Advanced Genetics: Dihybrid Crosses & Linkage
IB Biology HL requires mastery of genetics beyond simple monohybrid crosses. In a dihybrid cross involving two independently assorting loci (Mendel's Second Law), a double heterozygote AaBb crossed with AaBb produces offspring in the 9:3:3:1 phenotypic ratio (9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb). This is because independent assortment produces four equally likely gamete types: AB, Ab, aB, ab. However, when two genes are located on the same chromosome (linked genes), they do not assort independently. Linked genes tend to be inherited together, producing parental-type offspring more frequently than recombinant offspring. Recombination frequency (RF) = (number of recombinant offspring / total offspring) × 100% and equals the genetic map distance in centimorgans (cM). RF < 50% indicates linkage. At RF = 50%, genes behave as if independently assorting even if on the same chromosome (far apart). Sex-linked inheritance: genes on the X chromosome have no corresponding allele on the Y. Males (XY) are hemizygous—one copy determines the phenotype. X-linked recessive conditions (like haemophilia) appear more frequently in males. In crosses involving sex-linked traits, always write genotypes with X^A notation.