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Please contact Paul Szauter with comments and questions.
Please contact Paul Szauter with comments and questions. | ||||||||
| Testcrosses | ||
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A testcross is a cross of an individual drake to a drake known to be homozygous for the recessive allele of a gene or genes under study. Testcrosses are used to determine the genotype of a drake displaying the phenotype associated with the dominant allele of one or more genes. Testcrosses are also used to determine whether two genes are sorting independently, and if they are not, testcrosses allow the measurement of the map distance between genes. One gene. The simplest testcross involves only a single gene, and can be used to determine whether a drake displaying the phenotype associated with the dominant allele A is homozygous (A/A) or heterozygous (A/a). The expected outcomes are shown below. | ||
| Testcross to a/a for single gene | ||
| A/A X a/a | ---> | 100% A/* phenotype |
| A/a X a/a | ---> | 50% A/* phenotype |
| 50% a/a phenotype | ||
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| Two genes. A testcross can be carried out to determine the genotype of a drake with respect to two or more genes as shown below. | ||
| Testcross to a/a b/b for two unlinked genes | ||
| A/A B/B X a/a b/b | ---> | 100% A/* B/* phenotype |
| A/a B/B X a/a b/b | ---> | 50% A/* B/* phenotype |
| 50% a/a B/* phenotype | ||
| A/A B/b X a/a b/b | ---> | 50% A/* B/* phenotype |
| 50% A/* b/b phenotype | ||
| A/a B/b X a/a b/b | ---> | 25% A/* B/* phenotype |
| 25% A/* b/b phenotype | ||
| 25% a/a B/* phenotype | ||
| 25% a/a b/b phenotype | ||
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| Two genes, linked. If two genes are linked, the ratio of phenotypes observed in the offspring of a testcross will depend on the map distance between the genes and on their coupling relationship. If both recessive alleles are on one chromosome and both dominant alleles are on the other chromosome (A B/a b), the recessive alleles are said to be in coupling. If one recessive allele is on one chromosome and the other recessive allele is on the other chromosome (A b/a B), the recessive alleles are said to be in repulsion. | ||
| Dihybrid testcross to a b/a b for two linked genes 10 cM apart | ||
| A B/a b X a b/a b | ---> | 45% A/* B/* phenotype |
| 5% A/* b/b phenotype | ||
| 5% a/a B/* phenotype | ||
| 45% a/a b/b phenotype | ||
| A b/a B X a b/a b | ---> | 5% A/* B/* phenotype |
| 45% A/* b/b phenotype | ||
| 45% a/a B/* phenotype | ||
| 5% a/a b/b phenotype | ||
| Dihybrid testcross to a b/a b for two linked genes 20 cM apart | ||
| A B/a b X a b/a b | ---> | 40% A/* B/* phenotype |
| 10% A/* b/b phenotype | ||
| 10% a/a B/* phenotype | ||
| 40% a/a b/b phenotype | ||
| A b/a B X a b/a b | ---> | 10% A/* B/* phenotype |
| 40% A/* b/b phenotype | ||
| 40% a/a B/* phenotype | ||
| 10% a/a b/b phenotype | ||