📖 Notes
LESSON 13 OF 16
Mendelian Genetics & Inheritance
From Pea Plants to Blood Groups
Mendel's two laws explain everything from monohybrid crosses to codominant blood types — the same logic just scales up as more genes get involved.
Mendel's Laws
Segregation & Independent Assortment
- Segregation: alleles separate during gamete formation
- Independent Assortment: genes on different chromosomes sort independently
Key Terms
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Homozygous
Two identical alleles — AA or aa.
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Heterozygous
Two different alleles — Aa.
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Dominant
Expressed even in a heterozygote (Aa).
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Recessive
Only expressed in a homozygous recessive (aa).
Cross Ratios
3:1
Monohybrid (Aa × Aa) phenotype ratio
1:2:1
Monohybrid genotype ratio (AA:Aa:aa)
9:3:3:1
Dihybrid phenotype ratio
Punnett Square — Monohybrid Aa × Aa
REFERENCEA a A AA Aa a Aa aa Ratio: 3 dominant : 1 recessive
Beyond Simple Dominance
- Incomplete dominance: red + white = pink (blended phenotype)
- Codominance: blood type AB — both alleles fully expressed
- Sex-linked: colour blindness — X-linked recessive
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Blood group genetics
IA and IB are both dominant over i, but codominant to each other — IAIA or IAi gives Type A, IBIB or IBi gives Type B, IAIB gives Type AB, and ii gives Type O.