https://www.learner.org/courses/biology/textbook/gender/gender_4.html
Day 353 has no protein-coding genes (browser view). The rest of Chromosome Y is constitutive heterochromatin – the longest such stretch in the genome.
Not all animals use an X-Y sex determining system. The Y chromosome is unique to mammals, and the ancestral relationship between X and Y can still be detected. As the two diverge, the Y continues to accumulate mutations that it can’t fix the way the X chromosome can.
Click here to see all 8175345 letters of Day 353.