01Exceptions to the simple pattern
Incomplete dominance: blended heterozygote (red × white snapdragons → pink; 1:2:1 ratio).
Codominance: both alleles fully shown (AB blood type; roan cattle).
Multiple alleles: more than two alleles in the population (ABO: Iᴬ, Iᴮ, i).
Polygenic traits: many genes add up (height, skin color) → bell-curve variation.
Pleiotropy: one gene affects many traits (sickle-cell allele).
02Sex-linked and linked genes
X-linked recessive traits (color blindness, hemophilia) show up mostly in males, who have only one X. Affected males get the allele from their mothers.
Linked genes sit close together on one chromosome and are inherited together unless crossing over separates them. Recombination frequency maps distance: 1% = 1 map unit.
Mitochondrial DNA is inherited only from the mother.
03Notebook box
Recombination frequency = recombinant offspring / total × 100.
Worked example: 85 parental-type and 15 recombinant offspring. Distance? → 15/100 = 15 map units.