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LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson

Chapter 14

Mendel and the Gene Idea

Lectures by
Erin Barley
Kathleen Fitzpatrick
© 2011 Pearson Education, Inc.

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)

© 2011 Pearson Education, Inc.

• The “particulate” hypothesis is the idea that
parents pass on discrete heritable units
(genes)
• This hypothesis can explain the reappearance
of traits after several generations
• Mendel documented a particulate mechanism
through his experiments with garden peas

© 2011 Pearson Education, Inc.

Figure 14.1

Concept 14.1: Mendel used the scientific
approach to identify two laws of inheritance
• Mendel discovered the basic principles of
heredity by breeding garden peas in carefully
planned experiments

© 2011 Pearson Education, Inc.

Mendel’s Experimental, Quantitative
Approach
• 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 can be controlled
– Each flower has sperm-producing organs
(stamens) and an egg-producing organ (carpel)
– Cross-pollination (fertilization between different
plants) involves dusting one plant with pollen
from another
© 2011 Pearson Education, Inc.

Figure 14.2

TECHNIQUE
1

2
Parental
generation
(P)
3

Stamens
Carpel
4

RESULTS
First filial
generation
offspring
(F1)

5

Figure 14.2a

TECHNIQUE
1

2
Parental
generation
(P)

Stamens
3

Carpel
4

Figure 14.2b

RESULTS
First filial
generation
offspring
(F1)

5

• Mendel chose to track only those characters
that occurred in two distinct alternative forms
• He also used varieties that were true-breeding
(plants that produce offspring of the same
variety when they self-pollinate)

© 2011 Pearson Education, Inc.

• 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 or crosspollinate with other F1 hybrids, the F2 generation is produced

© 2011 Pearson Education, Inc.

The Law of Segregation
• When Mendel crossed contrasting, truebreeding 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

© 2011 Pearson Education, Inc.

Figure 14.3-1

EXPERIMENT
P Generation
(true-breeding
parents)

Purple
flowers

White
flowers

Figure 14.3-2

EXPERIMENT
P Generation
(true-breeding
parents)

F1 Generation
(hybrids)

Purple
flowers

White
flowers

All plants had purple flowers
Self- or cross-pollination

Figure 14.3-3

EXPERIMENT
P Generation
(true-breeding
parents)

Purple
flowers

White
flowers

F1 Generation
(hybrids)

All plants had purple flowers
Self- or cross-pollination

F2 Generation

705 purpleflowered
plants

224 white
flowered
plants

• 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
• The factor for white flowers was not diluted or
destroyed because it reappeared in the F2
generation

© 2011 Pearson Education, Inc.

• 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

© 2011 Pearson Education, Inc.

Table 14.1

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
• These concepts can be related to what we now
know about genes and chromosomes

© 2011 Pearson Education, Inc.

• First: alternative versions of genes account for
variations in inherited characters
• For example, the gene for flower color in pea
plants exists in two versions, one for purple
flowers and the other for white flowers
• These alternative versions of a gene are now
called alleles
• Each gene resides at a specific locus on a
specific chromosome

© 2011 Pearson Education, Inc.

Figure 14.4

Allele for purple flowers

Locus for flower-color gene

Pair of
homologous
chromosomes

Allele for white flowers

• Second: for each character, an organism
inherits two alleles, one from each parent
• Mendel made this deduction without knowing
about the role of chromosomes
• The two alleles at a particular locus may be
identical, as in the true-breeding plants of
Mendel’s P generation
• Alternatively, the two alleles at a locus may
differ, as in the F1 hybrids

© 2011 Pearson Education, Inc.

• Third: if the two alleles at a locus differ, then one
(the dominant allele) determines the organism’s
appearance, and the other (the recessive allele)
has no noticeable effect on appearance
• In the flower-color example, the F1 plants had
purple flowers because the allele for that trait is
dominant

© 2011 Pearson Education, Inc.

• ...

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