Pocket Notes — Principles of Inheritance and Variation

Mendel's 7 pea characters (memorise pairs)

Character	Dominant	Recessive
Stem length	Tall	Dwarf
Flower colour	Violet	White
Flower position	Axial	Terminal
Pod shape	Inflated	Constricted
Pod colour	Green	Yellow
Seed shape	Round	Wrinkled
Seed colour	Yellow	Green

Ratios cheat sheet

• Monohybrid F₂ phenotypic: 3 : 1
• Monohybrid F₂ genotypic: 1 : 2 : 1
• Dihybrid F₂ phenotypic: 9 : 3 : 3 : 1
• Test cross (Tt × tt): 1 : 1
• Incomplete dominance F₂ (Antirrhinum): 1 red : 2 pink : 1 white
• ABO blood group — 4 phenotypes, 6 genotypes.

Mendel's laws

1. Dominance — heterozygote shows dominant.
2. Segregation — allele pair separates in gametes (purity of gametes).
3. Independent assortment — genes on different chromosomes sort independently.

Definitions cheat

• Alleles — versions of a gene.
• Homozygous vs heterozygous.
• Genotype (genes) vs phenotype (looks).
• Test cross — × with homozygous recessive.
• Back cross — F₁ × either parent.

Non-Mendelian patterns

• Incomplete dominance — heterozygote intermediate (snapdragon).
• Co-dominance — both alleles expressed (ABO — AB blood).
• Multiple alleles — >2 alleles at one locus (I^A, I^B, i).
• Pleiotropy — 1 gene → many traits (PKU, sickle cell).
• Polygenic inheritance — many genes → 1 trait (skin colour, height).

Human blood groups

Genotype	Phenotype
I^AI^A, I^Ai	A
I^BI^B, I^Bi	B
I^AI^B	AB
ii	O

Sex determination

• Human/Drosophila: XY (male XY, female XX).
• Grasshopper: XO (male XO, female XX).
• Birds: ZW (male ZZ, female ZW).
• Honey bee: haplodiploidy.
• Human sex determined by father's sperm (X or Y).

Sex-linked (X-linked) traits

• Haemophilia — blood clotting factor missing (VIII or IX).
• Colour blindness — red-green.
• Pattern: sons of carrier mothers 50% affected; daughters usually carriers.

Chromosomal disorders quick recall

Syndrome	Chr change	Karyotype	Symbol
Down's	Trisomy 21	47, +21	Palmar crease, MR
Klinefelter's	Extra X	47, XXY	Male + breasts, sterile
Turner's	Missing X	45, XO	Female, short, sterile

Sickle-cell (must-know)

• Autosomal recessive.
• Beta-globin gene: GAG → GUG codon → Glu (6) → Val (6) substitution.
• HbS/HbS = disease; HbA/HbS = carrier (heterozygote advantage in malaria zones).

Pedigree symbols (for pedigree Qs)

• Circle = female · Square = male · Filled = affected · Line = mating · Horizontal from mating = offspring · Half-filled = carrier.