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CHAPTER 15
THE CHROMOSOMAL BASIS OF
INHERITANCE
History
• It was not until 1900 that biology finally caught up
with Gregor Mendel.
•Independently, Karl Correns, Erich von Tschermak, and
Hugo de Vries all found that Mendel had explained the
same results 35 years before.
• Around 1900, cytologists and geneticists and the
behavior of Mendel’s factors.
• Mendel’s hereditary factors are the __________
located on chromosomes.
• Around 1902, Walter Sutton, Theodor Boveri, and
others noted these parallels and a ____________
theory of inheritance began to take form.
Fig. 15.1
2. Morgan traced a gene to a specific
chromosome
• Thomas Hunt Morgan was the first to associate a specific
gene with a specific chromosome in the early 20th
century.
• Morgan’s model-Drosophila melanogaster, a fruit fly
- have three pairs of _______________and a pair of
______ chromosomes (XX in females, XY in males).
Red eyes
White eyes
Normal is called___________
Non-normal is called__________
Know these symbols
= _______
• Morgan’s experiments-
= ________
Red eye X White eye
F1
All Red eye
3:1 Red:white
F2
Conclude- classic _______________ genetics
But- all __________were white,
all females red
Morgan concluded that a
fly’s eye color was linked to
the ___________________.
Fig. 15.3
3. Linked genes tend to be inherited together
because they are located on the same chromosome
15.3
15.2
15.1
14
13.2
13.3
13.1
12
11
11.1
11.2
Chromosome- 1.5 x 108 base pairs containing about ________ genes
12
13.1
13.2
13.3
14
15
21
22
23.1
23.2
23.3
31.1
31.2
31.3
32
33.2 33.1
33.3
34
35.2
35.1
35.3
0.4% of a chromosome, containing 10 genes
• Genes located on the same chromosome,__________________,
tend to be inherited together because the chromosome is passed
along as a unit.
•Results of crosses with linked genes deviate from those expected
according to ____________________________.
Linked genes
• Morgan observed this
linkage in ___________
and ____________.
• Gray with wings is wild
type
Gray/wings X Black/wingless
P
F1
Gray/wings
X Black/wingless
_______ Equal numbers of each
Thus, body color and wing
size genes must be_________
_______
5:5:1:1
4. Independent assortment of chromosomes and crossing
over produce genetic recombinants
• A _____frequency of recombination is observed for
any two genes located on different (nonhomologous)
chromosomes.
• In contrast, _________genes, genes located on
the same_______________, tend to move together
through meiosis and ______________.
Morgan’s testcross for body color and wing shape did not
_____________to either independent assortment or
complete linkage.
• Independent assortment- expect 1:1:1:1 ratio
• Completely linked- expect 1:1:0:0
• But observe _______of the flies were recombinants
(5:5:1:1) , suggesting _______________linkage.
• The actual mechanism, crossing over during_____________,
results in the production of more types of gametes than one would
predict by _____________ rules alone.
Fig. 15.5a
These unexpected numbers (5:5:1:1) is the basis for
constructing a ________________________.
5. Geneticists can use recombination data to
map a chromosome’s genetic loci
• ________________an ordered list of the genetic loci
along a chromosome.
Sturtevant hypothesis- the ________________of recombinant
offspring reflects the distances between genes on a chromosome.
These genes are less likely to travel together
These genes are more likely to travel together
Sturtevant used _____________________________from fruit
fly crosses to map the relative position of genes along
chromosomes, a ___________________________.
•
Recombination frequencies (Rf) determine
the relative __________ of genes
•
Example: 3 fruit fly genes: body color (b),
wing size (vg), and eye color (cn).
•
Rf between cn and b is______
•
Rf between cn and vg is_______.
•
Rf between b and vg is _________
•
The only possible arrangement
of these three genes places the ___________
gene between the other two.
• Sturtevant expressed the distance between genes, the
recombination frequency, as______________.
• One map unit (sometimes called a_________________) is
equivalent to a _____ recombination frequency.
Fig. 15.6
• Some genes on a chromosome are so far apart that
a _____________between them is virtually certain.
• Rf reaches is its maximum value of ______
• genes act as if found on separate chromosomes and are
inherited independently.
• For example, seed color and flower color are far
enough apart that linkage is not observed.
A
a
A
A’
Meiosis
B
b
b
B
How do we map a gene if linkage is 50% ??
Answer- find another gene that does _______________
• Genes located far apart on a chromosome are mapped by adding
the ___________________________ between the distant genes
and ____________________genes.
Fig. 15.7
0 MU
22 ____or 22% ______
or 22 ___________
37 mu
48 MU
67 MU
Long
antennae
Short
antennae
Grey
body
Black
body
Normal
wings
Red
eyes
104 MU
Short
wings
Brown
eyes
• Map units indicate _____________distance and
order, not precise locations of genes.
• The frequency of crossing over is not actually uniform
over the length of a chromosome.
• A _____________________indicates the positions
of genes with respect to chromosomal features.
A stained chromosome
15.3
15.2
15.1
14
13.2
13.3
13.1
12
11
11.1
11.2
12
13.1
13.2
13.3
Gene A is located on this band
14
15
21
22
23.1
23.2
23.3
31.1
31.2
31.3
33.2
32
33.1
33.3
34
35.2
35.1
35.3
Gene B is located here
The distance
between A and B
can be_________
Both ________maps and physical maps are used to locate disease genes
6. The chromosomal basis of sex
Mammals use the X-Y system to determine sex
Other creatures may use other “systems”
Female XX
x
Male XY
•
•
XY or XX
Male Female
Each _____ has one X chromosome.
1/2 the _________have an X chromosome
1/2 sperm have a Y chromosome
Thus, each conception has about a __________
chance of producing a particular sex.
What determines sex in mammals??
• Answer- the ____ gene (sex determining region of
the Y chromosome)
• determines sex (male).
• located on the ____________________
• regulates many other “male” genes.
• If lack the SRY gene, __________are produced.
Female is the____________________
7. Sex-linked genes have unique patterns of
inheritance
• The X chromosomes is loaded with________.
• One defective gene spells trouble for male
• Why? Because male has only one copy of the gene
• In contrast, for a recessive sex-linked trait, a female will
likely have one ______________ gene and one normal one
• Heterozygous females will be_______________.
• Example- Duchenne muscular dystrophy affects one in 3,500
males born in the United States.
• Affected individuals rarely live past their early 20s.
• Absence of an X-linked gene called _____________
• The disease is characterized by a progressive weakening of the muscles
• Another example: Hemophilia is a __________________
trait defined by the absence of one or more clotting factors.
• prolonged bleeding because a firm clot forms slowly.
• Bleeding in muscles and joints can be painful and lead to
serious damage.
• Individuals can be treated with intravenous injections of the
missing protein.
• In female mammals only _____ X chromosome is
active.
• Therefore, males and females have the same effective
_______ (one copy ) of genes on the X chromosome.
• The inactivated X chromosome is a _______________.
• This inactivates most of its genes.
• reactivated in ovarian cells that produce ova.
• After Barr body formation, all descendent cells have
the same inactive X.
•If a female is _______________________for a sex-linked
trait, approximately half her cells will express one allele and the
other half will express the other allele.
Examples of X-linked diseases
• A _______________________mutation• A heterozygous woman will have patches of normal
skin and skin patches lacking sweat glands.
• _________________cats -orange and black pattern
• ½ cells express an orange allele while others have a
nonorange allele.
Fig. 15.10
How is the X chromosome inactivated?
• Answer- One X chromosome expresses the XIST
gene (X-inactive specific transcript) and XIST
RNA _______ the chromosome.
Inactivated X chromosome
Xist RNA
XIST RNA
• Xist is necessary and
________________ for X
inactivation (using 450kb YAC)
13
p
12
11
•Evidence: If insert Xist
transgene on autosome
results in an
______________autosome
12
13
14
q
21
24
mouse autosome
8. Alterations of chromosome number or
structure cause some genetic disorders
• ________________occurs when problems with the
meiotic spindle cause errors in daughter cells.
• This may occur if
tetrad chromosomes
do not separate
properly during
meiosis I.
• Alternatively, sister
chromatids may fail
to separate during
meiosis II.
Fig. 15.11
• Nondisjunction results in too many or to few
chromosomes termed ________________.
• some gametes receive two of the same type of
chromosome and another gamete receives no copy.
• ___________ cells - three copies of a particular
chromosome type and have 2n + 1 total chromosomes.
• _____________cells - one copy of a particular
chromosome type and have 2n - 1 chromosomes.
• Normally results in embryonic death, but some survive
Organisms with more than two ____________sets of chromosomes,
have undergone ________________e.g. 3n or 4n (rare in mammals)
• Polyploids are more nearly normal in phenotype than____________
Other chromosome problems
• A deletion - a chromosome
fragment lacking a
________________ is lost
during cell division.
• A _____________- a
fragment becomes attached
as an extra segment to a
sister chromatid.
Fig. 15.13c & d
• An inversion - a chromosomal
fragment ____________ to the
original chromosome but in the
reverse orientation.
• In_________________, a
chromosomal fragment joins a
nonhomologous chromosome.
•Some translocations are
___________others are not.
Results of chromosome errors
• Homozygous embryos with a large ___________ normally die
• A deletion in the X chromosome is ____________in males
• A _______________________________ can alter phenotype
because a gene’s expression is influenced by its location.
Example- Leukemia is due to a growth gene being placed
next to an active region resulting in cancer
• Approx. ________ of human embryos are aneuploid and die early
in development
Some are viable• Chromosome 15 trisomy- die at birth
• Chromsome 21 trisomy- Down’s syndrome
• Down syndrome- trisomy 21.
• One in 700 children born in the United States.
• result from nondisjunction during gamete production
• correlates with the age of the mother.
Fig. 15.14
Karyotype
Phenotype
Extra sex chromosomes are allowed
• _______________________________an XXY male, occurs once
in every 2000 live births.
• Male sex organs, but are__________.
• Feminine characteristics; normal intelligence.
• Males with an extra Y chromosome (XYY) tend to somewhat
_________ than average.
• Trisomy X (XXX), which occurs once in every 2000 live births,
produces _______________ females.
• Monosomy X or Turner’s syndrome (X0), which occurs once in
every 5000 births, produces phenotypic, but immature females.
• XYY and XXYY- males often found in ____________.
9. The phenotypic effects of some mammalian
genes depend on whether they were inherited
from the mother or the father________________
• A few genes (not most!!) are expressed only if derived
from the mother (or from the father)
• _____________________________
35.3
• A gene on ________ homologous chromosome is
silenced
• The ________________ status of a given gene depends
on whether the gene resides in a female or a male.
Examples of imprinted genes
15.3
15.2
15.1
14
• 1. Prader-Willi syndrome and Angelman syndrome are due a
deletion of a specific segment of ______________________.
13.2
13.3
13.1
12
11
11.1
11.2
12
13.1
13.2
13.3
14
• Prader-Willi syndrome - mental retardation, obesity, short
stature, and unusually small hands and feet.
15
21
22
23.1
23.2
23.3
31.1
31.2
31.3
33.2
• Inherit the abnormal chromosome from their father.
• Angelman syndrome -spontaneous laughter, jerky
movements, and other motor and mental symptoms.
• This is inherited from the mother.
• The difference between the disorders is due
to___________________________
2. ________________ syndrome, which leads to various
degrees of mental retardation, also appears to be subject to
genomic imprinting
35.2
32
33.1
33.3
34
35.1
10. Extranuclear genes exhibit a nonMendelian pattern of inheritance
• ________________genes are found on small circles
of DNA in mitochondria and ____________.
• These organelles________________ themselves.
• Their cytoplasmic genes do not display Mendelian
inheritance.
• They are not distributed to offspring during meiosis.
• A zygote inherits all its mitochondria only from the_______.
• Mitochondrial mutations may contribute to diabetes, heart
disease, and other diseases of aging.