sex chromosome vs XXY

sex chromosome and XXY both are nouns.

Word	Noun	Adjective	Verb	Adverb
sex chromosome	Yes	No	No	No
XXY	Yes	No	No	No

As nouns, XXY is a hyponym of sex chromosome; that is, XXY is a word with a more specific, narrower meaning than sex chromosome:

sex chromosome: (genetics) a chromosome that determines the sex of an individual
XXY: (genetics) abnormal complement of sex hormones in a male resulting in Klinefelter's syndrome

Other hyponyms of sex chromosome include X chromosome, XX, XXX, XY, XYY, Y chromosome.

sex chromosome (noun)	XXY (noun)
(genetics) a chromosome that determines the sex of an individual	(genetics) abnormal complement of sex hormones in a male resulting in Klinefelter's syndrome