Download The chromosome theory of inheritance

Genetics 2011
Outline of Chromosome Theory of
Inheritance
• Part 1:
Lecture 4
• Observations and experiments that placed the
hereditary material in the nucleus on the
chromosomes
The chromosome
theory of inheritance
• Part 2:
• Mitosis and Meiosis
ᯘ⩶ె ⪁ᖌ
• Part
P t 3:
3
http://lms.ls.ntou.edu.tw/course/136
1
• Gametogenesis
• Validation of the chromosome theory of inheritance
2
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Genetics 2011
Evidence that Genes Reside in the
Nucleus
• 1667 – Anton van Leeuwenhoek
• Microscopist
p
• Semen contains spermatozoa (sperm animals).
• Hypothesized that sperm enter egg to achieve
fertilization
• 1854-1874 – confirmation of fertilization
g union of eggs
gg and sperm
p
through
• Recorded frog and sea urchin fertilization using
microscopy and time
time-lapse
lapse drawings and
micrographs
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Evidence that Genes Reside in
Chromosomes
• 1880s – innovations in microscopy and
staining techniques identified thread-like
structures
• Provided a means to follow movement of
chromosomes during cell division
• Mitosis – two daughter cells contained same
number of chromosomes as parent cell (somatic
cells)
• Meiosis – daughter cells contained half the
number of chromosomes as the parents (sperm
and
d eggs))
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One Chromosome Pair Determines an
Individual’s Sex.
• After
fertilization
• Walter Sutton – Studied great lubber
grasshopper
• Parent cells contained 22 chromosomes
plus an X and a Y chromosome.
• Daughter
D
ht cells
ll contained
t i d 11
chromosomes and X or Y in equal
numbers.
5
• Cells with XX
were females.
• Cells with XY
were males.
Great llubber
G
bb grasshopper
h
(Brachystola magna)
Genetics 2011
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• Sex chromosome
• Sex
determination
i h
in
humans
• Provide basis for
sex determination
d t
i ti
• One sex has
matching pair.
• Other sex has one
of each type of
chromosome.
Photomicrograph of human
X and Y chromosome
Fig. 4.6a
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• Children
receive only an
X chromosome
from mother
but X or Y from
father.
7
8
Fig. 4.6b
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At Fertilization, Haploid Gametes
Produce Diploid Zygotes.
Drosophila Melanogaster
• Gamete contains one-half the number
of chromosomes as the zygote.
• Haploid – cells that carry only a single
chromosome set
• Diploid – cells that carry two matching
chromosome
h
sets
t
• n – the number of chromosomes in a
haploid cell
• 2n – the number of chromosomes in a
diploid cell
Mostt body
M
b d cells
ll are di
diploid
l id ((each
h
chromosome pair has one
maternal and one paternal
copy)
Meiosis Æ haploid (n) gametes
In Drosophila, 2n = 8, n = 4
In humans , 2n = 46 and n = 23
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The number and shape of chromosomes
vary from species to species.
n
2
2n
Drosophila melanogaster
Drosophila obscura
Drosophila virilus
Peas
M
Macaroni
i wheat
h t
Giant sequoia trees
Goldfish
Dogs
4
5
6
7
14
11
47
39
8
10
12
14
28
22
94
78
Humans
23
46
O
Organism
i
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10
Fig. 4.2
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Fertilization is the union of haploid
gametes to produce diploid zygotes
Fertilized eggs carry matching sets of
chromosomes,, one set from maternal gamete
g
and one set from paternal gamete
Gametes
G
t are haploid
h l id ((n)) – carry only
l a single
i l sett
of chromosomes
Zygotes are diploid (2n) – carry two matching set
of chromosome
Mitosis ensures that all cells of developing
i di id l h
individuals
have id
identical
ti l 2n
2 chromosome
h
sets
t
11
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Anatomy of a chromosome
Genetics 2011
Homologous chromosomes match in
size, shape, and banding patterns.
• Homologous chromosomes (homologs)
contain the same set of genes.
alleles.
• Genes may carry different alleles
• Nonhomologous chromosomes carry
completely
l t l unrelated
l t d sets
t off genes.
Metaphase chromosomes are classified by the position of the centromere
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Karyotypes can be produced by cutting
micrograph images of stained chromosomes and
arranging them in matched pairs
Human male karyotype
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15
Autosomes – pairs of nonsex chromosomes
Sex chromosomes and autosomes are arranged in homologous pairs
Note 22 pairs of autosomes and 1 pair of sex chromosomes
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Genetics 2011
There is variation between species in how
chromosomes determine an individual’s sex.
Genetics 2011
Complement of sex chromosomes
H
Humans
– presence off Y d
determines
t
i
sex
Drosophila – ratio of autosomes to X chromosomes determines sex
XXX
XX
XXY
XO
XY
XYY
OY
Dies
Normal
f
female
l
Normal
f
female
l
Sterile
male
l
Normal
male
l
Normal
male
l
Dies
Nearly
normal
Male
Normal
female
Kleinfelter
male
(sterile);
t ll thin
tall,
thi
Turner
female
(sterile);
webbed
bb d
neck
Normal
male
Normal
or nearly
normal
l
male
Dies
__________________________________________________
Chromosome Females
Males
Organism
__________________________________________________
XX-XY
XX
XY
XX
XY
M
Mammals,
l Drosophila
D
hil
XX-XO
XX
XO
Grasshoppers
ZZ-ZW
ZZ
ZW
ZW
ZZ
Fish, Birds, Moths
__________________________________________________
Drosophila
Humans
Table 4.1
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Mechanisms of sex determination
differ between species
Heterogametic
H
t
ti sex – gender
d
with two different kinds of
gametes (e.g.
(e g XY males in
humans, ZW females in
birds)
Homogametic sex – gender
with one type of gamete
(e.g. XX females in
humans, ZZ males in birds)
I some species,
In
i
gender
d is
i
determined by environment
(e g temperature)
(e.g.
E d off P
End
Partt 1
• You should have learned….
• What experiments proving the chromosome
theory
y of heredity.
y
• What are the genetic basis for sex
determination in humans and other
organisms.
Table 4.2
20
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Genetics 2011
The cell cycle is a repeating pattern
of cell growth and division
Genetics 2011
The cell cycle: An alternation
between interphase and mitosis
• N
Nuclear
l
division
di i i d
during
i mitosis
it i apportions
ti
chromosomes in equal fashion to two genetically
identical daughter cells
• Cell cycle – repeating pattern of cell growth and
division
• Interphase – period of cell cycle between
divisions/cells grow and replicate chromosomes
• G1 – gap phase – birth of cell to onset of
chromosome replication/cell growth
• S – synthesis phase – duplication of DNA
• G2 – g
gap
pp
phase – end of chromosome replication
p
to
onset of mitosis
21
Most of cell growth
occurs during G1 and G2
phases
Some terminally differentiated
cells stop dividing and arrest
in G0 stage
Chromosomes replicate to
form sister chromatids during
S phase
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Chromosome replication during S
phase of cell cycle
G1 phase –
chromosomes are not
duplicating or dividing
ƒ Length of time varies
in different cell types
S phase – duplication of
chromosome into
ssister
s e cchromatids
o a ds
Fig. 4.7 b
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Fig. 4.7a
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G2 phase – synthesis of
proteins required for
23
mitosis
I t
Interphase
h
• Within nucleus
• G1, S, and G2 p
phase – cell g
growth, p
protein
synthesis, chromosome replication
• Outs
Outside
de o
of nucleus
uc eus
• Formation of microtubules radiating out
into cytoplasm crucial for interphase
processes
• Centrosome – organizing
g
g center for
microtubules located near nuclear envelope
• Centrioles – pair of small darkly stained bodies
at center of centrosome in animals (not found in
24
plants)
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Mitosis has five stages that have
distinct cytological characteristics
St
Stages
off mitosis:
it i prophase
h
The five stages of mitosis and their major events
• Prophase - chromosomes condense and become visible
• Prometaphase – spindle forms and sister chromatids
attach to microtubules from opposite centrosomes
• Metaphase – chromosome align at the cell's equator
• Anaphase – sister chromatids separate and move to
opposite poles
• Telophase – chromosomes decondense and are
enclosed in two nuclei
Chromosomes condense and become visible
Centrosomes move apart
p toward opposite
pp
p
poles
Nucleoli begin to disappear
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Figure 4.8a
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St
Stages
off mitosis:
it i prometaphase
t h
St
Stages
off mitosis:
it i metaphase
t h
Nuclear envelope breaks down
Microtubules from centrosomes invade the nucleus and
connect to kinetochores in centromere of each chromatid
• Sister chromatids attach to microtubules from opposite
poles
• Mitotic spindle forms from three kinds of microtubules
(astral, kinetochore, and polar)
Chromosomes align
Ch
li on th
the metaphase
t h
plate
l t with
ith
sister chromatids facing opposite poles
Forces pushing and pulling chromosomes to or
from each pole are in balanced equilibrium
Figure 4.8c
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Figure 4.8b
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St
Stages
off mitosis:
it i anaphase
h
St
Stages
off mitosis:
it i ttelophase
l h
Rewind
R
i d off prophase
h
Nuclear envelope forms around each group of
chromatids
Centromeres of all chromosomes divide
simultaneously
y
Kinetochore microtubules shorten and pull
separated sister chromatids to opposite poles
(characteristic V shape)
• Nucleoli re-form
• Spindle fibers disperse
• Chromosomes decondense
Figure 4.8d
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Figure 4.8e
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Cytokinesis is the final stage of cell
division
Begins during anaphase but not completed until
after telophase
Parent cells split into two daughter cells with
identical nuclei
Cytokinesis: The cytoplasm divides
and produces two daughter cells
Animals have contractile ring
that contracts to form
cleavage furrow
Plants have cell plate that
forms near equator of cell
Organelles (e.g. ribosomes,
mitochondria, Golgi bodies)
are distributed to each
daughter cell
Fig. 4.9
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Figure 4.8f
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Cytokinesis does not always occur
after mitosis
Checkpoints
help regulate
cell cycle
• In fertilized
p
eggs,
gg , 13
Drosophila
rounds of mitosis
occur without
cytokinesis
• At each checkpoint,
prior events must be
completed before
the next step of the
cycle can begin
• Results in syncytial
embryo with
thousands of nuclei
within a single cell
•
Fig. 4.10
Details of cell
cell-cycle
cycle regulation
and checkpoint controls are
described in Chapter 17
34
33
Fig. 4.11
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Meiosis
Two general types of cells
in plants and animals
Genetics 2011
Chromosomes replicate once.
Nuclei divide twice.
Somatic cells make up vast majority of
cells in the organism
• In G0 or are actively going through mitosis
Germ cells are precursors to gametes
• S
Sett aside
id ffrom somatic
ti cells
ll d
during
i embryogenesis
b
i
• Become incorporated into reproductive organs
• Only cells that undergo meiosis produce haploid
gametes
35
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Fig. 4.12
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36
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The first three substages of prophase I:
Leptotene, zygotene, and pachytene
Overview of meiosis I
• Homologs pair
pair, exchange parts
parts, and then segregate
• Homologs pair and are held together by synaptonemal
complex
• Crossing-over (recombination) occurs during prophase I
• Sister chromatids remain intact throughout meiosis I
• Maternal and paternal homologs recombine and create
new combinations of alleles
• After recombination, homologs segregate to different
daughter cells
• Five substages to prophase I – leptotene, zygotene,
pachytene diplotene,
pachytene,
diplotene and diakinesis
• Depending on the species, length of time in prophase I
can be short or very long
38
37
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Crossing over during prophase
produces recombined chromosomes.
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How crossing over produces
recombined gametes
39
Fig. 4.14
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Figure 4.13
40
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The last two substages of prophase :
Diplotene and diakinesis
In metaphase I and anaphase I,
homologs move to opposite poles
Synaptonemal complex dissolves and chromatids
in each tetrad become visible
Note
N
t that
th t the
th centromeres
t
do
d nott divide
di id and
d sister
i t
chromatids are not separated
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Genetics 2011
M i i I is
Meiosis
i a reductional
d ti
l division
di i i
Genetics 2011
During
D
ring meiosis II,
II sister chromatids
separate
p
and move to opposite
pp
poles
p
43
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Feature Fig. 4.13
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M i i II is
Meiosis
i an equational
ti
l division
di i i
Feature Fig.
Fig 4.13
4 13
45
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Meiosis contributes
t genetic
to
ti diversity
di
it
in two ways.
• Independent assortment
of nonhomologous
chromosomes creates
different combinations
of alleles among
chromosomes.
• Crossing-over between
homologous
chromosomes
h
creates
t
different combinations
of alleles within each
chromosome.
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Mistakes in meiosis produce defective
gametes
H b id sterility
Hybrid
t ilit
Nondisjunction – mistakes in chromosome
segregation
g g
during
g meiosis I or II
• May result in inviable gametes or embryos
• Can also result in abnormal chromosome numbers in
viable individuals (e.g. trisomy 21, Down syndrome; or
XXY Klinefelter syndrome)
XXY,
• Hibrid animals carry
nonhomologous
chromosomes,
which can not pair
up!
y father
• Mule : donkey
and horse mother
Many hybrids between species (i
(i.e.
e donkey x
horse Æ mule) are sterile because
chromosomes cannot pair properly
48
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Gametogenesis in sexually
reproducing animals
E d off P
End
Partt 2
Germ line
G
li – specialized
i li d di
diploid
l id cells
ll sett aside
id d
during
i
embryogenesis
• You should have learned…
• The steps and mechanisms of mitosis
• The steps and mechanisms of meiosis
• Meiosis contributes to genetic diversity in
two ways
• Hybrid sterility
Gametogenesis – the formation of gametes
• IInvolves
l
meiosis
i i as wellll as specialized
i li d events
t before
b f
and after meiosis
• Different
ff
types off animals have variations on general
aspects of this process
• In humans, oogenesis produces one ovum from each
primary oocyte
49
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• In humans, spermatogenesis produces four sperm from
50
each primary spermatocyte
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Oogenesis in humans
Gametogenesis involved mitosis and
meiosis.
One ovum and 2 – 3
nonfunctional polar
bodies produced by
asymmetrical meiosis
• Oogenesis – egg formation in humans
• Diploid germ cells called oogonia multiply
by mitosis to produce primary oocytes.
• Primary oocytes undergo meiosis I to
produce one secondary oocyte and one
small first p
polar body
y ((which arrests
development).
• Secondary
y oocyte
y undergoes
g
meiosis II to
produce one ovum and one small second
polar body.
Long meiotic arrest may
contribute to
chromosome
segregation errors
(e g trisomies)
(e.g.
51
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Fig 4.18
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52
Genetics 2011
O
Oogenesis
i in
i human
h
Genetics 2011
N di j ti in
Nondisjuction
i human
h
• Asymmetric division:
• polar body (5% cytosol); primary oocyte (95%)
• Trisomy
• Discontinue division:
• Usually lethal
• Fetal stage (~6 month):
• 500,000 primary oocyte were produced
• Arrested in diplotene of meiosis I
• Down syndrome
y
• Trisomy 21
• Puberty
• Release 1 primary oocyte per cycle (~480/life)
• Complete meiosis I to metaphase of meiosis II
• Klinefelter syndrome
• Trisomy X
• Fertilization
• After sperm penetrating,
penetrating the oocyte completes meiosis II
quickly
• Sperm nucleus and oocyte nucleus fused
• Meiotic segregational errors: depend on age
• Amniocentesis
A i
t i
• Exam amniocytes
53
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Spermatogenesis in humans
Spermatogonia – diploid germ cells found only in
testis
• Divide by mitosis throughout lifespan of individual
After birth,
birth meiosis begins and spermatogonia
become primary spermatocytes
• Primary spermatocytes undergo symmetrical division at
meiosis I to produce two secondary spermatoctyes
• Secondary spermatocytes undergo symmetrical division
at meiosis II to produce two spermatids
p
mature to become sperm
p
• Spermatids
• Equal numbers of X and Y sperm are produced
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Spermatogenesis in humans
Four haploid sperm
produced by
symmetrical
meiosis of each
spermatocyte
Mitosis and meiosis
occur throughout
adult
life
d lt lif
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The chromosome theory of inheritance
How the chromosome theory
y of inheritance
explains Mendel's law of segregation
Walter Sutton – 1903,, chromosomes carryy
Mendel's units of heredity
1. Two copies
p
of each kind of chromosome
2. Chromosome complement is unchanged during
g y
transmission to progeny
3. Homologous chromosomes separate to different
gametes
4. Maternal and paternal chromosomes move to opposite
poles
5. Fertilization of eggs by random encounter with sperm
6. In all cells derived from fertilized egg, half of
chromosomes
h
are maternall and
dh
half
lf are paternall
57
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Validation of the chromosome theory
How the
chromosome
h
theory of
inheritance
explains
Mendel's
Mendel
s law of
independent
assortment
Table 4.4b
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display
59 Hartwell et
al., 4th ed., Chapter 4
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Prior to 1910, the chromosome theory of inheritance
was supported by two circumstantial lines of
evidence
1. Sex determination associates with inheritance of
particular chromosomes
p
2. Events in mitosis, meiosis, and gametogenesis ensure
constant numbers of chromosomes in somatic cells
This theory confirmed and validated by:
1. Inheritance of genes and chromosomes correspond in
every detail
2 Transmission
2.
T
i i off particular
ti l chromosome
h
coincides
i id with
ith
transmission of traits other than for sex determination 60
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N
Nomenclature
l t
for
f Drosophila
D
hil genetics
ti
Examples of notations for
Drosophila
G
Gene
symbol
b l identified
id tifi d b
by abnormal
b
l phenotype
h
t
• Gene symbol is chosen arbitrarily
• e.g.,
e g Cy is curly winged
winged, v is vermilion eyed,
eyed
etc.
• Cy,
Cy Sb
Sb, D are dominant (upper case letter)
letter).
• vg, y, e, are recessive (lower case letter).
• vg+ - wild-type
wild type recessive allele for vestigial gene
locus
• Cy+ - wild-type dominant allele for curly gene
locus
Wild-type
Wild
type allele denoted with superscript +
Recessive mutant allele denoted with lowercase
ƒ e.g. gene symbol
b l ffor white
hit gene is
i w
ƒ wild-type allele (w+) specifies brick-red eyes
ƒ mutant allele (w) specifies white eyes
Dominant mutant allele denoted with upper case
ƒ e.g. gene symbol for bar eyes is Bar
ƒ wild-type allele (Bar+) specifies normal eye
ƒ mutant allele (Bar) specifies abnormal eyes
61
62
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The Drosophila white gene is located on
the X chromosome
• See cross D – daughters inherit the phenotype
of their fathers, sons inherit the phenotype of
their mothers
T. H. Morgan (1910) discovered a white-eyed
Drosophila mutant and did a series of crosses
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Fig. 4.19
Crisscross inheritance occurs with
X-linked recessive traits
63
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Fig. 4.19
64
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Rare mistakes in
meiosis helped
confirm the
chromosome
theory (cont)
Rare mistakes in meiosis helped
confirm the chromosome theory
C. Bridges found 1/2000
male
l progeny off white
hit
females have red eyes
Chromosome segregation
Ch
ti iin
an XXY female
The three sex chromosomes
pair and segregate in two
ways producing progeny
ways,
with unusual sex
chromosome complements
Hypothesized that red-eyed
males arise from
mistakes in chromosome
segregation
(
(nondisjunction)
di j
ti ) during
d i
meiosis in white-eyed
females
Fig. 4.20b
Fig. 4.20a
65
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X and Y linked traits in humans are
identified by pedigree analysis.
D lt i
Daltonian
• X-linked traits exhibit five characteristics seen in
pedigrees.
• Trait appears in more males than females.
• Mutation and trait never pass from father to
son.
• Affected male does pass X-linked mutation to
allll daughters,
d
ht
who
h are h
heterozygous
t
carriers.
i
• Trait often skips a generation.
• Trait only appears in successive generations iff
sister of an affected male is a carrier. If so,
one half of her sons will show trait
trait.
67
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(Top)) Vi
(T
View off th
the world
ld tto a
person with normal color vision
(Bottom) View of the world to a
person with
ith red-green
d
colorblindness
• E. B. Wilson – 1911, assigned
gene for
f this
thi trait
t it to
t the
th X
chromosome
• 8% in male; 0.44% in female
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68
Genetics 2011
Example of sex
sex-linked
linked recessive trait in
human pedigree – hemophilia
The Russian imperial family of
C
Czar
Ni
Nicholas
h l II
II.
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Fig. 4.22 a
Genetics 2011
Example of sex-linked dominant trait in
human pedigree – hypophosphatemia
• Affected father has affected daughter
• Affected mother has 50% affected
children
70
Fig. 4.22 b
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Autosomal Genes Can Also Affect
Phenotypic Differences Between Sexs
• Sex-limited traits
• Aff
Affectt a structure
t t
or process
found in only one sex
• Stuck mutant in Drosophia
• Sex-influenced traits
John Adams and John Quincy Adams
• Appear in both sexes but hormonal differences
may cause difference
diff
• Pattern baldness
71
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• Heterozygous:
H t
Male
M l bald;
b ld F
Female
l normall
• Homozygous: Male early bald; Female late!
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72
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E d off P
End
Partt 3
E d off the
End
th class
l
• You should be able to….
• You should have learned…
• Gametogenesis in human
• the elegant experiments of Morgan and his
student Bridges in demonstrating that
genes occur on chromosomes and account
for Mendelian patterns of inheritance
• sex-linked genes in humans
• D
Describe
ib experiments
i
t proving
i th
the chromosome
h
theory of heredity.
• Explain the genetic basis for sex determination
in humans and other organisms.
• Identify several sex-linked genes in humans
and explain
p
their segregation
g g
properties.
p p
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Lectured by Han-Jia Lin
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Lectured by Han-Jia Lin