2.15—Inheritance terminology and monohybrid inheritance

A gene occupies a locus and its alternative forms are alleles. An individual's genotype is the allele combination; the phenotype is the observable result. Homozygous means two matching alleles, heterozygous means two different alleles, and dominance describes expression in a heterozygote—not importance.

For a monohybrid cross, write parental genotypes, list the alleles in their gametes, combine them in a Punnett square and convert genotypes to phenotypes. Pedigrees use symbols and family links to infer possible genotypes and whether an allele is likely dominant or recessive.

For Bb × Bb, the predicted genotypes are 1 BB : 2 Bb : 1 bb and, with complete dominance, phenotypes are 3 dominant : 1 recessive. Those are probabilities across many offspring, not a guarantee for four births.

Codominance or incomplete dominance changes the phenotype rule; sex-linked alleles require the X/Y chromosome context. Do not infer a person's exact genotype when the pedigree only supports several possibilities.