13.1 Chromosomal Theory and Genetic Linkages
Drosophila that are true breeding for the traits straight wings (S) and red eyes (R) are crossed with flies that are true breeding for curved wings (s) and brown eyes (r). A test cross is then made between the offspring and the true-breeding ssrr flies.
- Use the symbols S, s, R, and r to construct a representation of the parental genotypes in the test cross.
- If these genes are located on different chromosomes, use a Punnett square to construct a representation of the offspring of the test cross.
- Predict the distribution of genotypes and phenotypes resulting from the test cross.
- As it happens, these genes are both on chromosome II as shown below. Use the symbols S, s, R, and r to construct a representation of the parental and recombinant genotypes in the test cross.
- Suppose that 500 flies are produced in the test cross. Apply mathematical methods to calculate the expected number of recombinant offspring using the linear map units (LMU) shown in the diagram below.
Studies like the one described in question AP12.1 were carried out by Morgan and Sturtevant beginning in 1911. The discovery of linkage was made by Bateson and Punnett in 1900. They crossed a true-breeding purple (P) plant with long seeds (L) with a true-breeding red (r) plant with round seeds (l). They then performed a self-cross between the F1 generations. They obtained the F2 data shown below.
- Use the symbols P, p, L, and l to construct a representation of the F2 genotypes and complete the second column in the table.
- Complete the fourth column of the table above by recording the values of the predicted numbers of plants with each genotype.
- Apply a c2 test at the 95% confidence level to evaluate the claim that these data confirm linkage. The definition of the statistic and this table are provided in the AP Biology Exam.
Degrees of Freedom p 1 2 3 4 5 6 7 8 0.05 3.84 5.99 7.82 9.49 11.07 12.59 14.07 15.51 0.01 6.64 9.32 11.34 13.28 15.09 16.81 18.48 20.09
- At first, Bateson and Punnett did not see that these genes are located on the same chromosome and proceeded to measure the linkage distance between them, taking the first step toward creating a gene map. Justify the selection of data and the procedure from which data could be collected that would have provided the necessary evidence to confirm linkage and recombination.
Review the observations that provided researchers with evidence in support of the Chromosomal Theory of Inheritance.
- Evaluate the dependence of these observations on improvements in a critical technology during the period from 1850 to 1940. Identify this technology and describe how this technology allowed scientists to make the connection between chromosomes and genes. (As a hint, the name "chromosome" is taken from the Greek word chroma, which means colored or stained.)
- Mendel's laws of inheritance are explained by the chromosomal theory. Use these observations to justify:
- the law of segregation
- the law of independent assortment
Errors in the transmission of genetic information to future generations are essential. Otherwise, organisms could not evolve over time. Some errors in the synthesis of new DNA during S phase in either meiosis or mitosis are not repaired. These errors usually involve single nucleotides. Errors that occur during prophase I of meiosis that are not corrected can involve the exchange of sequences between homologous chromosomes (duplications) or even nonhomologous chromosomes (translocations). Duplications are usually retained, and the organism remains viable without a change in phenotype. Translocations are usually lethal or significantly alter phenotype. In eukaryotes, duplications and the shuffling of parental genes through recombination are important sources of variation.
Construct an explanation of the role of duplication as a source of raw material for future mutations and selection and contrast this type of variation with recombination.
Bacteria and Archaea reproduce asexually, and genetic material is in a closed loop. In both domains, genetic material is transferred horizontally, and polyploidy is common. Polyploidy is common in plants and occurs in invertebrate animals but is less common in vertebrates. In all domains, multiple copies of genes (gene duplication) are common.
Based on this information, compare and contrast the mechanisms that provide genetic variation in the three domains: Bacteria, Archaea, and Eukarya.