dihybrid

A dihybrid refers to an organism or cross that involves two pairs of alleles at two different genes. In genetics, a dihybrid cross typically examines how two traits are inherited when both parents are heterozygous for both genes, such as AaBb x AaBb. The classic example assumes complete dominance and independent assortment.

During meiosis, alleles for each gene segregate and assort independently, producing four types of gametes in equal proportions: AB, Ab, aB, and ab. When two such dihybrid individuals are crossed, the resulting offspring often display a phenotypic ratio of 9:3:3:1 for the two traits, assuming the genes are unlinked and affect the phenotype independently. This ratio reflects the combined probabilities of the different allele combinations in the offspring.

Deviations from the 9:3:3:1 ratio can occur if genes are linked on the same chromosome, reducing recombination between them, or if there are epistatic interactions, incomplete dominance, or codominance. The observed frequency of recombinants provides information about genetic distance and gene order, and crossing over during meiosis contributes to variation in dihybrid crosses.

Dihybrid analysis is a foundational concept in Mendelian genetics and is commonly used in teaching to illustrate segregation and independent assortment. It also lays groundwork for understanding genetic mapping, heredity of two traits, and how allele interactions shape phenotypes.