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Commonly confused
Homozygous vs heterozygous
It comes down to one thing: are the two copies of a gene the same, or different?
Homozygous
Both copies are the same: TT (homozygous dominant) or tt (homozygous recessive). A homozygous organism breeds true for that trait.
Heterozygous
The two copies are different: Tt. One dominant, one recessive. The dominant one shows. The recessive one is hidden, but it can still be passed on.
The trap
Students try to tell these apart by looking, but TT and Tt plants are both tall, exactly the same to the eye. For a dominant trait, you cannot tell homozygous from heterozygous by looking. Only tt shows itself, because the recessive trait finally appears.
Quick check
An organism with genotype Tt is:
- AHomozygous dominant
- BHomozygous recessive
- CHeterozygous
- DPure-breeding