Yes, ZYGOTE is a Scrabble word!

Letter values in Scrabble US: Z 10, Y 4, G 2, O 1, T 1, E 1.

Is it a valid word?

ZYGOTE is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

(genetics) the diploid cell resulting from the union of a haploid spermatozoon and ovum (including the organism that develops from that cell)

Also: fertilized ovum.