| MadSci Network: Genetics |
Not all animals possess the "traditional" XY chromosomal sex determination
mechanisms. Mammals including humans, Drosophila and many other species have
an X-Y mechanism that determines sex at fertilization. The male is the
heterogametic sex, containing an X chromosome and a Y chromosome. The female
is the homogametic sex carrying two X chromosomes. However, the mechanism
that determines maleness may vary among these animals. For example, in
mammals it is the presence of the Y chromosome, but in Drosophila, maleness
is not determined by the presence of Y but by the ratio of X chromosomes
to autosomes.
Other animals like grasshoppers, crickets, roaches and some other insects
have an X-O sex detrmination mechanism.Like the X-Y system, the male is the
heterogametic sex because males have two types of gametes: X and O (absence
of sex chromosome). Females have two X chromosomes.
A third sex determination mechanism is Z-W found in birds, some fish and
some insects such as butterflies and moths.The female is the heterogametic
sex; males are ZZ while females are possess two different sex chromosomes,
ZW.
Finally, some animals exhibit haplo-diploidy. Some species of bees and ants
have no sex chromosomes at all, yet sex determination has a chromosomal
basis.Females develop from fertilized eggs and are diploid (have two copies of
each chromosome).Males develop from unfertilized eggs (parthenogenetically) and
are haploid (have only one copy of each chromosome).
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