Studying the inheritance of two characters simultaneously THE DIHYBRID CROSS Studying the inheritance of two characters simultaneously © 2007 Paul Billiet ODWS

Mendel’s peas Character Trait Allele Seed shape Round R Wrinkled r Cotyledon colour Yellow Y Green y © 2007 Paul Billiet ODWS

Combinations Genotype Phenotype RRYY Round Yellow RRYy RrYY RrYy RRyy Round Green Rryy rrYY Wrinkled Yellow rrYy rryy Wrinkled Green © 2007 Paul Billiet ODWS

THE LAW OF INDEPENDENT ASSORTMENT It appears that the inheritance of seed shape has no influence over the inheritance of seed colour The two characters are inherited INDEPENDENTLY The pairs of alleles that control these two characters assort themselves independently © 2007 Paul Billiet ODWS

Dihybrid cross genetic diagram P Phenotypes Round Yellow seed X Wrinkled Green seed (Pure bred) Genotypes RRYY rryy meiosis Gametes RY ry fertilisation F1 RrYy Round Yellow Proportions 100% © 2007 Paul Billiet ODWS

Dihybrid cross genetic diagram F1 Phenotypes RrYy (Selfed) Genotypes Round Yellow Proportions 100% meiosis Gametes Y y R RY Ry r rY ry © 2007 Paul Billiet ODWS

Dihybrid cross genetic diagram Gametes Y y R RY Ry r rY ry fertilisation F2 Genotypes RRYY RRYy RrYY RrYy RRyy Rryy rrYY rrYy rryy © 2007 Paul Billiet ODWS

Dihybrid cross proportions Phenotypes Proportions Round Yellow 9/16 or 56.25% Round Green Wrinkled Yellow Wrinkled Green © 2007 Paul Billiet ODWS

Dihybrid cross proportions Phenotypes Proportions Round Yellow 9/16 or 56.25% Round Green 3/16 or 18.75% Wrinkled Yellow Wrinkled Green 1/16 or 6.25% © 2007 Paul Billiet ODWS

Dihybrid test cross In monohybrid crosses, to know if a dominant trait is homozygous (RR) or heterozygous (Rr) it is necessary to carry out a test cross This is done with a homozygous recessive (rr) individual The same is true for a dihybrid cross where the test cross is made with an individual which is homozygous recessive for both characters (rryy) © 2007 Paul Billiet ODWS

Dihybrid test cross Phenotypes Round Yellow X Wrinkled Green Genotypes RrYy rryy Gametes RY, Ry, rY, ry ry Genotypes RY Ry rY ry RrYy Rryy rrYy rryy Phenotypes Round Yellow Round Green Wrinkled Yellow Wrinkled Green Proportions 1/4 or 25% © 2007 Paul Billiet ODWS