Transcript 9.3 How Are Single Traits Inherited?

Chapter 9
Patterns of
Inheritance
Lectures by
Gregory Ahearn
University of North Florida
Copyright © 2009 Pearson Education, Inc.
9.1 What Is The Physical Basis Of
Inheritance?
 Inheritance occurs when genes are
transmitted from parent to offspring.
• The units of inheritance are genes, which are
segments of DNA of variable length.
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9.1 What Is The Physical Basis Of
Inheritance?
 Genes are segments of DNA at specific
locations on chromosomes.
• A gene’s physical location on a chromosome
is called its locus.
• Each member of a pair of homologous
chromosomes carries the same genes,
located at the same loci.
• Different versions of a gene at a given locus
are called alleles.
Copyright © 2009 Pearson Education Inc.
9.1 What Is The Physical Basis Of
Inheritance?
 The relationship among genes, alleles, and
chromosomes
a pair of
homologous
chromosomes
Both chromosomes carry the same allele
of the gene at this locus; the organism is
homozygous at this locus
gene loci
This locus contains another gene for which
the organism is homozygous
Each chromosome carries a different allele
of this gene, so the organism is
heterozygous at this locus
the chromosome
from the male
parent
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the chromosome
from the female
parent
Fig. 9-1
9.1 What Is The Physical Basis Of
Inheritance?
 Mutations are the source of alleles.
• Differences in alleles at a given locus are due
to mutations at that gene.
• If a mutation occurs in the cells that become
sperm or eggs, it can be passed on from
parent to offspring.
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9.1 What Is The Physical Basis Of
Inheritance?
 An organism’s two alleles may be the same
or different.
• A diploid organism has pairs of homologous
chromosomes with two copies of each gene at
a given locus.
• If both homologous chromosomes have the
same allele at a locus, the organism is said to
be homozygous.
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9.1 What Is The Physical Basis Of
Inheritance?
 An organism’s two alleles may be the same
or different (continued).
• If two homologous chromosomes have
different alleles at a locus, the organism is
heterozygous at that locus.
• The gametes of a homozygous individual are
all the same at a particular locus, while
gametes of a heterozygous individual would
contain half one allele and half the other allele.
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9.2 How Were The Principles Of
Inheritance Discovered?
 The patterns of
inheritance were
discovered by an
Austrian monk,
Gregor Mendel.
Fig. 9-2
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9.2 How Were The Principles Of
Inheritance Discovered?
 Doing it right: the secrets of Mendel’s
success
• Mendel was the first geneticist to employ three
key steps in his experimentation:
• Choosing the right organism for the work
• Designing and performing experiments
correctly
• Analyzing the data properly
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9.2 How Were The Principles Of
Inheritance Discovered?
 Mendel chose edible pea as the
experimental subject for his experiments in
inheritance.
• Pea egg cells in a pea flower fertilized by
sperm from the same flower is called selffertilization.
• When sperm from one organism fertilizes
eggs from a different organism, the process is
called cross-fertilization.
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9.2 How Were The Principles Of
Inheritance Discovered?
 Mendel chose edible pea as the
experimental subject for his experiments in
inheritance (continued).
• Mendel studied individual characteristics of
pea plants, such as flower color; these
characteristics are called traits.
• He followed the inheritance of these traits for
several generations, counting the numbers of
offspring with each type of trait.
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9.3 How Are Single Traits Inherited?
 True-breeding traits of organisms, such as
purple flower color, are always inherited by
all of their offspring that result from selffertilization.
• In one experiment, Mendel cross-fertilized
white-flowered plants with purple-flowered
plants.
• When he grew the resulting seeds, he found
all the first-generation offspring, or the F1
generation, produced purple flowers.
• What happened to the white color?
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9.3 How Are Single Traits Inherited?
 Cross of pea plants that are true-breeding
for white or purple flowers
pollen
Parental
generation (P)
pollen
cross-fertilize
true-breeding,
purple-flowered
plant
true-breeding,
white-flowered
plant
First-generation
offspring (F1)
all purple-flowered
plants
Fig. 9-4
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9.3 How Are Single Traits Inherited?
 The F2 generation
• Next, Mendel allowed the F1 flowers to selffertilize, collected the seeds, and grew the
second generation, called the F2 generation.
• Flowers in the F2 generation were threefourths purple and one-fourth white, in a ratio
of 3 purple to 1 white.
• This showed that the gene for white flowers
was “hidden” in the F1 generation, but
appeared again in the F2 generation.
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9.3 How Are Single Traits Inherited?
 Cross of F1 plants with purple flowers
Firstgeneration
offspring (F1)
self-fertilize
Secondgeneration
offspring (F2)
3/4 purple
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1/4 white
Fig. 9-5
9.3 How Are Single Traits Inherited?
PLAY
Animation—Crosses Involving Single Traits
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9.3 How Are Single Traits Inherited?
 All the white-flowered plants in the F2
generation only produced additional whiteflowered plants.
 Purple-flowered plants were of two types:
• About ⅔ were true-breeding for purple, while
⅔ produced both purple- and white-flowered
offspring (ratio 3 purple/1 white).
• Therefore, the F2 generation included ¼ truebreeding purple plants, ½ hybrid purple, and
¼ true-breeding white plants.
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9.3 How Are Single Traits Inherited?
 The inheritance of dominant and recessive
alleles on homologous chromosomes can
explain the results of Mendel’s crosses.
• Mendel’s results allow us to develop a fivepart hypothesis to explain the inheritance of
single traits.
1.Each trait is determined by pairs of distinct
physical units called genes.
• There are two alleles for each gene, one
on each homologous chromosome.
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9.3 How Are Single Traits Inherited?
2. When two different alleles are present in an
organism, the dominant allele may mask the
expression of the recessive allele; however,
the recessive allele is still present.
3. The two alleles of a gene segregate
(separate) from one another during meiosis;
this is known as Mendel’s law of
segregation.
4. Which allele ends up in any given gamete is
determined by chance.
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9.3 How Are Single Traits Inherited?
5. True-breeding (homozygous) organisms
have two copies of the same allele for a
given gene; hybrid (heterozygous)
organisms have two different alleles for a
given gene.
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9.3 How Are Single Traits Inherited?
 The distribution of alleles in gametes
homozygous parent
A
A
gametes
A
A
(a) Gametes produced by a homozygous parent
heterozygous parent
A
a
gametes
A
a
(b) Gametes produced by a heterozygous parent
Fig. 9-6
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9.3 How Are Single Traits Inherited?
 In pea plants, purple is dominant to white.
• Letters can be used to describe the alleles (P
is for the dominant allele; p is for the recessive
allele).
• Homozygous purple plants are PP;
homozygous white plants are pp.
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9.3 How Are Single Traits Inherited?
 Homozygous purple plants are PP;
homozygous white plants are pp.
purple parent
PP
P
+
P
all P sperm and eggs
white parent
pp
p
+
p
all p sperm and eggs
(a) Gametes produced by homozygous parents
Fig. 9-7a
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9.3 How Are Single Traits Inherited?
 F1 plants were produced by P and p
gametes, making Pp F1 hybrid offspring.
sperm
P
eggs
+
F1 offspring
p
Pp
P
Pp
or
p
+
(b) Fusion of gametes produces F1 offspring
Fig. 9-7b
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9.3 How Are Single Traits Inherited?
 Next, Mendel crossed two F1 hybrid plants
(Pp x Pp).
• This cross made three types of F2 offspring,
with the following allele composition
• ¼ were PP; ½ were Pp; ¼ were pp.
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9.3 How Are Single Traits Inherited?
 Fusion of gametes from the F1 generation
produces F2 offspring.
gametes from F1 Pp plants
sperm
F2 offspring
eggs
P
+
P
PP
P
+
p
Pp
p
+
P
Pp
p
+
p
pp
(c) Fusion of gametes from the F1 generation produces F2 offspring
Fig. 9-7c
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9.3 How Are Single Traits Inherited?
 Mendel’s hypothesis was that two plants
may look alike, called its phenotype, but
have a different allele composition, called its
genotype.
 In this case, purple plants had PP or Pp
genotypes, but their phenotype (purple
color) was the same.
 The F2 generation could be described as
having three genotypes (¼ PP, ½ Pp, and ¼
pp) and two phenotypes (¾ purple and ¼
white).
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9.3 How Are Single Traits Inherited?
 Simple “genetic bookkeeping” can predict
the genotypes and phenotypes of offspring.
• The Punnett square method is a convenient
way to predict the genotypes and phenotypes
of offspring.
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9.3 How Are Single Traits Inherited?
 The Punnett square
method
Pp
self-fertilize
P
1
2
1
2
1
2
p
P
sperm
1
2
eggs
1
4
PP
1
4
Pp
1
4
pP
1
4
pp
p
Fig. 9-8
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9.3 How Are Single Traits Inherited?
PLAY
Animation—Punnett Square
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9.3 How Are Single Traits Inherited?
 Mendel’s hypothesis can predict the
outcome of new types of single-trait
crosses.
• Mendel predicted the outcome of crossfertilizing Pp plants with homozygous
recessive plants (pp)—there should be equal
numbers of Pp (purple) and pp (white)
offspring.
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9.3 How Are Single Traits Inherited?
 A Punnet square
shows how this “test
cross” results in the
predicted offspring.
pollen
PP or Pp
pp
all eggs
sperm unknown
if PP
if Pp
p
all
sperm
p
P
p
eggs
1
2
P
1
2 Pp
sperm
all Pp
1
2
eggs
p
1
2 pp
Fig. 9-9
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9.4 How Are Multiple Traits Inherited?
 Mendel next crossed pea plants that differed
in two traits, such as seed color (yellow or
green) and seed shape (smooth or
wrinkled).
• He knew from previous crosses that smooth
and yellow were both dominant traits in peas.
• His first cross was a true-breeding plant with
smooth, yellow seeds (SSYY) to a truebreeding plant with wrinkled, green seeds
(ssyy).
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9.4 How Are Multiple Traits Inherited?
 Traits of pea plants
studied by Gregor
Mendel
Trait
Dominant form
Recessive form
Seed
shape
smooth
wrinkled
Seed
color
yellow
green
Pod
shape
inflated
constricted
Pod
color
green
yellow
purple
white
Flower
color
Flower
locaat leaf
tion
junctions
Plant
size
tall
(about
6 feet)
at tips of
branches
dwarf
(about 8 to
16 inches)
Fig. 9-10
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9.4 How Are Multiple Traits Inherited?
 All the offspring of this cross (F1 generation)
were SsYy and had smooth, yellow seeds
(both dominant traits).
 F1 plants were allowed to self-fertilize and
produced F2 offspring in the phenotypic ratio
9:3:3:1.
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9.4 How Are Multiple Traits Inherited?
 Mendel concluded that multiple traits are
inherited independently.
• Mendel realized that these results could be
explained if the genes for seed color and seed
shape were inherited independently.
• The independent inheritance of two or more
distinct traits is called the law of independent
assortment.
• Multiple traits are inherited independently
because the alleles of one gene are
distributed to gametes independently of the
alleles of other genes.
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9.4 How Are Multiple Traits Inherited?
 Predicting
genotypes and
phenotypes
SsYy
self-fertilize
eggs
1 SY
4
sperm
1 Sy
4
1 sY
4
1 sy
4
1 SY
4
1 Sy
4
1 sY
4
1
16 SSYY
1
16 SSYy
1
16 SsYY
1
16 SsYy
1
16 SSyY
1
16 SSyy
1
16 SsyY
1
16 Ssyy
1
16 sSYY
1
16 sSYy
1
16 ssYY
1
16 ssYy
1
16 sSyY
1
16 sSyy
1
16 ssyY
1
16 ssyy
1
4
sy
Fig. 9-11
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9.4 How Are Multiple Traits Inherited?
PLAY
Animation—Multiple Traits
PLAY
Animation—Multiple Traits
Copyright © 2009 Pearson Education Inc.
9.4 How Are Multiple Traits Inherited?
 Independent
assortment of
alleles
S
pairs of alleles on
homologous chromosomes
in diploid cells
s
Y
y
chromosomes replicate
S
Y
s
y
replicated homologues
pair during metaphase
S
of meiosis I,
orienting like this
or like this
s
y
Y
meiosis I
S
Y
s
y
S
y
s
Y
S
Y
s
y
S
y
s
Y
meiosis II
S
S
s
Y
Y
SY
S
s
y
y
sy
s
S
y
y
s
Y
Y
Sy
sY
independent assortment produces four equally
likely allele combinations during meiosis
Fig. 9-12
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9.4 How Are Multiple Traits Inherited?
 In an unprepared world, genius may go
unrecognized.
• Gregor Mendel presented his theories of
inheritance in 1865.
• His experiments made little impact on science
during his lifetime.
• It was not until 1900 that three biologists—
Carl Correns, Hugo de Vries, and Erich
Tschermak—rediscovered Mendel’s work and
acknowledged its importance to science.
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