Cell division & heredity · Topic 16 of 17

Mendelian Genetics & Punnett Squares

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Biology · heredity

Each gene comes in alleles, one from each parent; a dominant allele masks a recessive one, and Punnett squares predict offspring ratios.

01 · DominanceRecessive shows only when bb

02 · Don't confuseGenotype = alleles · phenotype = trait

03 · Hybrid crossBb × Bb → 3:1 look, 1:2:1 alleles

Gene: a DNA segment on a chromosome that codes for a trait must know

Allele: one version of a gene — one comes from each parent must know

Homozygous = same alleles (BB, bb) · heterozygous = different (Bb) must know

Punnett square: gametes on two sides; each box = an equal chance

Mendel (pea plants): F1 all dominant; F1 × F1 → F2 in a 3 : 1 ratio

Carrier: heterozygous (Bb) — no recessive trait, but can pass b on

Bb × Bb → 1 BB : 2 Bb : 1 bb → 3 dominant : 1 recessive

Dihybrid AaBb × AaBb: 16 boxes → 9 : 3 : 3 : 1 phenotypes

Dominant (B) masks recessive (b); the recessive trait shows only as bb must know

Law of segregation: each gamete gets only one allele of each gene

Independent assortment: genes for different traits sort separately

Independent chances multiply: aabb = 1/4 × 1/4 = 1/16

Gg × gg → 1 Gg : 1 gg → 50% green, 50% brown

Ee × Ee, 12 kits → expect about 9 upright (75%), 3 floppy (25%)

PpSs × PpSs → 9 purple-spotted : 3 purple-plain : 3 white-spotted : 1 white-plain

Genotype = the alleles (Bb) · phenotype = the observable trait must know

Commonly confused

Each child is a fresh 1-in-4 chance — not exactly 1 of every 4 children

Show 1 more fact

Test cross with bb: any recessive offspring → the parent was Bb

Pearl

A capital letter sets the look; it takes two lowercase letters to show the recessive trait.

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