Homozygous vs Heterozygous: Difference Explained for NEET
Biology · Principles of Inheritance and Variation · NEET
Homozygous means the two alleles for a gene are the SAME (like TT or tt). Heterozygous means the two alleles are DIFFERENT (like Tt). Every diploid organism has two alleles for each gene, one from each parent, so it is either homozygous or heterozygous for that gene. Memory hook: "Homo = same, Hetero = different" (homo means same, like homogeneous milk).
Homozygous genotypes (TT, tt) have two identical alleles and breed true; the heterozygous genotype (Tt) has two different alleles and is a carrier. Crossing two heterozygotes (Tt x Tt) gives the classic 1:2:1 genotype ratio and 3:1 phenotype ratio.
Your doubts, answered
Is Tt homozygous or heterozygous?
Tt is HETEROZYGOUS. The two alleles are different: one capital T (tall allele) and one small t (dwarf allele). Heterozygous always means the two letters are NOT the same. Only TT and tt (same two letters) are homozygous.
What is the difference between homozygous and heterozygous in simple words?
You have two alleles for each gene, one from your mother and one from your father. If both alleles are the same, you are homozygous (TT or tt). If the two alleles are different, you are heterozygous (Tt). That is the only difference: same alleles vs different alleles.
What is the difference between homozygous dominant and homozygous recessive?
Homozygous dominant means both alleles are the dominant (capital) form, like TT. Homozygous recessive means both alleles are the recessive (small) form, like tt. Both are homozygous because the two alleles match. TT shows the tall trait; tt shows the dwarf trait.
Can a heterozygous plant be true-breeding?
No. A true-breeding (pure line) plant must be homozygous (TT or tt) so it always gives offspring of the same type. A heterozygous plant (Tt) is NOT true-breeding, because when selfed it gives a mix of TT, Tt and tt offspring. This is a common NEET trap.
Why does a heterozygous Tt plant look tall and not medium?
In complete dominance, the dominant allele (T) fully masks the recessive allele (t). So Tt looks exactly the same as TT (both tall). The recessive dwarf trait only appears when there is NO dominant allele, that is only in tt. (Exception: in incomplete dominance, like the snapdragon, the heterozygote looks in-between.)
If two heterozygous plants (Tt x Tt) are crossed, how many offspring are homozygous?
The offspring come in the ratio 1 TT : 2 Tt : 1 tt. So 2 out of 4 (half) are homozygous (1 TT + 1 tt) and 2 out of 4 (half) are heterozygous (Tt). Remember: the F2 genotype ratio is 1:2:1, but the phenotype ratio is 3 tall : 1 dwarf.
⚠️ The NEET trap ✗ A true-breeding plant must be homozygous recessive (tt). ✓ A true-breeding plant is homozygous, but it can be homozygous dominant (TT) OR homozygous recessive (tt). Both breed true. 🧠 NEET 2016 asked this. "True breeding = near homozygous and produces offspring of its own kind" is correct; "always homozygous recessive" is the trap option. True-breeding tall (TT) exists too, so recessive-only is wrong.
Real NEET questions
NEET 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: Crossing true-breeding tall (TT) with true-breeding dwarf (tt) gives an all-tall Tt F1. Selfing Tt x Tt gives genotypes TT : Tt : tt in the ratio 1 : 2 : 1, that is tall homozygous : tall heterozygous : dwarf. Note the genotype ratio is 1:2:1 while the phenotype ratio is 3 tall : 1 dwarf.
NEET 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 generations, so it is essentially homozygous and gives offspring like itself. It can be homozygous dominant (TT) or homozygous recessive (tt), so option D (always recessive) is wrong.
NEET 2023
A heterozygous pea plant with violet flowers was crossed with a homozygous pea plant with white flowers. Violet is dominant over white. Which one represents the expected combinations among 40 progenies?
A · All 40 produced violet flowers
B · All 40 produced white flowers
C · 30 produced violet and 10 produced white flowers
D · 20 produced violet and 20 produced white flowers ✓
Solution: Heterozygous violet (Vv) crossed with homozygous white (vv) is a test cross. Vv x vv gives 1 Vv (violet) : 1 vv (white), a 1:1 ratio. Out of 40 progeny this is 20 violet and 20 white.
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.
Homozygous means both alleles for a gene are the same. Example: TT (both tall alleles) or tt (both dwarf alleles). A homozygous TT tall plant and a homozygous tt dwarf plant are both true-breeding.
What is heterozygous with an example?
Heterozygous means the two alleles are different. Example: Tt has one tall allele (T) and one dwarf allele (t). In complete dominance, Tt looks tall because T masks t, but it still carries the hidden dwarf allele.
Is TT homozygous or heterozygous?
TT is homozygous (homozygous dominant). Both alleles are the same capital-T dominant form. tt is also homozygous (homozygous recessive). Only Tt, with two different alleles, is heterozygous.
How do you write homozygous and heterozygous genotypes?
Use two letters for the two alleles. Same two letters = homozygous (TT, tt, AA, aa). Two different letters = heterozygous (Tt, Aa). Capital = dominant allele, small = recessive allele.
Does heterozygous mean the trait is in-between the two parents?
Only in incomplete dominance (like the pink snapdragon flower). In normal complete dominance, a heterozygote (Tt) looks exactly like the homozygous dominant (TT), not in-between, because the dominant allele fully hides the recessive one.
