Mendelian Genetics
Inheritance through allele pairs: genotype is the allele combination (homozygous or heterozygous), phenotype is the observable trait, dominant alleles mask recessive ones, and a Punnett square predicts offspring ratios.
A heterozygous organism (Bb) shows the dominant trait but can pass the hidden recessive allele on — the definition of a carrier. Sex-linked traits ride the X chromosome, which is why disorders like red-green colorblindness appear far more often in males.