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A tall pea plant could be TT or Tt, and looking at it will not tell you which. Mendel's answer was to cross it with a dwarf plant and read the offspring. That is a test cross. A back cross is a broader idea: crossing a hybrid with one of its parents. The two overlap, and questions often test exactly that overlap. This post defines both, reads Mendel's own back-cross counts, and works through how to decide a genotype from the results.
Where is this in the syllabus?
The NMC syllabus for NEET (UG) 2026 lists "Mendelian Inheritance" under heredity and variation. The NCERT Class 12 chapter Principles of Inheritance and Variation (reprint 2026-27) defines a test cross: an organism showing the dominant phenotype, whose genotype is to be found, "is crossed with the recessive parent instead of self-crossing". Its Figure 4.5 uses violet (V) and white (v) flowers. We searched the chapter text and the term "back cross" does not appear, so treat it as an extra term from reference books and question banks. Check the syllabus for your exam year.
What is the difference?
Test cross: unknown dominant-phenotype individual × homozygous recessive. Its purpose is to reveal the unknown genotype.
Back cross: an F1 hybrid × either of its parents, dominant or recessive.
A back cross to the recessive parent is also a test cross.
A back cross to the dominant parent is not a test cross, because it cannot reveal the genotype.
Why must the tester be recessive?
A tt plant gives only t gametes, so every offspring's phenotype shows the allele that came from the unknown parent.
TT × tt gives all Tt: all tall.
Tt × tt gives 1 Tt : 1 tt: half tall, half dwarf.
Now try a dominant tester. TT × TT and Tt × TT both give all tall offspring, so the two possibilities look identical.
What did Mendel's own back crosses show?
In his 1865 paper (English translation on MendelWeb), Mendel crossed round yellow (AB) with wrinkled green (ab) peas, then crossed the hybrids with both parents in four experiments. Crossing with the round yellow parent (experiments 1 and 3) gave only round yellow seeds: 98 and 94. Crossing with the wrinkled green parent gave four kinds:
Experiment 2: 31 round yellow, 26 round green, 27 wrinkled yellow, 26 wrinkled green.
Experiment 4: 24, 25, 22 and 27 of the same four kinds.
Experiment 2 has 110 seeds, so a 1:1:1:1 ratio predicts 27.5 of each. Every count is within 4 of that. Experiments 1 and 3 were back crosses to the dominant parent and showed only one kind of seed. Experiments 2 and 4 were back crosses to the recessive parent, in other words test crosses, and they revealed four kinds of gametes in about equal numbers. Mendel ran them both ways round: in his words, "The first and second experiments had primarily the object of proving the composition of the hybrid egg cells, while the third and fourth experiments were to decide that of the pollen cells."
Worked example: reading a test cross
Sample numbers for a hypothetical class exercise:
Unknown tall × dwarf gives 52 tall and 48 dwarf. That is close to 1:1, so the unknown is Tt.
Unknown tall × dwarf gives 10 tall and no dwarf. The unknown is probably TT. How sure can you be? If it were Tt, each offspring would have a 1/2 chance of being tall, so the chance of 10 tall in a row is (1/2)¹⁰ = 1/1024, about 0.001. More offspring make the conclusion stronger; they never make it certain.
Dihybrid: RrYy × rryy. The tester gives only ry gametes, so the four gamete types of RrYy show up directly: 1:1:1:1. If a dihybrid test cross gives 45 : 5 : 5 : 45 instead, the parental types dominate and the genes are probably linked, with 10 recombinants in 100, or 10 per cent. Our post on linkage and crossing over takes this further.
Which mistakes cost marks?
Calling every F1 × parent cross a test cross. Only the recessive-parent version is.
Expecting 3:1 from a test cross. 3:1 comes from selfing or intercrossing Tt × Tt.
Writing four gamete types for rryy. It makes only ry.
Expecting 9:3:3:1 from a dihybrid test cross. It gives 1:1:1:1 when genes assort independently.
Calling a genotype certain from a few offspring. State it as likely.
A practice routine
Draw the Punnett squares for VV × vv and Vv × vv from NCERT's Figure 4.5.
Write Mendel's four experiments as genotype crosses (for example AaBb × aabb) and predict each result before checking the counts above.
Label each cross as test cross, back cross, both or neither: Tt × tt, where tt is a parent of the Tt plant; Tt × TT, where TT is a parent; Tt × Tt; RrYy × rryy, where rryy is an unrelated tester.
Make up one sample result for each and say what genotype it points to.
For the general cross method, see our post on Mendelian genetics problems, and for the person behind the counts, read about Mendel's pea experiments.
When your cross is set up right but the ratio still comes out wrong, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
Today, explain aloud why a back cross to the dominant parent cannot tell TT from Tt.
