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BIOLOGY
A GUIDE TO THE NATURAL WORLD
FOURTH EDITION
DAVID KROGH
The First Geneticist:
Mendel and His Discoveries
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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.
• In his plants, each of these characters came in
two varieties or traits, one of them dominant,
the other recessive.
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The Experimental Subjects
• His experiments involved breeding pea plants.
• He started with plants that had a given set of
traits.
• He then observed which of those traits showed
up in succeeding generations.
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Cross Pollination
Parent
generation
X
(yellow seeds)
(green seeds)
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 pollenbearing stamens with tweezers so that
self-fertilization is no longer possible.
2. Next, gather pollen from
another plant by dabbing
its anthers with a
paintbrush.
crosspollination
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.
offspring
(yellow seeds)
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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.
• Mendel realized that the phenotypes in his
plants were being controlled by their genotypes.
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Phenotypes and Genotypes
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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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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 member of an allele pair resides on one
chromosome.
• The other allele resides on a second
chromosome that is homologous to the first.
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Chromosomes and Phenotypes
possible pairing of
homologous chromosomes
dominant
allele
recessive
allele
location of
gene for
seed color
maternal paternal maternal paternal maternal paternal
homozygous
heterozygous
homozygous
dominant
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.
• Genes that coded for green pea color, for
example, were retained in their existing form
over many generations.
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Genes Retain Their Character
(a) P generation crosses
female
YY
YY
male
YY
YY
yy
yy
yy
yy
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).
P generation
Y
2. The cells of the pea plants that give rise
to gametes start to go through meiosis.
y y
Y Y
y
Y
female
gametes
y
male
gametes
y
y
Y
Yy
Yy
Y
Yy
Yy
Yy
Yy
Yy
possible
outcomes in
fertilization
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.
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.
F1 generation
(b) How to read a Punnett square
female gametes
p
P
p
pp
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).
P
male gametes
male gametes
P
female gametes
p
P
p
Pp
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
11.4 Another Generation for Mendel
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Law of Segregation
• A third insight of Mendel’s was that alleles
separate prior to the formation of gametes.
• Although Mendel did not know it, the physical
basis for this is that the alleles he 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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Figure 11.7
Mendel’s Experiments and Probability
PLAY
Animation 11.1: Mendel’s Experiments and Probability
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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, non-homologous
chromosomes.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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
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Incomplete Dominance
• Not all inheritance works through the principles
Mendel perceived in his peas.
• 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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Incomplete Dominance
P generation
RR
red
rr
white
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
RR
Rr
Rr
rr
egg
r
1 :
2
: 1
red
pink white
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Figure 11.10
Variations on Mendel
PLAY
Animation 11.2: Variations on Mendel
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
11.8 Lessons from Blood Types:
Co-dominance
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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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Codominance
Blood type
(phenotype) . . .
. . . has these surface
glycolipids . . .
. . . and is
produced by
these genotypes
Surface glycolipids
on red blood cells
A
AA or AO
B
BB or BO
AB
AB
O
OO
no surface glycolipids
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Figure 11.11
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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11.9 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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
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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Polygenic Inheritance
(a) Continuous variation in human height
(b) The bell curve
Number of individuals
80
beak depth
60
40
20
0
5
6
7
8
9
10
11
12
13
14
Beak depth (mm)
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Figure 11.13
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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11.10 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.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Genes and Environment
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Figure 11.14
11.11 One Gene, Several Effects:
Pleiotropy
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Pleiotropy
• Pleiotropy is a phenomenon in which one gene
has many effects.
• Genes work in an interrelated fashion, such that
a single gene is likely to have multiple effects.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.