Titzaveh: Sanctification

How do you make a person holy? This week's parsha gives the instructions.  

You dress him in special clothes. Clothes make the man.

You put a sign in his head: HOLY to GD

He wears golden bells on the hem of his tunic

He bears a breastplate of justice and prophecy

You designate places that only the holy one can go, on holy occasions

There he does holy things: with animal parts and blood and oil and bread

You smear him with oil and sprinkle blood on him 

And you write it all down  so that it is remembered forever.
And you make it hereditary.  That is the wild card.  How do you know that Cohen generations down the road will be holy? Does the nature of holiness change over time?

The story of  Shemayah and Abtalion. informs the point
Yoma 71b


 Our 
Rabbis taught: It happened with a high priest 
that as he came forth from the Sanctuary, all 
the people2 followed him, but when they saw 
Shemayah3 and Abtalion, they forsook him 
and went after Shemayah and Abtalion. 
Eventually Shemayah and Abtalion visited 
him, to take their leave of the high priest. He 
said to them: May the descendants of the 
heathen come in peace!4 — 

They answered him: May the descendants of 
the heathen, who do the work of Aaron, 
arrive in peace, but the descendant of Aaron, 
who does not do the work of Aaron, he shall 
not come in peace!5 

It is noteworthy that Hillel is introduced in Pirkei Avoth as  the student of  Shemayah and Abtalion.

הלל ושמאי קבלו מהם הלל אומר הוי מתלמידיו של אהרן אוהב שלום ורודף שלום אוהב את הבריות ומקרבן לתורה:

Translation:

Hillel and Shammai received from them. Hillel would say: Be of the disciples of Aaron--a lover of peace, a pursuer of peace, one who loves the creatures and draws them close to Torah

The heredity of the holiness is not genetic, it is an educational process and a home environment, like the holiness of Israel.