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Chromosomes
Chapter 13
1
Chapter 13 Learning Objectives
13.1 Sex Linkage and the Chromosomal Theory of
Inheritance
– Describe sex-linked inheritance in fruit flies.
– Understand pedigree of sex-linked traits in humans
13.2 Sex Chromosomes and Sex Determination
– Describe the relationship between sex chromosomes and
sex determination.
– Explain the genetic consequences of dosage
compensation (Barr Bodies) in mammals.
13.5 Selected Human Disorders
– Describe the consequences of nondisjunction in humans
– Describe how genetic disorders are detected
2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Normal / Wild Type
Mutant Type
© Cabisco/Phototake
• T.H. Morgan – 1910
– Working with fruit fly, Drosophila melanogaster
– Discovered a mutant male fly with white eyes instead
of red
– Crossed the mutant male to a normal red-eyed
female
• All F1 progeny red eyed = dominant trait
3
1. Morgan crossed F1 females x F1 males
2. F2 generation contained red and whiteeyed flies
1. But all white-eyed flies were male
3. Testcross of a F1 female with a whiteeyed male showed the viability of whiteeyed females
4. Morgan concluded that the eye color
gene resides on the X chromosome
4
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Parental generation
male
Xr
Y
XR
Parental
generation
female
XR
F1 progeny all had red eyes
F1 generation
male
y
XR
F1 generation
female
Xr
F2 female progeny had red eyes, only males had white eyes
5
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Testcross
Parental generation
male
Xr
Y
XR
F1 generation
female
Xr
The testcross revealed that white-eyed females
are viable. Therefore eye color is linked to the
X chromosome and absent from the Y chromosome
6
Sex Chromosomes
• Sex determination in Drosophila is based on the
number of X chromosomes
– 2 X chromosomes = female
– 1 X and 1 Y chromosome = male
• Sex determination in humans is based on the
presence of a Y chromosome
– 2 X chromosomes = female
– Having a Y chromosome (XY) = male
7
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
chromosome
XXchromosome
Y chromosome
Y chromosome
© BioPhoto Associates/Photo Researchers, Inc.
35,000 ×
• Humans have 46 total chromosomes
– 22 pairs are autosomes
– 1 pair of sex chromosomes
– Y chromosome highly condensed
• Recessive alleles on male’s X have no active
counterpart on Y
– “Default” for humans is female
• Requires SRY gene on Y for “maleness”
8
Sex Linkage
• Certain genetic diseases affect males to a
greater degree than females
• X-linked recessive alleles
– Red-green color blindness
– Hemophilia
9
Hemophilia
• Disease that affects a single protein in a
cascade of proteins involved in the formation of
blood clots
• Form of hemophilia is caused by an X-linked
recessive allele
– Heterozygous females are asymptomatic carriers
• Allele for hemophilia was introduced into a
number of different European royal families by
Queen Victoria of England
10
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The Royal Hemophilia Pedigree
George III
Generation
Edward
Duke of Kent
Prince Albert
I
Frederick
Victoria
III
No hemophilia
III
Queen Victoria
King
Edward VII
II
Louis II
Grand Duke of Hesse
Alice
Duke of
Hesse
Alfred
Helena
Arthur
Leopold
Beatrice
No hemophilia
German
Royal
House
King
George V
Irene
Czar
Nicholas II
Czarina
Alexandra
Earl of
Athlone
Princess
Alice
?
IV
Duke of
Windsor
Waldemar
Earl of
Mountbatten
King
George VI
Queen
Elizabeth II
Margaret
Prince
Philip
VI
Prince
Charles
Anne
Andrew
Henry
Prince
Sigismond
Prussian
Royal
House
V
Princess
Diana
Prince
Henry
Edward
Anastasia
Russian
Royal
House
Alexis
?
Maurice
?
Viscount
Tremation
Leopold
Queen
Eugenie
?
?
Alfonso
Alfonso
King of
Spain
Jamie Juan
Gonzalo
?
KingJuan
Carlos
?
No evidence
of hemophilia
No evidence
of hemophilia
Spanish Royal House
British Royal House
VII
William
Henry
11
X-Linked Disease Pedigree
12
http://www.macalester.edu/academics/psychology/whathap/ubnrp/visionwebsite04/twotypes.html
Question 14
A women who is colorblind marries a man who has
normal color vision. Their sons will —
a. All be like the father because the trait is Xlinked
b. All be like the mother because the trait is Xlinked
c. All have normal color vision
Question 13
What chance will a daughter have of being color blind if
she has a normal mother and a color blind father?
a. 0%
b. 25%
c. 50%
d. 75%
e. 100%
Dosage compensation
• Ensures an equal expression of genes
from the sex chromosomes even though
females have 2 X chromosomes and
males have only 1
• In each female cell, 1 X chromosome is
inactivated and is highly condensed into a
Barr body
• Females heterozygous for genes on the X
chromosome are genetic mosaics
15
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Second gene causes patchy distribution of pigment:
white fur = no pigment, orange or black fur = pigment
2nd gene responsible for
depositing pigment on fur
Example of epistasis
• Calico cat
• X-chromosome
inactivation in
females
Allele for black
fur is in activated
Allele for orange
fur is in activated
X-chromosome
allele for
orange fur
X-chromosome
allele for
black fur
Inactivated X
chromosome
becomes barr body
Inactivated X
chromosome
becomes barr body
Nucleus
(top): © Kenneth Mason
Nucleus
16
Nondisjunction
• Failure of homologues or sister chromatids
to separate properly during meiosis
• Aneuploidy – gain or loss of a
chromosome
– Monosomy – loss
– Trisomy – gain
– In all but a few cases, do not survive
17
Nondisjunction
18
http://www.nature.com/scitable/content/aneuploids-can-be-produced-through-nondisjunction-in-4918
• Smallest autosomes can present as 3
copies and allow individual to survive
– 13, 15, 18, 21 and 22
– 13, 15, 18 – severe defects, die within a few
months
– 21 and 22 – can survive to adulthood
– Down Syndrome – trisomy 21
• May be a full, third 21st chromosome
• May be a translocation of a part of chromosome 21
• Mother’s age influences risk
19
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
6
2
9
8
7
13
14
19
20
4
3
15
21
10
16
5
11
17
22
© Colorado Genetics Laboratory, University of Colorado, Anschutz Medical Campus
X
12
18
Y
20
Nondisjunction of sex chromosomes
• Do not generally experience severe
developmental abnormalities
• Individuals have somewhat abnormal features,
but often reach maturity and in some cases may
be fertile
• XXX – triple-X females
• XXY – males (Klinefelter syndrome)
• XO – females (Turner syndrome)
• OY – nonviable zygotes
• XYY – males (Jacob syndrome)
21
Detection
• Pedigree analysis used to determine the
probability of genetic disorders in the
offspring
• Amniocentesis collects fetal cells from the
amniotic fluid for examination
• Chorionic villi sampling collects cells from
the placenta for examination
22
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Uterus
Amniotic
fluid
Hypodermic syringe
23
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cells from
the chorion
Ultrasound
device
Uterus
Suction
tube
Chorionic villi
24
Question 7
Nondisjunction can occur in either the 1st or 2nd meiotic
division when homologous chromosomes or sister
chromatids do not separate.
a. This is true
b. This is false
Question 12
Which of the following statements is false regarding
sex linkage traits?
a. Women can be carriers because they can be
heterozygous for the trait
b. Sex-linked traits are more common in men
c. Males inherit X-linked sex linkage traits from
their fathers
d. Males are never carriers since they only
receive one X chromosome
e. All are true
Question 17
Lisa was told that her baby has Down syndrome
because it had two copies of the same chromosome
from its dad. Which of the following would be most
likely to have caused the Down syndrome?
a. XYY father
b. Deletion
c. Amniocentesis
d. Karyotyping
e. Nondisjunction
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