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Dihybrid Inheritance
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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 - Y = yellow s = wrinkled - y = green
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The Cross Parental Phenotype Smooth, yellow X Wrinkled, green
Parental Genotype SSYY X ssyy Gametes SY SY SY SY sy sy sy sy SY sy SsYy Fertilisation All SsYy F1 genotype F1 Phenotype All smooth and yellow
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9 smooth yellow: 3 smooth green : 3 wrinkled yellow : 1 wrinkled green
The Cross F1 Phenotype Smooth, yellow X smooth yellow F1 Genotype SsYy X SsYy Gametes SY Sy sY sy SY Sy sY sy SY Sy sY sy SSYY SSYy SsYY SsYy SSyy Ssyy ssYY ssYy ssyy Fertilisation F2 genotype 9 smooth yellow: 3 smooth green : wrinkled yellow : 1 wrinkled green F2 Phenotype
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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.
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4 smooth yellow: 4 smooth green : 4 wrinkled yellow : 4 wrinkled green
Test Cross F1 Phenotype Smooth, yellow X wrinkled green F1 Genotype SsYy X ssyy Gametes SY Sy sY sy sy sy sy sy SY Sy sY sy SsYy Ssyy ssYy ssyy Fertilisation F2 genotype 4 smooth yellow: 4 smooth green : wrinkled yellow : 4 wrinkled green F2 Phenotype
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Test Cross SY sy SsYy F1 Phenotype Smooth, yellow X wrinkled green
F1 Genotype SSYY X ssyy Gametes SY SY SY SY sy sy sy sy SY sy SsYy Fertilisation F2 genotype All smooth yellow F2 Phenotype
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ALWAYS REMEMBER! Always keep the same alleles in the same order
E.g. if you have SsBb 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.
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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.
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Practice Practice Practice
Patterns of Life Pg
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