Transcript 15_Lectures_PPT

Chapter 15
The Chromosomal Basis of
Inheritance
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 15.1: Mendelian inheritance has its
physical basis in the behavior of chromosomes
• The chromosome theory of inheritance states
that:
– Mendelian genes have specific loci (positions)
on chromosomes
– It is the chromosomes that undergo
segregation and independent assortment
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LE 15-2
P Generation
Yellow-round
seeds (YYRR)
Green-wrinkled
seeds (yyrr)
Meiosis
Fertilization
Gametes
All F1 plants produce
yellow-round seeds (YyRr)
F1 Generation
Meiosis
LAW OF SEGREGATION
LAW OF INDEPENDENT ASSORTMENT
Two equally
probable
arrangements
of chromosomes
at metaphase I
Anaphase I
Metaphase II
Gametes
F2 Generation
Fertilization among the F1 plants
Morgan’s Experimental Evidence: Scientific Inquiry
• The first solid evidence associating a specific
gene with a a specific chromosome came from
Thomas Hunt Morgan, an embryologist.
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Morgan’s Choice of Experimental Organism
• Characteristics that make fruit flies a convenient
organism for genetic studies:
– They breed at a high rate
– A generation can be bred every two weeks
– They have only four pairs of chromosomes
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• Morgan noted wild type, or normal, phenotypes
that were common in the fly populations
• Traits alternative to the wild type are called
mutant phenotypes
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Correlating Behavior of a Gene’s Alleles with
Behavior of a Chromosome Pair
• In one experiment, Morgan mated male flies with white
eyes (mutant) with female flies with red eyes (wild
type)
– The F1 generation all had red eyes
– The F2 generation showed the 3:1 red:white
eye ratio, but only males had white eyes
• Morgan determined that the white-eye mutant allele
must be located on the X chromosome
• Morgan’s finding supported the chromosome
theory of inheritance
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P
Generation
F1
Generation
F2
Generation
P
Generation
Ova
(eggs)
Sperm
F1
Generation
Ova
(eggs)
F2
Generation
Sperm
Concept 15.2: Linked genes tend to be inherited together because
they are located near each other on the same chromosome
• Genes located on the same chromosome that
tend to be inherited together are called linked
genes
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How Linkage Affects Inheritance: Scientific Inquiry
• Morgan did other
experiments with
fruit flies to see how
linkage affects
inheritance of two
characters
P Generation
(homozygous)
Wild type
(gray body,
normal wings)
Double mutant
(black body,
vestigial wings)
b b vg vg
b+ b+ vg+ vg+
F1 dihybrid
(wild type
(gray body,
normal wings)
b+ b vg+ vg
Double mutant
(black body,
vestigial wings)
TESTCROSS
b b vg vg
Ova
965
944
Wild type
Black(gray-normal) vestigial
206
Grayvestigial
185
Blacknormal
Sperm
Parental-typeRecombinant (nonparental-type)
offspring
offspring
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• From the results, Morgan reasoned that body
color and wing size are usually inherited together
in specific combinations (parental phenotypes)
because the genes are on the same chromosome
• However, nonparental phenotypes were also
produced
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Recombination of Unlinked Genes: Independent
Assortment of Chromosomes
• Parental types - Offspring with a phenotype matching
one of the parental phenotypes.
• Recombinant types or Recombinants - Offspring
with nonparental phenotypes (new combinations).
– A 50% frequency of recombination is observed for
any two genes on different chromosomes
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LE 15-UN278-2
Gametes from yellow-round
heterozygous parent (YyRr)
Gametes from greenwrinkled homozygous
recessive parent (yyrr)
Parental-type
offspring
Recombinant
offspring
Recombination of Linked Genes: Crossing Over
• Crossing over of homologous chromosomes
explains how recombinants can sometimes occur
to otherwise linked genes.
Animation: Crossing Over
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LE 15-6
Testcross
parents
Black body,
vestigial wings
(double mutant)
Gray body,
normal wings
(F1 dihybrid)
Replication of
chromosomes
Replication of
chromosomes
Meiosis I: Crossing
over between b and vg
loci produces new allele
combinations.
Meiosis I and II:
No new allele
combinations are
produced.
Meiosis II: Separation
of chromatids produces
recombinant gametes
with the new allele
combinations.
Recombinant
chromosomes
Sperm
Ova
Gametes
Ova
Testcross
offspring
Sperm
965
Wild type
(gray-normal)
944
Blackvestigial
Parental-type offspring
206
Grayvestigial
185
Blacknormal
Recombinant offspring
Recombination
391 recombinants
=
 100 = 17%
frequency
2,300 total offspring
Linkage Mapping: Using Recombination Data:
Scientific Inquiry
• The farther apart two genes are, the higher the
probability that a crossover will occur between
them, and therefore the higher the recombination
frequency
Recombination
frequencies
9%
9.5%
17%
b
Chromosome
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cn
vg
• A linkage map is a genetic map of a chromosome
based on recombination frequencies
• Distances between genes can be expressed as
map units; one map unit, or centimorgan,
represents a 1% recombination frequency
Recombination
frequencies
9%
9.5%
17%
b
Chromosome
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cn
vg
LE 15-8
I
II
Y
X
IV
III
Mutant phenotypes
Short
aristae
0
Long aristae
(appendages
on head)
48.5
Gray
body
Vestigial
wings
Cinnabar
eyes
Black
body
57.5
67.0
Red
eyes
Wild-type phenotypes
Brown
eyes
104.5
Normal
wings
Red
eyes
Concept 15.3: Sex-linked genes exhibit unique
patterns of inheritance
• An organism’s sex is an inherited phenotypic
character determined by the presence or
absence of certain chromosomes
• In humans and other mammals, there are two
varieties of sex chromosomes, X and Y
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LE 15-9
Parents
Ova
Sperm
Zygotes
(offspring)
The X-Y system
The X-0 system
The Z-W system
The haplo-diploid system
Inheritance of Sex-Linked Genes
• The sex chromosomes have genes for many
characters unrelated to sex
• A gene located on either sex chromosome is
called a sex-linked gene
• Sex-linked genes follow specific patterns of
inheritance
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LE 15-10
Sperm
Ova
Sperm
Ova
Sperm
Ova
• Some disorders caused by recessive alleles on
the X chromosome in humans:
– Color blindness
– Duchenne muscular dystrophy
– Hemophilia
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X inactivation in Female Mammals
• In mammalian females, one of the two X
chromosomes in each cell is randomly inactivated
during embryonic development
• If a female is heterozygous for a particular
gene located on the X chromosome, she will
be a mosaic for that character
Two cell populations
in adult cat:
Active X
Early embryo:
X chromosomes
Orange
fur
Cell division
Inactive X
and X
Inactive X
chromosome
inactivation
Allele for
orange fur
Active X
Allele for
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black fur
Black
fur
Abnormal Chromosome Number
• In nondisjunction, pairs of homologous
chromosomes do not separate normally during
meiosis
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LE 15-12
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n+1
n+1
n–1
n–1
n+1
n–1
n
Number of chromosomes
Nondisjunction of homologous
chromosomes in meiosis I
Nondisjunction of sister
chromatids in meiosis I
n
• Aneuploidy results from the fertilization of
gametes in which nondisjunction occurred
– Trisomic zygote has three copies of a
particular chromosome
– Monosomic zygote has only one copy of a
particular chromosome
– Polyploidy is a condition in which an organism
has more than two complete sets of
chromosomes
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Aneuploidy
• Trisomy 21 (Down’s Syndrome) affects about
one out of every 700 children born in the United
States.
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Alterations of Chromosome Structure
• Breakage of a chromosome can lead to four types
of changes in chromosome structure:
– Deletion removes a chromosomal segment
– Duplication repeats a segment
– Inversion reverses a segment within a
chromosome
– Translocation moves a segment from one
chromosome to another
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LE 15-14
A deletion removes a chromosomal
segment.
A duplication repeats a segment.
An inversion reverses a segment
within a chromosome.
A translocation moves a segment
from one chromosome to another,
nonhomologous one.
Deletion
Duplication
Inversion
Reciprocal
translocation
Aneuploidy of Sex Chromosomes
• Nondisjunction of sex chromosomes produces a
variety of aneuploid conditions
• Klinefelter syndrome is the result of an extra
chromosome in a male, producing XXY individuals
• Monosomy X, called Turner syndrome, produces
X0 females, who are sterile; it is the only known
viable monosomy in humans
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Disorders Caused by Structurally Altered
Chromosomes
• One syndrome, cri du chat (“cry of the cat”),
results from a specific deletion in chromosome 5
• A child born with this syndrome is mentally
retarded and has a catlike cry; individuals usually
die in infancy or early childhood
Chromosomal Disorders Website
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