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.

step-by-step explanation:

given perimeter: 28cm

length: 9cm

atp,

2(l+w)=p of the reactangle

2(9+x)=28

18+2x=28

2x=28-18

x=10/2

x=5

therefore width is 5 cm