Transcript ppt

The Chromosomal
Basis of Inheritance
PowerPoint Lectures for
Biology, Seventh Edition
Chapter 15
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Overview: Locating Genes on Chromosomes
• Genes
– Are located on chromosomes
– Can be visualized using certain techniques
Figure 15.1
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Concept 15.1: Mendelian inheritance has its
physical basis in the behavior of chromosomes
• Several researchers proposed in the early
1900s that genes are located on chromosomes
• The behavior of chromosomes during meiosis
was said to account for Mendel’s laws of
segregation and independent assortment
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The chromosomal basis of Mendel’s laws
P Generation
Starting with two true-breeding pea plants,
we follow two genes through the F 1 and F2
generations. The two genes specify seed
color (allele Y for yellow and allele y for
Y
green) and seed shape (allele R for round
and allele r for wrinkled). These two genes are
on different chromosomes. (Peas have seven
chromosome pairs, but only two pairs are
illustrated here.)
Yellow-round
seeds (YYRR)
Green-wrinkled
seeds (yyrr)
Y
R
r
R
r
y
Meiosis
Fertilization
y
R Y
Gametes
y
r
All F1 plants produce
yellow-round seeds (YyRr)
R
R
y
F1 Generation
y
r
r
Y
Y
Meiosis
LAW OF SEGREGATION
r
R
Y
1 The R and r alleles segregate
R
at anaphase I, yielding
two types of daughter
cells for this locus.
Y
y
Y
y
LAW OF INDEPENDENT ASSORTMENT
r
y
Y
1 Alleles at both loci segregate
in anaphase I, yielding four
types of daughter cells
R
depending on the chromosome
arrangement at metaphase I.
Compare the arrangement of
the R and r alleles in the cells
y
on the left and right
r
R
y
y
Metaphase II
Y
y
Y
Y
R
R
r
Y
r
r
1 yr
4
F2 Generation
3 Fertilization
recombines the
R and r alleles
at random.
Y
Y
r
1
YR
4
Figure 15.2
R
Anaphase I
r
Y
Gametes
r
r
R
2 Each gamete
gets one long
chromosome
with either the
R or r allele.
Two equally
probable
arrangements
of chromosomes
at metaphase I
1 yr
4
2 Each gamete gets
a long and a short
chromosome in
one of four allele
combinations.
y
y
R
R
1
yR
4
Fertilization among the F1 plants
9
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:3
:3
:1
3 Fertilization results
in the 9:3:3:1
phenotypic ratio in
the F2 generation.
Scientists build on the discoveries of their predecessors:
mid-1800’s Gregor Mendel - Laws of
Inheritance; named "factors" as hereditary
information
1879- Walter Fleming - stained cells and
identified chromosomes
1902 Walter Sutton- "Chromosomal Theory of
Heredity"
• chromosomes are the carriers of traits
• each chromosome carries many traits
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Scientists build on the discoveries of their predecessors:
1905 E.B Wilson - American biologist
identified sex chromosomes in insects
Human: total 23 pairs of
chromosomes
• 1 pair of sex
chromosomes XX or XY;
(inherit 1 from each
parent)
• your 22 other pairs are
called autosomes, the
body chromosomes that
carry most of your traits
All the chromosomes of an individual cell can be visualize with a
karyotype.
how to make one and what they look like
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The Chromosomal Basis of Sex
• 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
44 +
XY
22 + Sperm
X
44 +
XX
(a) The X-Y system
Figure 15.9a
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44 +
XX
Parents
22 +
Y
22 +
XY
Ova
Zygotes
(offspring)
44 +
XY
• Different systems of sex determination
– Are found in other organisms
22 +
XX
22 +
X
76 +
ZW
76 +
ZZ
(b) The X–0 system
(c) The Z–W system
Figure 15.9b–d
16
16
(Diploid)
(Haploid)
(d) The haplo-diploid system
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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
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Morgan’s Experimental Evidence: Scientific Inquiry
• Thomas Hunt Morgan
– Provided convincing evidence that
chromosomes are the location of Mendel’s
heritable factors
won the Nobel Prize in Physiology or Medicine 1933
"for his discoveries concerning the role played by the
chromosome in heredity"
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Morgan’s Choice of Experimental Organism
• Morgan worked with fruit flies Drosophila melanogaster
– Because they breed at a high rate
– A new generation can be bred every two
weeks
– They have only four pairs of chromosomes
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• Morgan first observed and noted
– Wild type, or normal, phenotypes that were
common in the fly populations
• Traits alternative to the wild type
– Are called mutant phenotypes
Figure 15.3
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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
mutant
wt
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• Morgan determined
– That the white-eye mutant allele must be
located on the X chromosome
EXPERIMENT Morgan mated a wild-type (red-eyed) female
with a mutant white-eyed male. The F1 offspring all had red eyes.
CONCLUSION
P
Generation
X
F1
Generation
Morgan then bred an F1 red-eyed female to an F1 red-eyed male to
produce the F2 generation.
RESULTS
The F2 generation showed a typical Mendelian
3:1 ratio of red eyes to white eyes. However, no females displayed the
white-eye trait; they all had red eyes. Half the males had white eyes,
and half had red eyes.
F2
Generation
Figure 15.4
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Since
all F1 offspring had red
eyes, the mutant
white-eye trait (w) must be
recessive to the wild-type
red-eye trait (w+).
Since the recessive trait—
white eyes—was
expressed only in males in
the F2 generation,
Morgan hypothesized that
the eye-color gene is
located on the X
chromosome and that
there is no corresponding
locus on the Y
chromosome.
• Morgan’s discovery that transmission of the X
chromosome in fruit flies correlates with
inheritance of the eye-color trait
– Was the first solid evidence indicating that a
specific gene is associated with a specific
chromosome
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• Some recessive alleles found on the X
chromosome in humans cause certain types of
disorders
– Color blindness
– Duchenne muscular dystrophy
– Hemophilia
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Morgan did other experiments with fruit flies
– To see how the inheritance of two different
characters is affected by gene location
• Morgan determined that
–
Genes that are close together on the same chromosome are
linked and do not assort independently
–
Unlinked genes are either on separate chromosomes of are far
apart on the same chromosome and assort independently
b+ vg+
Parents
in testcross
Most
offspring
X
b vg
b vg
b vg
b+ vg+
b vg
or
b vg
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b vg
Recombination of Unlinked Genes: Independent
Assortment of Chromosomes
• When Mendel followed the inheritance of two
characters
– He observed that some offspring have
combinations of traits that do not match either
parent in the P generation
Gametes from yellow-round
heterozygous parent (YyRr)
Independent
assortment
Gametes from greenwrinkled homozygous
recessive parent (yyrr)
YR
yr
Yr
yR
Yyrr
yyRr
yr
YyRr
yyrr
Parentaltype offspring
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Recombinant
offspring
Due to the appearance of recombinant phenotypes,
– Some process must occasionally break the
physical connection between genes on the
same chromosome
– Crossing over of homologous
chromosomes was the mechanism behind
the recombination
– Crossover animation, narrated
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• “Recombinant “offspring
– Are those that show new combinations of the
parental traits
• When 50% of all offspring are recombinants
– Geneticists say that there is a 50% frequency
of recombination
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Gene Linkage
• Each chromosome
– Has hundreds or
thousands of genes
• Linked genes tend to be
inherited together
because they are
located near each other
on the same
chromosome
•
http://highered.mcgrawhill.com/sites/dl/free/0072835125
/126997/animation5.html
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Linked genes
– Exhibit recombination frequencies less than 50%
Testcross
parents
b+ vg+
Gray body,
normal wings
b vg
(F1 dihybrid)
Replication of
chromosomes
b+ vg
Meiosis I: Crossing
over between b and vg
loci produces new allele
combinations.
Black body,
vestigial wings
b vg (double mutant)
Replication of
chromosomes
b vg

b+vg+
vg
b
b vg
vg
b
b vg
b vg
Meiosis II: Segregation
of chromatids produces
recombinant gametes
with the new allele
combinations.
Gametes
b vg
Meiosis I and II:
Even if crossing over
occurs, no new allele
combinations are
produced.
Recombinant
chromosome
Ova
Sperm
b+vg+
b vg
b+
vg
b vg+
b vg
b+ vg+
Testcross
offspring
Sperm
b vg
Figure 15.6
b vg
944
965
BlackWild type
(gray-normal) vestigial
b+ vg+
b vg+
b vg
b vg
b+ vg
206
Grayvestigial
b+ vg+
b vg
b vg+ Ova
185
BlackRecombination
normal
b vg+ frequency
b vg
Parental-type offspring Recombinant offspring
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
391 recombinants
= 2,300 total offspring 
100 = 17%
Linkage Mapping: Using Recombination Data:
Scientific Inquiry
• A genetic map
– Is an ordered list
of the genetic
loci along a
particular
chromosome
– Can be
developed using
recombination
frequencies
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A linkage map
– Is the actual map of a chromosome based on
recombination frequencies
APPLICATION
A linkage map shows the relative locations of genes along a chromosome.
TECHNIQUE
A linkage map is based on the assumption that the probability of a crossover between two
genetic loci is proportional to the distance separating the loci. The recombination frequencies used to construct
a linkage map for a particular chromosome are obtained from experimental crosses, such as the cross depicted
in Figure 15.6. The distances between genes are expressed as map units (centimorgans), with one map unit
equivalent to a 1% recombination frequency. Genes are arranged on the chromosome in the order that best fits the data.
RESULTS In this example, the observed recombination frequencies between three Drosophila gene pairs
(b–cn 9%, cn–vg 9.5%, and b–vg 17%) best fit a linear order in which cn is positioned about halfway between
the other two genes:
Recombination
frequencies
9.5%
9%
17%
Chromosome b
cn
vg
The b–vg recombination frequency is slightly less than the sum of the b–cn and cn–vg frequencies because double
crossovers are fairly likely to occur between b and vg in matings tracking these two genes. A second crossover
Figure 15.7 would “cancel out” the first and thus reduce the observed b–vg recombination frequency.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Crossing Over is a normal part of meiotic division
• The farther apart genes are on a chromosome
–
The more likely they are to be separated during crossing over
CROSSING OVER: alleles on chromosomes become rearranged
during meiosis (click for animation) "Chromosome Mapping" is
now possible. The crossover % is directly related to a gene's
position on the chromosome.
example: the relative abundance of four genes on a chromosome can be mapped from the
following data on crossover frequencies:
GENES
Frequency of Crossover
B and D
5%
C and A
15%
A and B
30%
C and B
45%
C and D
50%
Which of the following represents the relative positions of the four genes on the
chromosome?
a. ABCD
b. ADCB
c. CABD
d. CBAD
e. DBCA
HUMAN: chromosome maps online
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Gene Mapping
• Many fruit fly genes
– Were mapped initially using recombination
I
frequencies
Y
II
Try this online gene
mapping tutorial, if you’re
feeling ambitious
http://www.dnaftb.org/11/pr
oblem.html
Figure 15.8
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
X
IV
III
Mutant phenotypes
Short
aristae
Black
body
0
Long aristae
(appendages
on head)
Cinnabar Vestigial
eyes
wings
48.5 57.5 67.0
Gray
body
Red
eyes
Normal
wings
Wild-type phenotypes
Brown
eyes
104.5
Red
eyes