UNIT 05 Sex Linkage, and Sex Determination

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UNIT 05Sex Linkage, and Sex Determination
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MECHANISMS OF SEX DETERMINATION XY method XO
method The ZW method Environmental Sex
Determination Systems
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Environmental Sex Determination
Turtles - Eggs incubated above 32 C develop into
females - Eggs incubated below 28 C develop into
males - Eggs incubated between 28 C and 32 are a
mix of sexes
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SEX DETERMINATION IN HUMANS
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A small portion of the Y chromosome contains a
gene SRY (sex-determining region Y). The
identification of the SRY region relies of
finding individuals who's phenotypic sex is at
odds with their chromosomal makeup (sex reversed
individuals). 1. Males who are XX Dr. David
Page (Whitehead Inst. For Biomedical Research)
found 20 XX males who had a small piece of the
short arm of chromosome Y attached to one of
their X chromosomes. 2. He also found 6 XY
females in whom the Y chromosome was missing this
small piece.
SRY
Y
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The SRY gene produces a DNA binding protein that
binds to two genes. -inhibits production of p450
aromatase which converts the male hormone,
testosterone, to the female hormone estradiol. -
enhances a gene which promotes testicle
development and represses female reproductive
structures.
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Structure of the Y-chromosome 1) The
Y-chromosome has few genes 2) There is a region
at either end that has homology to the X
chromosome and allows pairing with the
X-chromosome during cell divisions.
PARHomologous to X
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DOSAGE COMPENSATION AND X-CHRMOSOME
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Barr Bodies
A. Gene dosage varies between the sexes in
mammals B. X chromosomes are inactivated at
random early in development. An X is randomly
chosen in each cell for inactivation early in
development (in humans, day 16 post-fertilization)
. C. Barr bodies are darkly staining bodies in
the nuclei of females but not males 1. Barr
bodies are condensed X-chromosomes 2. All X
chromosomes except one become inactivated 3. XX
females have one Barr body. People with three X
chromosomes have 2 Barr bodies.
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Barr Bodies
FEMALE Cheek cells
MALE
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BARR BODIES
BARR BODIES REPRESENT INACTIVATED AND CONDENSED
X-CHROMOSOMES
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Barr Bodies
Determining chromosome numbers for X and Y
chromosomes 1) X-chromosome number of Barr
bodies 1 2) Y chromosome can be determined by
fluorescent staining of the nucleus. Y appears
as a brightly fluorescing spot.
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Barr Bodies
Females are mosaics. In some cells, one X
chromosome is expressed. In other cells, the
other X is expressed.
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Calico and Tortoiseshell cats
B
black
b
Brown or yellow
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Anhidrotic ectodermal dysplasia
Heterozygous female Areas where sweat glands
are Absent shown in blue
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ANEUPLOIDY INVOLVING X AND Y CHROMOSOMES
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Nondisjunction in meiosis involving the X
chromosome
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KLEINFELTERS SYNDROME XXY
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TURNERS SYNDROME
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TURNERS SYNDROME XO
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POLY X SYNDROME XXX, XXXX, XXXXX
BARR BODIES
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XYY syndrome
Thought at one time to be associated with
aggression and antisocial behavior because it was
first identified in an institution for the
criminally insane. Found in the normal
population Early learning disabilities.
Slightly reduced IQ scores (10 points). Possible
association with acne. Surveys of the possible
association of an extra Y with antisocial
behavior have not been done because of the moral
implications.
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SEX DETERMINATION IN DROSOPHILA
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XY determination in Drosophila
Autosomes non-sex chromosomes A
IIIIIIV AAIIIIIIIIIIIVIV
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In Drosophila, females are XX and males are
XY Unlike humans, the Y chromosome does not
determine maleness. It is an X-autosome balance
that determines sex
SUPER OR META FEMALE
XXX/AA 3/2 1.5 XX/AA 2/2 1.0 XX/AAA
2/3 0.67 XY/AA 1/2 0.5 XY/AAA 1/3 0.33
FEMALE
INTERSEX
MALE
SUPER OR META MALE
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SEX LINKAGE
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White eye in Drosophila is found on the
X-chromosome
Red eye is dominant to white eye
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X-linked inheritance of white eyes in Drosophila
Red-eyed female ? white-eyed male
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Queen Victoria and the royal family of England
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Fig. 11.13b Pedigree of Queen Victoria (III-2)
and her descendants, showing the X-linked
recessive inheritance of hemophilia
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Dominant faulty tooth enamel
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Pedigree showing the transmission of the X-linked
dominant trait of faulty tooth enamel
DY
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Y-linked or holandric genes
Hypertrichosis of the ear a probable Y-linked
gene
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Holandric Genes

• hgporcupine man (long rigid hairs over the
  body), Hgnormal

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SEX LIMITED AND SEX-INFLUENCED GENES
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Sex-influenced traits
PATTERN BALDNESS
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Sex-influenced inheritance of pattern baldness in
humans
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TAIL FEATHERING
SEX LIMITED TRAITS
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SEX LIMITED TRAITS
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END