According to the principal of dominance, if a recessive gene for tallness is paired with another recessive gene for tallness, the organism
a.) will be tall.
b.) may be tall or short.
c.) will be short

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The correct answer for the blank is-

A.) will be tall.

According to Mendel's law of dominance, when true breeding parents for a contrasting characteristic (such as tall plant and dwarf plant height) are allowed to breed, the trait that is dominant, appears in the next generation.

Dominant trait (depicted by capital letter allele) masks the expression of a recessive trait (depicted by small letter allele).

Recessive trait only appears when the genotype is homozygous recessive (that is having both recessive alleles).

In this question, recessive gene for tallness is paired with another recessive gene. This form a homozygous recessive genotype.

Thus, the organism will be tall.