Sex Determination

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Chapter 7 Sex Determination
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Sex determination in Chlamydomonas
2 () 2 (-)
Fig. 7.1
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Few Terms
Monoecious (hermaphroditic)- Produce both male
and female reproductive organs. Dioecious-
Produce only male or only female reproductive
organs.
Sex determination in Corn (Zea mays)

• Monoecious
• Male parts stamen and pollen
• Female parts pistil

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Sex determination in Caenorhabditis elegans
XX (homogametic sex) Has testes and ovaries (XA
1)
X- (heterogametic sex) Has testes only (XA 0.5)

• 2 types of sperm
• X-containing
• X-lacking

Fig. 7.4b
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Sex determination in humans
female karyotype
male karyotype
XX female (homogametic sex) XY male
(heterogametic sex)
Fig. 7.6
An aside In some organisms the female is the
heterogametic sex (ZZ male, ZW female).
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What determines maleness and femaleness in humans?
2 possibilities
1. Two X chromosomes are needed to make a
female. If you have one X chromosome then you
default to male. No, not the mechanism.
2. The Y chromosome is needed to make a male.
Lack of the Y chromosome and you default to
female. Yes, this is the mechanism.
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Klinefelter Syndrome (47,XXY)

• 1 in 500 males
• Have male genitalia
• Decreased libido
• Underdeveloped testes
• Sparse facial body hair

• Sterile (few exceptions)
• Slight breast development
• Usually tall and skinny
• Learning-disabled (slight)

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Turner Syndrome (45,X)

• 1 in 3,000 females
• Female external genitalia
• Underdeveloped ovaries
• Sterile
• Short stature (avg. 47)
• Webbed neck

• Broad, shield-like chest
• Wide-spaced nipples
• Low-set ears
• Low posterior hairline
• Learning-disabled (slight)

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20
21
22
X
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47, XXX Syndrome
47, XYY Syndrome

• 1 in 1,000 males
• Phenotypically normal.
• Above average height.
• Lower I.Q.
• Usually fertile.
• Previously thought to show tendency toward
  violence but not proven.

• 1 in 1,200 females
• Phenotypic expression variable.
• Tendency for increased height and lower I.Q.
• Sterility may result.

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Nondisjunction
a
b
a
b
a
a
b
b
heterozygous disomy gametes
nulliomy gamete
b
a
a
b
a
a
b
b
homozygous disomy gamete
Fig. 8.1
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X-chromosome
Y-chromosome
Duchenne Muscular Dystrophy locus
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More Detailed Y Chromosome Map
SRY (testes determining factor)
Gadgetry (MAC-locus)
Channeling flipping (FLP locus)
Catching and throwing (BLZ-1 locus)
Self-confidence (BLZ-2) (note-unlinked to ability)
Ability to remember and tell jokes (GOT-1 locus)
Sports page (BUD-1)
Addiction to death and destruction movies (T-2
locus)
Air guitar (RIF)
Ability to identify aircraft (DC10)
Preadolescent fascination with Arachnida and
reptilia (MOM-4U)
Spitting (P2E)
Sitting on john reading (SIT locus)
Leave toilet seat up locus
Inability to express affection over the phone
(ME-2)
Selective hearing loss (HUH?)
Total lack of recall for important dates (OOPS)
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More Detailed X Chromosome Map
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Dosage Compensation
To compensate for females having 2 X chromosomes
vs. males having only 1 X chromosome could do one
of two things.

• Double the amount of transcription of
  X-chromosome genes in males.

OR

• Inactivate one of the X-chromosomes in females.

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Dosage Compensation (cont.)
46, XX 1 Barr body 45, X 0 Barr body 47, XXY
1 Barr body 47, XXX 2 Barr bodies 48, XXXX 3
Barr bodies
Barr bodies N-1 (N X cs. present)
Fig. 7.10

• Mechanism
• XIC (X-inactivation center)
• XIST (X-inactive specific transcript)

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X-Inactivation (Lyon hypothesis)
ectodermal dysplasia

• Which X chromosome is inactived?
• Is X-inactivation permanent?
• Is the same X chromosome inactivated in all cells?

calico cat
mosaic
Fig. 7.12
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Sex determination in Drosophila
sterile female, usually die
XXY normal female XO sterile male
normal female
both male and female morphology
sterile male
normal male
sterile male
Fig. 7.13
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It is the ratio of X chromosomes to the haploid
set of autosomes that determines sex in
Drosophila.
XA 1.0 normal female (2X2A) (3X3A) XA
0.5 normal male (1XY2A) XA 0.5 sterile
male (1X2A) XA 1.5 metafemale (3X2A) XA lt
0.5 metamale (1XY3A) XA gt 0.5 and lt 1.0
intersex (2X3A)
Dosage compensation in Drosophila- Males
transcribe X-linked genes at twice the level
found in females.
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Sex Determination in Some Reptiles
high temp male
high temp female
both high and low temp female