Dihybrid Crosses

A dihybrid cross involves breeding organisms that differ in two traits, whereas monogenic traits and monohybrid crosses examine only one trait. In Mendel's pea experiments, dihybrid crosses tracked seed shape and color simultaneously. The F1 generation produces dihybrids—organisms heterozygous for both traits—displaying dominant phenotypes for each characteristic.

The 9:3:3:1 ratio emerges from sixteen possible allele combinations during F2 self-fertilization. Nine combinations produce both dominant traits, three produce one dominant and one recessive trait, three produce the opposite combination, and one produces both recessive traits. This ratio demonstrates that each fertilization event is equally probable.

The consistent 9:3:3:1 ratio indicates that inheriting one trait does not influence the likelihood of inheriting another. This observation established Mendel's law of independent assortment, showing that traits segregate independently during gamete formation. The principle applies when genes are on separate chromosomes.

Parental plants are homozygous for both traits, such as RRYY (round, yellow) and rryy (wrinkled, green). All F1 offspring are dihybrids with RrYy genotypes, displaying round and yellow phenotypes. These heterozygous F1 plants carry one dominant and one recessive allele for each trait.

Genes on separate chromosomes assort independently, but genes close together on the same chromosome exhibit linkage, making them more likely to inherit together. Recombination during meiosis can break linkage by swapping homologous chromosome segments. Mendel's traits showed independent assortment because they were either unlinked or far apart, allowing recombination to mimic independent inheritance.

Recombination occurs during prophase I of meiosis when chromosome pairs cross over and exchange genetic segments. Loci far apart on a chromosome are more likely separated by recombination events, causing them to inherit independently. This process explains why Mendel observed independent assortment for traits that might otherwise show linkage.

Most of Mendel's studied traits are determined by loci on different chromosomes or far apart on the same chromosome. For example, pod color and pea shape are on chromosomes 5 and 7, respectively. Recombination events between distant loci on chromosome 1 and chromosome 4 caused linked genes to behave as if independently assorted.