Biology · Principles of Inheritance and Variation · NEET
A true-breeding (pure line) plant is a plant that always gives offspring of its own kind, generation after generation, for a chosen trait. This happens because it is homozygous (it carries two identical alleles, like TT or tt), so its gametes all carry the same allele. Memory hook: "Pure line = same babies every time." Mendel made his peas true-breeding by self-pollinating them for many generations first.
Both a homozygous dominant (TT) and a homozygous recessive (tt) plant are true-breeding: each gives offspring identical to itself on selfing. True-breeding means homozygous — not only recessive.
Your doubts, answered
What is a true-breeding plant in the simplest words?
It is a plant that gives offspring exactly like itself for a trait, every time, over many generations. If a true-breeding tall plant self-pollinates, all its offspring are tall. There are no surprises, because the plant is homozygous (two identical alleles). NCERT says a true-breeding line shows stable inheritance and expression of a trait for several generations.
Is a true-breeding plant homozygous or heterozygous?
It is homozygous. This is the key idea. It has two identical alleles for the trait (TT or tt), so every gamete it makes carries the same allele. That is why the offspring always look the same. A heterozygous plant (Tt) is NOT true-breeding, because it makes two kinds of gametes and gives mixed offspring.
Can a true-breeding plant be homozygous recessive?
Yes, but it does not have to be. A true-breeding plant only needs to be homozygous. It can be homozygous dominant (TT, tall) OR homozygous recessive (tt, dwarf). Both breed true. The 2016 NEET answer rejects the option 'always homozygous recessive' for exactly this reason. Remember: true-breeding = homozygous, not only recessive.
How did Mendel get true-breeding pea plants?
He self-pollinated the pea plants for several generations. Self-pollination slowly removes the mixed (heterozygous) forms and leaves only pure homozygous plants. After many rounds, a trait like tall stayed tall every time. Only then did Mendel start his crosses. He selected 14 such true-breeding varieties, forming 7 pairs of contrasting traits.
What is the difference between true-breeding and pure line?
There is no difference for NEET. 'True-breeding' and 'pure line' mean the same thing: a homozygous plant that gives offspring like itself over generations. NCERT uses 'true-breeding line' as one term. Do not treat them as two separate concepts in the exam.
Why did Mendel need true-breeding plants for his experiments?
He needed a clean, fixed starting point. If parents were mixed (heterozygous), the results would be messy and he could not track how traits pass down. True-breeding parents are 100% predictable, so any new pattern in the F1 and F2 offspring is caused by the cross, not by hidden variation. This is why the P (parent) generation is always true-breeding.
⚠️ The NEET trap ✗ A true-breeding plant is always homozygous recessive in its genetic makeup. ✓ A true-breeding plant is near-homozygous and produces offspring of its own kind. It can be homozygous dominant (TT) OR homozygous recessive (tt) — not only recessive. 🧠 True-breeding = HOMOZYGOUS (either TT or tt). The word 'always recessive' is the trap NTA plants — a true-breeding tall plant (TT) breeds true too.
Real NEET questions
2016
A true breeding plant is
A · One that is able to breed on its own
B · Produced due to cross-pollination among unrelated plants
C · Near homozygous and produces offspring of its own kind ✓
D · Always homozygous recessive in its genetic constitution
Solution: A true-breeding line, after continuous self-pollination, shows stable inheritance of a trait for several generations, i.e., it is essentially homozygous and gives offspring identical to itself. It need NOT be homozygous recessive — it can be homozygous dominant too — so option D is wrong. This makes C correct.
2020
How many true breeding pea plant varieties did Mendel select as pairs, which were similar except in one character with contrasting traits?
A · 14 ✓
B · 8
C · 4
D · 2
Solution: Mendel selected 14 true-breeding pea varieties. These formed 7 pairs of contrasting traits (such as round/wrinkled seeds, yellow/green seeds, tall/dwarf plants). 7 pairs × 2 = 14 varieties, so the answer is 14.
2016
A tall true breeding garden pea plant is crossed with a dwarf true breeding garden pea plant. When the F1 plants were selfed the resulting genotypes were in the ratio
Solution: True-breeding tall (TT) × true-breeding dwarf (tt) gives an all-tall Tt F1. Selfing Tt × Tt gives TT : Tt : tt in the ratio 1 : 2 : 1 = Tall homozygous : Tall heterozygous : Dwarf. Note: the phenotype ratio is 3 tall : 1 dwarf, but this question asks for genotypes, so A is correct.
Solved Principles of Inheritance and Variation NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Both terms mean a homozygous plant that gives offspring like itself over many generations. NEET uses them interchangeably.
Why is a true-breeding plant always homozygous?
Because it carries two identical alleles for the trait. All its gametes carry the same allele, so all offspring get the same combination and look identical to the parent.
Can a heterozygous plant be true-breeding?
No. A heterozygous plant (Tt) makes two kinds of gametes and gives mixed offspring on selfing (3 tall : 1 dwarf), so it does not breed true.
How many true-breeding pea varieties did Mendel use?
14 varieties, arranged as 7 pairs of contrasting traits. This is a frequently asked NEET fact (asked in 2020).
What generation is true-breeding in a cross?
The P (parent) generation is true-breeding. Mendel started every cross with pure, homozygous parents to keep results predictable.
