1. Chapter 04 Lecture Outline
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3. What is true about sexual reproduction?
Each parent contributes a haploid gene set to offspring.
Sexual reproduction consists of mitosis only.
Sexual reproduction involves the union of diploid gametes.
Sexual reproduction produces a haploid zygote.
All of the above.
Location: 4.1 Sex Is Determined by a Number of Different Mechanisms
Concept Check 1

4. 4.1 Mechanisms of Sex Determination Among Various Species
Different mechanisms of sex determination
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5. Sex determination and sex chromosomes
Sex determination is the mechanism by which an individual develops into a female or a male
In many species, the process relies on sex chromosomes that are different in males and females
for example, X and Y
Other mechanisms exist as well
Environmental (temperature)
Behavioral interactions

6. X-Y sex determination Found in mammals, including humans
44 + XY
X-Y sex determination
Found in mammals, including humans
Male is X-Y – the heterogametic sex
Two kinds of sperm are produced
Either X or Y plus 22 autosomes
Female is X-X – the homogametic sex
All eggs are the same, with X
X plus 22 autosomes
The sperm determines the sex of the zygote
44 + XX +

7. A human with an XY chromosome pair appears female
A human with an XY chromosome pair appears female. What might explain this person’s condition?
This person suffers from Turner syndrome.
This person suffers from Klinefelter syndrome.
This person has a mutated Sry gene.
This person has an extra copy of the Sry gene.
The XY determination was an error because it is impossible for a human XY individual to be female.
Location: 4.1 Sex Is Determined by a Number of Different Mechanisms
Concept Check 3

8. X-Y sex determination The Y chromosome promotes male development
How do we know that two Xs do not directly promote female development?
Because XXY individuals are male
and X0 individuals are female
A single gene on the Y chromosome is responsible for male development – the Sry gene
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9. X-0 sex determination Found in many insects, such as grasshoppers
Male is X-0 (has just one X chromosome)
Some insects have XY males (ex: Drosophila)
Female is X-X
In this system (even with XY males), it is the ratio of X chromosomes to autosomes that determines sex
1 X / 2n autosomes is male
2 X / 2n autosomes is female
22 + X
22 + XX

10. Z-W sex determination Found in birds and some fish
76 + ZZ
Found in birds and some fish
Male is Z-Z – the homogametic sex
Female is Z-W – the heterogametic sex
76 + ZW
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11. Haplodiploid sex determination
Male honeybee (Drone)
Haploid – 16 chromosomes
Female honeybee
Diploid – 32 chromosomes
© Scott Camazine/Photo Researchers
© Photo by Rob Flynn/USDA
Sex is determined by the number of sets of chromosomes
Found in bees, wasps, and ants
Example: The honeybee
The male (a drone) is produced from unfertilized eggs that are haploid – one set of 16 chromosomes
The females (workers and the queen) grow from fertilized eggs that are diploid – two chromosome sets, 32 total
So only the female arises through sexual reproduction

12. Temperature-dependent sex determination
Found in some reptiles and some fish
Example: the American alligator
Eggs incubated at 33oC grow into males
If eggs are incubated a few degrees below or above 33oC, they grow into females
Male and female alligators have the same chromosome composition

13. Behavioral sex determination
Behavioral interactions can determine sex
Example: Clownfish (Amphiprion)
Protandrous hermaphrodites – switch from male to female
Many fish live in each anemone – one female, one male, and many small juveniles
When the large female dies, the male switches sex to take over the female role, and one juvenile becomes a male
Male and female clownfish have the same chromosome composition

14. Dioecious Plants
Most plants produce both male and female gametes from the same individual – called monoecious
But some plants are dioecious – each individual plant is either male or female
Examples: Holly, willow, and ginkgo
Example: White campion, Silene latifolia
Males are XY, females are XX
But for other species, cannot recognize different sex chromosomes, so mechanism is mysterious
© Rolf Nussbaumer Photography/Alamy
(a) American holly (I. opaca)
Female
Male
© Arco Images GmbH/Alamy
(b) Female and male flowers on separate individuals in white campion
(S. latifolia)

15. 4.2 Dosage Compensation and X-Chromosome Inactivation in Mammals
How dosage compensation is achieved in different animal species
X-chromosome inactivation in mammals
How X-chromosome inactivation may affect the phenotype of female mammals
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16. Dosage compensation Important genes lie on the X chromosome
Many of these encode proteins that interact with proteins encoded by autosomal genes
How do both females and males keep the levels of X chromosome genes in balance with the levels of autosomal genes?
Dosage compensation – the mechanism that keeps levels of expression of X chromosome genes in balance with those of autosomal genes for both sexes

17. Dosage compensation mechanisms vary for different species
Some - increased expression in heterogametics (XY)
Example: Drosophila males increase X expression
Others - decreased expression in homogametics (XX)
Example: C. elegans females lower X expression
X-Chromosome inactivation (XCI) is another way to do this, used by mammals

19. X chromosome inactivation
Mary Lyon, 1961
Proposed that female mammals inactivate one of their X chromosomes in each somatic cell
The X is chosen at random in different cells early in development, then maintained
Known as the Lyon Hypothesis
Evidence:
Barr and Bertram saw condensed structure in somatic cell nuclei – the Barr body
Female mammals often exhibit variegation
Such as calico cats
© Courtesy of I.Solovei, University of Munich (LMU).
Barr
body
Active
X-chromosome

20. Mouse with
patches of
black and
white fur
Further
development
Random X chromosome
inactivation
Barr bodies
White fur
allele
Black fur
Early embryo—
all X chromosomes
active b B

21. X-chromosome inactivation
Human individuals that are not 46 (XY) or 46 (XX) still have only one active X chromosome
Klinefelter syndrome – XXY; one Barr body
Turner syndrome – X0; no Barr body
Triple X syndrome – XXX; two Barr bodies
What is the mechanism to “count” the number of X chromosomes in cells?

22. X-inactivation center and Xist
Mechanism of counting relies on a region of the X-chromosome called the X-inactivation center (Xic)
Contains the X-inactive specific transcript (Xist) gene
Xist is active on the condensed chromosome in Barr body
Three phases for inactivation
Initiation – X-chromosome is selected for inactivation
Spreading
Xist expressed on chromosome to be inactivated
Xist transcripts coat chromosome
Proteins recruited to chromosome to condense it
Maintenance through mitosis and beyond

23. Initiation: Occurs during embryonic development.
The number of X-inactivation centers (Xics) is
counted and one of the X chromosomes remains
active and the other is targeted for inactivation.
To be
inactivated
Xic
Xic
Spreading: Occurs during embryonic
development. It begins at the Xic and
progresses toward both ends until the
entire chromosome is inactivated. The
Xist gene encodes an RNA that coats the
X chromosome and recruits proteins that
promote its compaction into a Barr body.
Xic
Xic
Further
spreading
Barr
body
Maintenance: Occurs from embryonic
development through adult life. The
inactivated X chromosome is maintained
as such during subsequent cell divisions.

24. 4.3 Properties of the X and Y Chromosomes in Mammals
Features of the X and Y chromosomes in mammals
How pseudoautosomal inheritance occurs
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25. Properties of mammalian X and Y
Some genes are unique to X or Y – sex-linked genes
Those on X only are called X-linked
Those on Y only are called Y-linked or holandric
Inheritance patterns different than autosomal genes
Pseudoautosomal genes are found on both X and Y
Inheritance patterns similar to autosomal genes
Some regions of X and Y without genes share homology
Help in pairing X and Y chromosomes during meiosis I
Mic2 gene X Y

26. 4.4 Transmission Patterns for X-Linked Genes
Morgan’s experiment localizing an eye color gene to the X chromosome
Crosses involving X-linked genes
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27. Inheritance patterns of X-linked genes
Inheritance patterns of X-linked genes differ from autosomal genes
Called X-linked inheritance
Fathers transmit the X only to daughters, and sons receive their X only from their mothers
A male is said to be hemizygous for X-linked genes
As opposed to homozygous or heterozygous
Since there is only one copy
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28. Morgan’s Experiments with Flies
The Chromosome Theory of Inheritance was confirmed by Thomas Hunt Morgan in the early 1900s
He confirmed that a particular gene was localized to a chromosome
Morgan induced eye color mutations in Drosophila melanogaster
Obtained a white-eyed fly (rather than normal red)
Called the new mutation white
He studied its inheritance pattern after breeding with red-eyed flies
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29. Experimental level
F1 generation
From
F2 generation
Conceptual level x
1. Cross the white-eyed male to a
true-breeding red-eyed female.
2. Record the results of the F1 generation.
This involves noting the eye color and
sexes of many offspring.
3. Cross F1 offspring with each other to
obtain F2 offspring. Also record the
eye color and sex of the F2 offspring.
4. In a separate experiment, perform a
testcross between a white-eyed male
and a red-eyed female from the F1
generation. Record the results.
XwY x Xw+Xw+
Xw+Y x Xw+Xw
Xw+Y male offspring and Xw+Xw female
offspring, both with red eyes
1 Xw+Y : 1 XwY : 1 Xw+Xw+ : Xw+Xw
1 red-eyed male : 1 white-eyed male :
2 red-eyed females
XwY x Xw+Xw
1 Xw+Y : 1 XwY : 1 Xw+Xw : XwXw
1 red-eyed female : 1 white-eyed female

30. The Data Cross Results Test Cross Results
Original white eyed-male to red-eyed females
F1 generation: All red-eyed flies
F1 male to F1 females
F2 generation: ,459 red-eyed females
1,011 red-eyed males
0 white eyed-females
782 white-eyed males
Test Cross
Results
White-eyed males to F1 females
Testcross: red-eyed females
132 red-eyed males
88 white eyed-females
86 white-eyed males
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31. Interpreting the Data
White eyed male crossed to red eyed female produced all red-eyed flies in F1
Conclude that red eye is dominant
If gene is autosomal can predict that in F2 (from cross of the F1 generation flies) should get
75% males with red and 25% with white eyes
75% females with red and 25% with white eyes
3:1 ratio overall of red-eyed to white-eyed flies
BUT – he got zero white-eyed females!
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32. All white-eyed flies should be male All females should be red-eyed
As with autosomal genes, a Punnett square can be used to predict the outcome of a mating if a gene is sex-linked
If the gene is X-linked expect different ratios than if it was autosomal
All white-eyed flies should be male
All females should be red-eyed
3:1 ratio of red-eyed to white-eyed flies
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F1 male is Xw+Y
F1 female is Xw+Xw
Male gametes
Xw+ Y
Xw+Xw+
Xw+Y
Xw+
Red, female
Red, male
Female gametes
Xw+Xw
XwY Xw
Red, female
White, male

33. Decreased survival of white-eyed flies
However
The experimental ratio of red eyes to white eyes in the F2 generation is (2, ,011):782 or about a 4.4:1 ratio instead of 3:1 ratio
How can the lower-than-expected number of white-eyed flies be explained?
Decreased survival of white-eyed flies
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34. Sex linked traits can be confirmed by test crosses
Testcross – a phenotypically dominant individual is mated with recessive individual
Male gametes
Xw+ Y Xw
Red, female
Red, male
White, female
White, male
Xw+Xw
Xw+Y
XwXw
XwY
Testcross:
Male is XwY
F1 female is Xw+Xw
Female gametes
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35. Note in Morgan’s experiment:
The observed data of the testcross are 129:132:88:86
This ~ 1.5:1.5:1:1 ratio deviates from the predicted 1:1:1:1 ratio
Again, lower-than-expected number of white-eyed flies can be explained by a lower survival rate for those flies
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36. Pedigrees Can Identify X-linked Genes
Example: Duchenne muscular dystrophy (DMD)
Seen in both dogs and humans
Gene for DMD is on the X chromosome
Encodes dystrophin protein, needed for muscle structure
Damages heart and breathing muscles
Survival is rare beyond the early 30s
X-linked recessive pattern
Disease is rare in females; they may be carriers
Carrier mothers have ~50% affected sons
Refer to figure 4.10
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37. Pedigree of family with DMD
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Unaffected,
presumed heterozygote
Affected
with DMD
I-1
I-2
II-1
II-2
II-3
II-4
II-5
II-6
III-1
III-2
III-3
III-4
III-5
III-6
III-7
III-8
IV-1
IV-2
IV-3
IV-4
IV-5
IV-6
IV-7

38. Reciprocal Crosses
Reciprocal cross – two crosses that differ in which sex carries the trait
Example: Duchenne muscular dystrophy (DMD)
Reciprocal crosses demonstrate that it is X-linked in dogs as well
X-linked genes behave differently in reciprocal crosses
Refer to figure 4.11
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39. Figure 4.11 Reciprocal cross XDXD XdY XdXd XDY Sperm Sperm Xd Y XD Y
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Reciprocal cross
XDXD
XdY
XdXd
XDY
Sperm
Sperm Xd Y XD Y
XdY
(affected
with muscular
dystrophy)
XDXd
(unaffected,
carrier)
XDY
(unaffected)
XDXd
(unaffected,
carrier) XD Xd
Egg
XDXd
(unaffected,
carrier)
XDY
(unaffected)
XdY
(affected
with muscular
dystrophy)
XDXd
(unaffected,
carrier) XD Xd
© AP Images.
Male golden retriever with
X-linked muscular dystrophy
(b) Examples of X-linked muscular
dystrophy inheritance patterns
Figure 4.11