Patterns of Inheritance – Monohybrid Crosses

Monohybrid Cross Basics

• Considers inheritance at a single gene locus with two alleles.

• Use a Punnett square to predict offspring genotypes/phenotypes.

• Genotypic ratio for crossing two heterozygotes (Aa \times Aa) is 1\,AA : 2\,Aa : 1\,aa.

• Phenotypic ratio (complete dominance) becomes 3 \text{ dominant} : 1 \text{ recessive}.

Key Terminology

• Autosomal: Gene located on any of the 22 non-sex chromosome pairs.

• X-linked: Gene located on the X chromosome; males (XY) are hemizygous.

• Codominance: Heterozygote expresses both homozygote phenotypes fully (e.g. roan cattle).

• Incomplete dominance: Heterozygote shows an intermediate phenotype (e.g. pink snapdragons).

• Punnett square: Grid that displays gamete combinations to forecast genotypes.

• Test cross: Cross an organism with dominant phenotype to a homozygous recessive partner to reveal unknown genotype.

Autosomal Complete Dominance Example

• Widow’s peak (W) dominant to round hairline (w) .

• Two heterozygous parents (Ww) : chance of widow’s-peak child = 75\% .

X-linked Recessive Example

• Carrier mother (X^HX^h) × normal father (X^HY) .
  • Sons: 50\% affected (X^hY) .
  • Daughters: 0\% affected, 50\% carriers.

• Sex of offspring directly influences probability of expressing trait.

Codominance Example

• Red (R) and White (W) alleles in Shorthorn cattle are codominant.

• Red bull (RR) × Roan cow (RW) → calves: 50\%\,RR (red), 50\%\,RW (roan).

Incomplete Dominance Example

• Snapdragon: Red (FR) incompletely dominant to White (FW) .

• Pink (FRFW) × Pink (FRFW) produces
  1\,\text{red} : 2\,\text{pink} : 1\,\text{white} phenotypic ratio.

Test Cross Logic

• Dominant-phenotype individual (unknown: BB or Bb) × homozygous recessive (bb) .
  • All dominant offspring → parent is BB.
  • Any recessive offspring → parent is Bb.

Quick Problem Patterns

• Heterozygote × homozygous recessive (complete dominance) → 50\% dominant, 50\% recessive.

• Two carriers of autosomal recessive disease → 25\% affected, 50\% carriers, 25\% unaffected.

• X-linked recessive: affected male × normal (non-carrier) female → all daughters carriers, all sons normal.

Recap

• Punnett squares and test crosses allow prediction/confirmation of genotypes.

• Mode of dominance (complete, co-, incomplete, X-linked) determines expected phenotypic ratios.