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F2 segregation

Independent assortment of the 4 possible alleles gives 16 possible combinations, as shown in the diagram above. Since round (S) and yellow (Y) are dominant alleles, the ratio of phenotypes seen would be 9:3:3:1 – 9 round and yellow (S-Y-), 3 round and green (S-yy), 3 wrinkled and yellow (ssY-) and 1 wrinkled and green (ssyy). 

BUT: this is only true for unlinked genes, e.g. genes located on different chromosomes or far away from one another (see next slide).