What is represented by a pair of lowercase letters, such as tt, in a punnett square?

"tt" represents an allele pair for a recessive trait.

Step-by-step explanation:

A pair of alphabet in a punnet square is used to represent an allele pair (representing genotype for a given trait)

Usually a dominant trait is represented by  an alphabet in capital letter while a recessive form of the same trait is represented by the same alphabet in small letters.

For example

If "T" represents the dominant trait "being Tall"

then "t" will represent the recessive trait "being short" or " not being tall"

Hence, "tt" represents an allele pair for a recessive trait.

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step-by-step explanation:

fist on the left goes with the third on the right. second on the left goes with the fourth on the right. third on the left goes with the second on the right. the fourth on the left goes with first on the right