Heterogameity or heterogamy

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Heterogameity or heterogamy Sexual
dimorphism in chromosome shape and or number so
that two different gametes are produced.
Heterogameity Designation Male
(2N - 1) female 2N
X0/XX Male (2N - 1 Y) female 2N
XY/XX Female (2N - 1 Z) male 2N
WZ/WW Note Y denotes a morphologically
distinguishable chromosome found only in males.
Z denotes one found only in females. X and
W chromosomes may not be distinguishable from
other chromosomes. This is an old
nomenclature. No functional equivalence among
Y, X, W, or Z chromosomes in different
species is necessarily implied.
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• Some general properties of heterogametic
  chromosome
• pairs
• Usually a size difference (Y often smaller)
• Limited synapsis at MI (often end-to-end rather
• than lateral).
• Often a difference in the amount or distribution
  of
• heterochromatin (very dense chromatin).
• Often chromsome-specific genes on X but not Y,
• Y but not X, etc.

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Some common misconceptions about
heterogameity Heterogametic chromosome pairs
are sometimes called sex determination systems.
This is jargon that has made its way into
textbooks. Sometimes one of the sex
chromo- somes does indeed determine sex, but
sometimes sex can be genetically determined by
other factors. In some cases of heterogameity
the sexual dimorphism in chromosome shape or
size is so small as to be undetectable by
simple inspection under the light microscope.
This does not necessarily mean that
genetic differences between the sexes do not
exist, merely that we cannot see them.
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Drosophila X/O is male.
Human X/O is female (Turner syndrome)
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Notice that the Y is much smaller than the X.
This is the usual situation, but it is not
always the case.
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Actual male-determining gene
PAR pseudoautosomal regions. NRY
nonrecombining regions.
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Is sex (gender phenotype) necessarily genetically
determined? In some organisms, temperature or
other environmental cues determine gender.
These curves are from different reptiles.
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Environmentally-influenced gender in a fish
species
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Sex-limited traits
Sex-influenced traits Autosomal, but expressed
Autosomal, but
sex only in one sex. Example
influences phenotype. cock-feathered
phenotype Example
male pattern in chickens. Both sexes can
baldness. Heterozygote be
hen-feathered, but only
Bb is bald in men but not males can be
cock-feathered. women.
BB is bald in
both
sexes, bb is not bald in either
sex.
p 72 - 73 in text
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