1. When a F1 generation of a monohybrid cross is allowed self-fertilize, the expected phenotypic ratio is
(1) 2:1 (2) 3:1
(3) 4:1 (4) 3:4
Monohybrid Cross: Detailed Explanation
Parent Generation
Consider a monohybrid cross between two true-breeding parents:
Parent 1:
AA (homozygous dominant)Parent 2:
aa (homozygous recessive)F1 Generation
All offspring are heterozygous: Aa
Phenotype: all show the dominant trait.
F1 Self-Fertilization
Aa × Aa
Gametes: Each F1 produces
A and a in equal frequency (1:1)Punnett Square
A |
a |
|
A |
AA |
Aa |
a |
aA |
aa |
F2 Generation Ratios
- Genotypic ratio:
1 AA : 2 Aa : 1 aa - Phenotypic ratio:
AAandAashow dominant phenotype (3 parts)
aashows recessive phenotype (1 part)
3 dominant : 1 recessive → 3:1
Why Other Options Are Incorrect
- 2:1 – occurs in special cases like lethal alleles, not standard Mendelian crosses [web:11].
- 4:1 and 3:4 – do not match Mendelian segregation for one gene with complete dominance [web:13].
✅ Correct Answer: Option (2) 3:1
