With Alleles: Dominant and Recessive come two words that save a lot of explaining: one for the letters and one for the look.
Key idea
The genotype is the pair of alleles (TT, Tt or tt). The phenotype is the trait that shows (tall or short). Two alleles alike (TT or tt) is homozygous; two different (Tt) is heterozygous.
| Genotype | Name | Phenotype (T = tall dominant) |
|---|---|---|
| TT | homozygous dominant | tall |
| Tt | heterozygous | tall |
| tt | homozygous recessive | short |
Why it works
Because one dominant allele is enough to show the trait, TT and Tt look the same. The phenotype alone does not tell you which genotype a tall plant has. A short plant is different: it must be tt, since any T would make it tall. So the recessive phenotype always tells you the genotype.
Worked example: Name them
In pea plants, yellow seeds (Y) are dominant over green (y). Give the genotype name and phenotype of Yy and yy.
- Yy: two different alleles, heterozygous; one Y, so yellow.
- yy: two alike, homozygous recessive; green.
Answer: Yy: heterozygous, yellow. yy: homozygous recessive, green.
Worked example: Tall: which one?
A tall pea plant is crossed with a short one. Half the offspring are short. What was the tall plant's genotype?
- Short offspring are tt, so they got a t from each parent.
- The tall parent must have had a t to give: Tt.
Answer: Tt.
Common mistake
Students think TT and Tt must look different, or write a phenotype as letters ("the plant is Tt"). Letters are the genotype. Describe the phenotype in words (tall, purple, round). With complete dominance, TT and Tt have the same phenotype.