March 11,
2019 by Jenny Graves, The Conversation
The
highly unusual "semi-identical" Australian twins reported last week
are the result of a rare event. It's thought the brother and sister (who have
identical genes from their mother but not their father) developed from an egg
fertilised by two different sperm at the same moment.
In
humans, it's the sperm that determines whether an embryo is pushed along a male
or female development pathway. But in birds, it's the other way around. Eggs
are the deciding factor in bird sex.
There are
other fascinating aspects of bird sex that are not shared with humans. Female
birds seem to have some capacity to control the sex of their chicks. And occasionally
a bird that is female on one side and male on the other is produced – as in
recent reports of this cardinal
in the United States.
X and Y,
Z and W chromosomes
So what
is it about bird chromosomes that makes bird sex so different from human sex?
In
humans, cells in females have two copies of a large, gene-rich chromosome
called X. Male cells have one X, and a tiny Y chromosome.
Birds also
have sex chromosomes, but they act in completely the opposite way. Male birds
have two copies of a large, gene-rich chromosome called Z, and females have a
single Z and a W chromosome. The tiny W chromosome is all that is left of an
original Z, which degenerated over time, much like the human Y.
When
cells in the bird ovary undergo the special kind of division (called
"meiosis") that produces eggs with just one set of chromosomes, each
egg cell receives either a Z or a W.
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