Homozygous and heterozygous
Two alleles that match, or two that differ. Those are the only two possibilities, and each has a name you will meet in every question from here on.
Homozygous
The two alleles are the same. Also called pure for that characteristic, because it breeds true.
TT, tt
Heterozygous
The two alleles are different. Also called hybrid for that characteristic.
Tt
Why the difference matters
It does not change what the plant looks like. It changes what the plant can pass on.
| Pair of alleles | Called | Gametes it can make |
|---|---|---|
| TT | Homozygous dominant | All carry T — one kind only |
| tt | Homozygous recessive | All carry t — one kind only |
| Tt | Heterozygous | Half carry T, half carry t — two kinds |
Only a heterozygote can surprise you
A homozygous parent has only one kind of gamete to offer, so its contribution is fixed. A heterozygous parent offers two, and that is where every ratio in this chapter comes from. Cross two homozygotes and the offspring are uniform; cross two heterozygotes and you get 3 : 1.
Same pair or different pair
Homozygous — both alleles the same, one kind of gamete, breeds true. Heterozygous — the two alleles different, two kinds of gamete, can produce offspring unlike itself.