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) $.</p></li><li><p>Pink$ (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.