Learn Italian: Mio nonno è altruista, e lo sei anche tu. - My grandfather is altruistic, and you are too.

Here, lo is a pro-predicative pronoun. It replaces the adjective altruista from the first half of the sentence to avoid repetition. While in English we simply say "you are too" and leave a blank space, Italian requires lo to represent that dropped adjective. You can think of it as meaning "you are it too."

Would lo change to la if the sentence were about my grandmother?

No. When the pro-predicative lo replaces an adjective or an indefinite noun, it is completely invariable. Even if the sentence were Mia nonna è altruista, you would still say e lo sei anche tu.

Since nonno is masculine, why does altruista end in -a?

Adjectives ending in -ista (like altruista, ottimista, pessimista) have the same form for both masculine and feminine in the singular. They do change in the plural, however, becoming -isti for masculine and -iste for feminine.

Why do we say mio nonno instead of il mio nonno?