Why is the sex chromosomes of humans and most animals written as XY while for birds it is ZW
In: Biology
To differentiate them. The sex-determination works in opposite way: humans (and other mamalians) offspring sex is determined by the sperm that has either X or Y chromosome. Male chromosomes are XY and female XX. Male birds have ZZ chromosomes and females have ZW and the chromosomes in the egg determine the sex of the offspring.
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