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Arrangement of Linked Genes
When two pairs of genes are linked, the linkage may be of either of two types in an Individual heterozygous for both pairs (dihybrid) : (1) the two dominant genes (e g., R and Ro of sweet pea) may be located on one member of the chromosome pair; with the two recessive genes (e.g., r and ro) on the other, or (2) the dominant allele of one pair and the recessive of the other (R and ro) may be located on one chromosome of the pair, with the recessive of the first gene pair and the dominant of the second gene (e.g., r and Ro) on the other chromosome. The first arrangement, with two dominants on same chromosome is called cis arrangement. This type of interrelationship of dominant and recessive linked genes (e.g., RRo/rro) is called coupling phase. The second arrangement, one dominant and one recessive gene on the same chromosome is called trans arrangement. Such type of interrelationship of dominant and recessived genes (e.g., Rro/rRo) is called repulsion phase.