Key points
Alleles, dominance, Mendel’s ratios
Mendel’s garden ratios are the grammar. Human traits rarely stay that clean — say so in the same answer.
- Allele: a version of a gene. Homozygous: same allele both sides; heterozygous: different.
- Dominant allele shows in AA and Aa; recessive shows only in aa.
- Monohybrid F2 teaching ratio: 3:1 phenotype, 1:2:1 genotype (classic yellow/green pea).
- Dihybrid F2: 9:3:3:1 when two genes assort independently (yellow-round × green-wrinkled teaching cross).
| Trait (teaching) | Dominant | Recessive |
|---|---|---|
| Earlobe | Free / hanging | Attached |
| PTC tasting | Taster | Non-taster |
| Cleft chin | Present | Absent |
Limits
When Mendel is not enough
- Codominance: both alleles show (ABO: IA and IB together give AB; IO is recessive).
- Incomplete dominance: heterozygote is intermediate.
- Polygenic / multifactorial: height, skin colour, many diseases — many loci plus environment.
- Linkage: genes on one chromosome travel together; recombination frequency < 0.5 flags linkage; complete linkage RF = 0.
Examples
- Achondroplasia is often taught as a dominant dwarfism phenotype — do not treat every ‘simple’ human trait as a pea gene.
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