Botany · Ch 2 — Classical Genetics
Dihybrid Cross
Dihybrid Cross
A dihybrid cross follows two characters, each controlled by a separate gene with two alleles, through a cross - for example seed shape (round R dominant over wrinkled r) together with cotyledon colour (yellow Y dominant over green y). Crossing a pure-breeding round-yellow parent (RRYY) with a pure-breeding wrinkled-green parent (rryy) produces an F1 generation that is uniformly round and yellow, with genotype RrYy. When these F1 dihybrids form gametes, the two gene pairs assort independently of each other during meiosis, because the genes lie on separate chromosomes; this independent behaviour is what Mendel's Law of Independent Assortment describes - segregation of one pair of characters is independent of the segregation of the other pair. As a result, each RrYy plant produces four kinds of gametes in equal (1/4 each) proportion - RY, Ry, rY and ry - rather than just the two parental combinations. When these four gamete types combine randomly at fertilisation, they generate sixteen possible zygote combinations that collapse, by dominance, into just four visible phenotype classes in the ratio 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green - Mendel's classic 9:3:3:1 dihybrid ratio, an idealised probability outcome of segregation, independent assortment and random fertilisation acting toge …
What this figure shows. A diagram tracing a cross between homozygous round-yellow (RRYY) and homozygous wrinkled-green (rryy) parents through meiosis to the F1 generation (RrYy, round-yellow), showing that because the R/r and Y/y genes sit on separate chromosomes and assort at random, the F1 plant forms four different kinds of gametes - RY, Ry, rY and ry - in eq …
What this figure shows. A full 4x4 Punnett square for the F1 x F1 dihybrid self-cross, with the four gamete types (RY, Ry, rY, ry) from each parent arranged along both edges, filling in all sixteen genotype combinations and grouping them into the four F2 phenotype classes - round yellow, round green, wrinkled yellow, wrinkled green - …
What this figure shows. A flowchart contrasting the two seed-shape alleles at the molecular level: the dominant R allele encodes an active starch-branching enzyme (SBEI) that converts linear, unbranched starch (amylose) into branched starch (amylopectin), giving smooth round seeds with balanced osmotic pressure, while the recessive r allele (interrupted by an extra 0.8 kb DNA insertion) encodes an inactive enzyme, so starch stays unbranched and the seed shrivels into a w …