Dihybrid Inheritance Dihybrid Cross This is between organisms

Dihybrid Inheritance

Dihybrid Cross • This is between organisms with 2 different characteristics, i. e. containing 2 different pairs of contrasting alleles. • Mendel did such a cross with homozygous smooth (S) and yellow (Y) seeds, with homozygous wrinkled (s) and green (y) seeds. – S = smooth – s = wrinkled - Y = yellow - y = green

Parental Phenotype The Cross Smooth, yellow Parental Genotype Gametes SY SSYY SY SY X Wrinkled, green X SY ssyy sy sy SY Fertilisation sy Ss. Yy F 1 genotype All Ss. Yy F 1 Phenotype All smooth and yellow sy sy

F 1 Phenotype The Cross Smooth, yellow F 1 Genotype Gametes Ss. Yy SY Sy Fertilisation F 2 genotype F 2 Phenotype SY Sy s. Y sy s. Y SY SSYy Ss. YY Ss. Yy X smooth yellow X sy Sy SSYy SSyy Ss. Yy Ssyy Ss. Yy SY Sy s. Y Ss. Yy ss. YY ss. Yy s. Y sy Ss. Yy Ssyy ss. Yy ssyy 9 smooth yellow: 3 smooth green : 3 wrinkled yellow : 1 wrinkled green sy

Dihybrid Test or Back Cross • This is used to work out the genotype of an individual which shows the dominant trait. • If the parent in question is heterozygous you get a ratio of 1: 1: 1: 1 • If it is homozygous, all the offspring will show the dominant phenotype for both characteristics.

Test Cross F 1 Phenotype Smooth, yellow F 1 Genotype Gametes Ss. Yy SY Sy Fertilisation F 2 genotype F 2 Phenotype sy sy s. Y SY Ss. Yy sy Sy Ssyy X wrinkled green X ssyy sy sy s. Y ss. Yy sy sy ssyy 4 smooth yellow: 4 smooth green : 4 wrinkled yellow : 4 wrinkled green sy

Test Cross F 1 Phenotype Smooth, yellow F 1 Genotype Gametes SSYY SY SY Fertilisation F 2 genotype F 2 Phenotype sy sy SY SY Ss. Yy X wrinkled green X ssyy SY SY Ss. Yy sy sy SY Ss. Yy All smooth yellow sy SY Ss. Yy sy

ALWAYS REMEMBER! • Always keep the same alleles in the same order – E. g. if you have Ss. Bb you must keep the S’s first, then the B’s • Many students find it hard to work out how many gametes there should be for each genotype. • If 2 gametes are the same it is not necessary to repeat yourself.

ALWAYS REMEMBER! • Dihybrid punnet squares are very busy so give yourself enough room to read you answers. • When you have 2 letters that look the same as capital and lowercase letters, make sure you exaggerate the differences. This will help you to avoid mistakes in interpretation.

Practice • Patterns of Life • Pg 186 -187