Transcript SexDetermination

Extra credit problem for
Lecture #6
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please use the index cards
provided.
• Print your name and
section # at top of card.
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How is sex
determined in
Drosophila
melanogaster?
Sex Determination in
Drosophila and other
organisms
X chromosome: Autosome
Balance
In Drosophila, sex is determined by the ratio of
the number of X chromosomes to the number of
sets of autosomes
Other kinds of sex determination
•
•
•
•
•
Grasshoppers, crickets, roaches: XO
Bees, ants, wasps: haplodiploidy
Snakes: ZZ/ZW
Lizards: both XX/XY and ZZ/ZW systems
Crocodiles, most turtles, and some lizards, sex is
determined by the incubation temperature during
a critical period of embryo development. This is
known as temperature-dependent sex
determination (TSD).
Some lizards?
Turtles
Lizards,
Turtles, Crocodilians
In some organisms, sex is not
‘determined’ at all!
Some fish change sex as they
get older or larger. In clown
fish, largest group member is
female, second largest is male,
and all others are nonbreeding.
Sometimes the largest fish in the
group will become male, all
others will be female (wrasses).
Coral Goby
Gobiodon histrio
Monogamous pairs
If mate dies or
leaves, resident
will court next fish
to come by, one
will change sex to
accommodate.
And some species are all female!
Asexual Cnemidophorus species are
all female; reproduce via
parthenogenesis.
Diploid or triploid eggs develop
directly from oocytes via premeiotic endomitosis, no
cellular/nuclear division.
At meiosis, identical rather than
homologous chromosomes pair,
and then segregate.
All offspring are females genetically
identical to their mothers.
Cnemidophorus species tend to
have extremely low levels of
genetic diversity.
From A. J. Collum, Creighton Univ, http://biology.creighton.edu/faculty/cullum/CnemmyInfo.html
Genomic Imprinting
Differential expression of genetic traits depending on
whether the trait has been inherited from a mother
or a father.
Disease can result from imprinting
QuickTime™ and a
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QuickTime™ and a
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are needed to see this picture.
QuickTime™ and a
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are needed to see this picture.
Paternal SNRPN
gene active
(chrom. 15)
Short stature
Mild retardation
Poor muscle tone
Compulsive
eating
Maternal SNRPN
Paternal SNRPN
Gene inactive by
Gene inactive by
imprinting
mutation
Fertilization
Prader-Willi
Syndrome
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Both copies
inactive
Genetic Linkage and Mapping
Meiosis: Prophase I
Closer look
Notation
Aa
Diploid
Adult
Haploid gametes
(single chromatid)
A
— —————
•
— —————
•
a
A
__•_______
A
__•_______
a
__•_______
a
__•_______
Two homologous
chromosomes,
four chromatids total
1/2 A : 1/2 a
Dihybrid Cross
AA BB Female
A
B
— —————
P:
•
— —————
•
A
Gametes:
aa bb Male
—— —————
•
—— —————
•
B
a
— —————
•
— —————
•
a
Egg (A B)
A
—•—————
b
—— —————
•
—— —————
•
b
Sperm (a b)
B
——•————
a
—•—————
b
——•————
F1 adult
after the S phase
A
— —————
F1:
•
— —————
•
a
B
—— —————
•
—— —————
•
b
A
— —————
F1:
•
— —————
•
a
A
—•—————
A
4 gamete types,
equally frequent
—•—————
a
—•—————
a
—•—————
B
—— —————
•
—— —————
•
b
B
——•————
b
——•————
Parental
Recombinant
B
——•————
Recombinant
b
——•————
Parental
F2 of dihybrid cross when
genes not linked
• What proportion of the gametes of the
double heterozygote are recombinant?
50 %