Tu madre es alta. "Sí, la es" vs. "Sí, lo es"
Do they both work here because madre is feminine? Seems like "lo es" means "it/he/she" is regardless of gender. But could gender come into play and use la? Gracias amigos.
9 Answers
Lo - Pronombre objeto neutro - Spanish Neuter Direct Object Pronoun
Lo is the Spanish neuter direct object pronoun, used to mean "it" when referring to something non-specific or with no gender, such as an idea, adjective, situation, or clause.
Note that lo is also the masculine direct object pronoun, which refers to something or someone specific.
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Compare:
Debiera ir a la tienda, pero lo olvidé.
I was supposed to go to the store, but I forgot [to go].
vs
Juan me dio una pluma, pero la olvidé en el autobús.
Juan gave me a pen, but I forgot it [the pen] on the bus.
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No parece feliz, pero lo es.
She doesn't seem happy, but she is [happy].
vs
La felicidad es muy importante. Búscala todos los días.
Happiness is very important. Seek it [happiness] every day.
.
Como es rico, lo tiene fácil.
Since he's rich, he has it easy.
vs
Me gusta la torta de chocolate. Es fácil hacerla.
I like chocolate cake. It's easy to make it.
Lo es refers to the fact that she is tall. It means "it she is." The fact that she is a woman does not affect the word lo because lo refers to an abstract concept.
Another example would be if someone said to me, "Eres americana?" I could say "lo soy." I would not say la soy, even though I am a female. 
Sí, lo es .... NO other possibility Jeezz 
Thank you, Guillermo. Why "lo" and not "la"? - Marianne-
I don't really know the technicalities, but you always use ""lo" in these cases. It does not matter the gender of the person or the adjective you are referring to.
You would use lo.
Man: Eres una mandona. (You are a bossy woman.)
Woman: No lo soy. (I am not.)
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Tu madre es alta.
Sí, lo es.
It doesn't matter whether the subject is feminine or not. When saying that someone "is that thing" which you called them, you always use LO. The "lo" refers to "it" that the subject was called and not the subject itself.
¡Sí, lo es!
I usually think of "lo" as "thing" and not "it".
"Yes, she is that thing that you just mentioned (her "tallness")
"thing" or "tallness" is neither masculine nor feminine.
"Ella es bonita. Sí, lo es"
She is pretty. Yes, she is that!
"That" is neither masculine nor feminine.
"Eres americana?" Si, lo soy."
Are you American (female) ? Yes, I am "what-you-just-said"
Does this make sense?
Wow. This really confuses me.
It's "Sí, lo es" because we are talking about the height and not the woman?
I would think that the woman was the subject and we would say "Sí, LA es (alta)."
After school I will look all through this thread. Lots of great replies so far guys. I appreciate all your help, and I agree with Sanlee. If "la es" never works to refer back to a feminine then there are some parts of Spanish that are still beyond my understanding.
I am totally confused. If "Ella es bonita. Sí, la es" is not correct according to Benz and Guillermo, (see comments above) , then I understand even less than I thought about these kind of sentences. I need help.
