Ayesha M. Khan
Spring 2013
The mechanism of sex determination controls
the inheritance of sex-linked characteristics.
 Most genes on the X chromosome are
different from genes on the Y chromosome.
 Males and females do not possess the same
number of alleles at sex-linked loci. This
difference in the number of sex-linked alleles
produces the distinct patterns of inheritance
in males and females.

In sexual reproduction, parents contribute
genes to produce an offspring that is
genetically distinct from both parents. In
eukaryotes, sexual reproduction consists of
meiosis, which produces haploid gametes,
and fertilization, which produces a diploid
zygote.
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We define the sex of an individual in terms of
the individual’s phenotype—ultimately, the
type of gametes that it produces.
The cells of female humans normally have
two X chromosomes, and the cells of males
have one X chromosome and one Y
chromosome.
◦ A few rare examples: Genetically female, having
two X chromosomes but male phenotype.
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Dioecious-organism has either male or female
reproductive structures.
Monoecious-organisms that bear both male
and female reproductive structures.
Hermaphroditism-both sexes are present in
the same individual.
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Hermann Henking-1891: Peculiar structure
termed the X body found in nuclei of male insect
cells.
Clarence
E.
McClung-1901:
Accessory
chromosome, which eventually became known as
the X chromosome.
Nettie Stevens and Edmund Wilson-1905: In
grasshoppers and other insects, the cells of
females have two X chromosomes, whereas the
cells of males have a single X.
◦ Two X chromosomes were found in female cells, whereas
a single X chromosome plus a smaller chromosome,
which they called Y, was found in male cells
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Stevens and Wilson also showed
that the X and Y chromosomes
separate into different cells in
sperm formation; half of the
sperm receive an X chromosome
and half receive a Y. All egg
cells produced by the female in
meiosis
receive
one
X
chromosome.
A sperm containing a Y
chromosome unites with an Xbearing egg to produce an XY
male,
whereas
a
sperm
containing an X chromosome
unites with an X-bearing egg to
produce an XX female.
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XX-XO system:
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XX-XY system:
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• XX – female
• XO – male
• grasshoppers
• XX – female
• XY – male
• Mammals
ZZ-ZW system:
• ZW- female
• ZZ-male
• birds, moths, some
amphibians, and
some fishes.
Heterogametic sex: two different
types of gametes with respect to
the sex chromosomes.
Homogametic sex: gametes that
are all the same with respect to the
sex chromosomes.
The X and Y chromosomes are
homologous only at pseudoautosomal
regions.
Chromosomal Sex-Determination
Systems
Haplodiploidy system:
No sex chromosomes; sex is based on the number of
chromosome sets found in the nucleus of each cell.
Males develop from unfertilized eggs,
and females develop from fertilized eggs.
•Haploid set – male
•Diploid set – female
•Bees, wasps, and ants
Genic Sex-Determining System
No sex chromosomes, only the sex-determining
genes.
Genotypes at one or more loci determine the sex
of an individual
No obvious differences in the chromosomes of
males and females.
In both genic sex determination and chromosomal sex
determination, sex is controlled by individual genes; the
difference is that, with chromosomal sex determination, the
chromosomes that carry those genes appear different in males
and females.
Environmental Sex Determination
Sex is determined fully or in part by environmental
factors.
Marine mollusk Crepidula fornicata (slipper limpet)
-sequential hermaphroditism each individual animal can be both
male and female, although not at the same time.
-In limpets sex is determined environmentally by the limpet’s
position in the stack.
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Although most snakes and lizards have sex
chromosomes, in many turtles, crocodiles,
and alligators, temperature during embryonic
development determines sexual phenotype.