NEET 2024 Botany Question 131 Solution

In genetics, a test cross is the cross of an organism showing a dominant phenotype (whose genotype may be homozygous or heterozygous) with an organism that is homozygous recessive for the same trait.

If the black seed plant is homozygous dominant ($BB$): $$BB \times bb \rightarrow 100\%\; Bb\;(\text{all black})$$ If the black seed plant is heterozygous ($Bb$): $$Bb \times bb \rightarrow 50\%\; Bb\;(\text{black}) : 50\%\; bb\;(\text{white})$$ Because the recessive parent donates only recessive alleles ($b$), the phenotype of the progeny directly reveals the alleles donated by the unknown dominant parent.

The genotype crossed with the black seeded plant is homozygous recessive ($bb$), which is Option (4).