes, which
always separate in meiosis.
• This concept is known as the law of
segregation.
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Law of Segregation
• An organism that has two identical alleles
of a gene for a given character is said to be
homozygous for that character.
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Law of Segregation
• An organism that has differing alleles for a
character is said to be heterozygous for that
character.
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Law of Segregation
• Dominant: expressed in the heterozygous
condition.
• Example: the yellow color of peas present
in the heterozygous Yy condition.
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Law of Segregation
• Recessive: not expressed in the
heterozygous condition.
• Example: the green color of peas absent in
the Yy condition.
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Law of Segregation
YY
Yy
Yy
yy
Three genotypes yield . . .
two phenotypes.
yellow
green
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Figure 11.7
11.5 Crosses Involving Two
Characters
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Crosses Involving Two Characters
• Mendel observed that the genes for the
different characters he studied were passed
on independently of one another.
• This was because the genes for these
characters resided on separate, nonhomologous chromosomes.
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Crosses Involving Two Characters
• The physical basis for what he found is the
independent assortment of chromosome
pairs during meiosis.
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11.6 Reception of Mendel’s Ideas
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Reception of Mendel’s Ideas
• Gregor Mendel published his work, but the
significance of it was never recognized in
his lifetime.
• It was only rediscovered 16 years after his
death, in 1900.
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11.7 Incomplete Dominance and
Codominance
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Incomplete Dominance
• Incomplete dominance operates when
neither allele for a given gene is completely
dominant, with the result that heterozygous
genotypes can yield an intermediate
phenotype (such as pink snapdragons).
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P generation
rr
white
RR
red
1. The starting plants are a
snapdragon homozygous for
red color (RR) and snapdragon
homozygous for white color (rr).
F1 generation
2. When these plants are crossed,
the resulting Rr genotype yields
only enough pigment to produce
a flower that is pink—the only
phenotype in the F1 generation.
Rr
100% pink
R sperm r
F2 generation
3. In the F2 generation, alleles
combine to produce red, pink,
and white phenotypes.
R
egg
RR
Rr
Rr
rr
r
1 : 2 : 1
red pink white
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Figure 11.10
Variations on Mendel
Animation 11.2: Variations on Mendel
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Codominance
• In some instances, differing alleles of the
same gene will have independent effects in
a single organism.
• Such is the case with the gene that codes for
the type A and B glycolipids that extend
from the surface of human red blood cells.
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Codominance
• An individual who has one A and one B
allele will have type AB blood.
• In such a situation, neither allele is
dominant; rather, each is having a separate
phenotypic effect.
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Table 11.3
Human Blood Types
This blood type
(phenotype) . . .
. . . has these surface glycolipids . . .
. . . and is produced by
these genotypes
A
AA or AO
B
BB or BO
AB
AB
O
OO
(no surface glycolipids)
The familiar ABO human blood-typing system refers to glycolipid molecules that extend from the
surface of red blood cells. People whose blood is “type A” have A extensions on their blood cells. It
is also possible to have only B extensions (and be type B); to have both A and B extensions (and be
type AB); or to have none of these extensions (and be type O). Note that a person whose genotype
is AO is phenotypically type A; likewise, a person whose genotype is BO is phenotypically type B.
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Table 11.3
Codominance
• When differing alleles of a single gene have
independent effects on the phenotype of an
individual, the alleles are said to be
codominant.
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11.8 Multiple Alleles and Polygenic
Inheritance
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Polygenic Inheritance
• Human beings and many other species can
have no more than two alleles for a given
gene, each allele residing on a separate,
homologous chromosome.
• However, many allelic variants of a gene
can exist in a population.
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Polygenic Inheritance
• Most traits in living things are governed by
many genes.
• These genes often have several allelic
variants.
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Polygenic Inheritance
• Polygenic inheritance means the
inheritance of a genetic character is
determined by the interaction of multiple
genes, with each having a small additive
effect on the character.
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Polygenic Inheritance
• Polygenic inheritance tends to produce
continuous variation in phenotypes, in
which there are no fixed increments of
difference between individuals.
• Human skin, for example, comes in a range
of colors in which one color shades
imperceptibly into the next.
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Polygenic Inheritance
• The traits produced in polygenic inheritance
tend to manifest in bell-curve distributions,
in which most individuals display near
average trait values rather than extreme trait
values.
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Continuous Variation and the
Bell Curve
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Figure 11.12
Polygenic Inheritance
• Gene interactions and gene–environment
interactions are so complex in polygenic
inheritance that predictions about
phenotypes are a matter of probability, not
certainty.
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11.9 Genes and Environment
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Genes and Environment
• The effects of genes can vary greatly in
accordance with the environment in which
the genes are expressed.
• An organism’s genotype and environment
interact to produce that organism’s
phenotype.
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Genes and Environment
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Figure 11.13