PATTERNS OF INHERITANCE

1. Codominance • 2 alleles produce 3 distinct phenotypes without mixing (AB has characteristics of both AA and BB) • Aa x Aa gives phenotypic ratio of 1:2:1 • (i.e. cattle coat colour: CRCW has equal numbers of red and white hairs intermingled) • equal ratio of male:female (not sex linked) • multiple alleles (2 alleles co-dominant over recessive) produces 4 distinct phenotypes without mixing) • (i.e. ABO blood types: IAIA/IAi, IBIB/IBI, IAIB, ii are all distinct, AB codes for 2 antigens) • Multifactorial (Polygenic) • many phenotypes approaching a continuous distribution • (i.e. height in humans) • equal ratio of male:female (not sex linked) • Sex-linked dominant/recessive • 2 distinct phenotypes (XAY, XAXa, XAXA all have the same phenotype) • XAY x XAXa gives 1 XAXA: 1 XAY: 1 XAXa: 1 XaY • unequal ratio of male:female (suspect sex linkage) • if trait determined by recessive allele, it skips a generation PATTERNS OF INHERITANCE • Simple autosomal ominant/recessive • Monohybrid Cross • 2 distinct phenotypes (Aa same phenotype as AA) • Aa x Aa gives phenotypic ratio of 3:1 • equal ratio of male:female (not sex linked) • Dihybrid Cross • 4 distinct phenotypes (A_B_, A_bb, aaB_, aabb all look different) • AaBb x AaBb gives phenotypic ratio of 9:3:3:1 • equal ratio of male:female (not sex linked) • Gene Linkage • 2 phenotypes if no crossing over(if A and B are linked, only gametes are AB and ab) • AaBb x AaBb produces 1 AABB: 1 aabb (any Aabb, aaBb a result of crossing over) • Incomplete dominance • 2 alleles produce 3 distinct phenotypes (Aa appears intermediate between AA and aa) • Aa x Aa gives phenotypic ratio of 1:2:1 • (i.e. heterozygous snapdragons are pink, intermediate between red and white) • equal ratio of male:female (not sex linked)