In which cross is an organism showing a dominant phenotype (genotype unknown) crossed with the recessive parent?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Test cross
A plant with a dominant phenotype may be homozygous (TT) or heterozygous (Tt); its looks do not tell which. To find out, Mendel crossed it with the recessive parent (tt). If all the offspring show the dominant trait, the plant is TT; if half of them show the recessive trait, the plant is Tt. This cross is called a test cross.
Option (1) Test cross — Correct. In a test cross the organism with the dominant phenotype and unknown genotype is crossed with the recessive parent, instead of being self-crossed. The offspring reveal its genotype.
Option (2) Out crossing — Incorrect. Out-crossing is a term from animal breeding: mating animals of the same breed that have no common ancestors for several generations. It is not used to find out a genotype.
Option (3) Back cross — Incorrect. A back cross is a cross of the F1 hybrid with either of its parents, dominant or recessive. The cross in the question, made with the recessive parent to reveal an unknown genotype, has its own name: the test cross.
NCERT Extract:
Inheritance of One Gene – “… Therefore, to determine the genotype of a tall plant at F2, Mendel crossed the tall plant from F2 with a dwarf plant. This he called a test cross. In a typical test cross an organism (pea plants here) showing a dominant phenotype (and whose genotype is to be determined) is crossed with the recessive parent instead of self-crossing. The progenies of such a cross can easily be analysed to predict the genotype of the test organism.”