我 是 生物学家。 我的 专业 是 生物学。

can use different grammer pattern 1,noun, biology+家 or biology+学家 ，means Biologists，others like 动物学家，物理学家，化学家，科学家，哲理家... the sentences could be: 我是生物学家 我是生物方面的专家（方面--aspect，专家--specialist） 2,adv+verb 专门研究，专攻，擅长+verb... the sentences could be" 我专门研究生物 我专攻生物 我擅长研究生物 also there is a lot of other expression can use for this sentences,but that use more common:)

For someone who specializes in a certain field or who is an expert, we have a suffix : 家, an equivalent to 【-ist】in English， as in 【scientist】. We have 科学家，企业家，思想家，政治家，文学家，书法家， and, curiously, 野心家 which is funny as it is derogative. " I am a scientist who specializes in biology" would be 我是生物学家， as biology is 生物学。 Well, some people are so modest they don't feel like calling themselfves XXX家, they might say 我是搞/做生物的。（I do biology for a living.）

I specialize in..... means 我专于.... or 在...方面专业， or 擅长于... I am a scientist who specializes in biology : 我是生物学方面的科学家

我是一個專攻生物學的科學家