This type of sex — determination pattern is common in birds. butterflies and moths. It was discovered by .1. Seiler in 19 14 in 1110111. It is the reverse of XY system. 1Therefore, it is called Vs.7. mechanism

(a)Female (ZW): Female is heterouametic XY. Female produces two kinds of eggs X and V in equal proportions.

(b)Male (ZZ): The male is homogametic XX. Each sperm is

carrying an X — chromosome. So all the sperms are similar. The kind of egg determines the sex of the offspring. When al. X — carrying egg is fertilized by the sperm, a male offspring is produced. When a V — carrying egg is fertilized by the sperm a female offspring is produced. Sex ratio is 1 .




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