Key Terms
Gene
The unit of heredity that encodes a trait. Each organism carries two copies per gene (one from each parent).
Allele
A specific version of a gene. Round vs.
Dominant
The allele that shows up in the phenotype when at least one copy is present. Abbreviated with a capital letter (R).
Recessive
The allele that only shows up when two copies are present; hidden by the dominant allele. Lowercase (r).
Phenotype
What you can observe — the physical trait.
Genotype
The actual allele combination underneath — the genetic code.
Homozygous
Two identical alleles (RR or rr).
Heterozygous
Two different alleles (Rr).
Homozygous dominant
RR — shows dominant phenotype. Heterozygous: Rr — shows dominant phenotype (same as RR outwardly).
Example
YYRR x yyrr (yellow-round x green-wrinkled) F1: all YyRr (yellow and round; both dominant traits show)
Genotype ratio
1 YY : 2 Yy : 1 yy Phenotype ratio: 3 yellow : 1 green
Problem
A plant shows the dominant phenotype. You can't tell from looking whether it's YY or Yy.
Solution
Cross it with a homozygous recessive individual (yy).
If unknown parent is YY
All offspring show dominant trait (Yy). If unknown parent is Yy: offspring are 1/2 dominant (Yy) and 1/2 recessive (yy).
Dominance hierarchy
C+ > cch > ch > c