A cross made between two pea plants produces $50\%$ tall and $50\%$ short pea plants. The gene combination of the parental pea plants must be

In pea plants, tallness ($T$) is a dominant trait and shortness ($t$) is a recessive trait. A plant with a short phenotype must have the homozygous recessive genotype ($tt$). To produce $50\%$ tall ($Tt$) and $50\%$ short ($tt$) plants, we perform a cross between a heterozygous tall plant ($Tt$) and a homozygous short plant ($tt$). Let's analyze the options:

• Option A ($Tt \times Tt$): This cross yields a phenotypic ratio of $3:1$ ($75\%$ tall and $25\%$ short).
• Option B ($TT \times Tt$): This cross yields $100\%$ tall plants ($50\%$ $TT$ and $50\%$ $Tt$).
• Option C ($Tt \times tt$): The gametes from $Tt$ are $T$ and $t$, and from $tt$ is only $t$. The offspring are $50\%$ $Tt$ (tall) and $50\%$ $tt$ (short). This matches the given condition.
• Option D ($TT \times tt$): This cross yields $100\%$ tall plants ($Tt$).