- In corn (i.e. maize, a diploid species), imagine that alleles for resistance to a particular pathogen are recessive and are linked to a locus that affects tassel length (short tassels are recessive to long tassels). Design a series of crosses to determine the map distance between these two loci. You can start with any genotypes you want, but be sure to specify the phenotypes of individuals at each stage of the process and specify which progeny will be considered recombinant. You do not need to calculate recombination frequency.
- In a mutant screen in Drosophila, you identified a gene related to memory, as evidenced by the inability of recessive homozygotes to learn to associate a particular scent with the availability of food. Given another line of flies with an autosomal mutation that produces orange eyes, design a series of crosses to determine the map distance between these two loci and specify which progeny will be considered recombinant. You do not need to calculate recombination frequency.
- Imagine that methionine heterotrophy, chlorosis (loss of chlorophyll), and absence of leaf hairs (trichomes) are each caused by recessive mutations at three different loci in Arabidopsis. Given a triple mutant, and assuming the loci are on the same chromosome, explain how you would determine the order of the loci relative to each other.
- Three loci are linked in the order B-C-A. If the A-B map distance is 1 cM, and the B-C map distance is 0.6 cM, given the lines AaBbCc and aabbcc, what will be the frequency of Aabb genotypes among their progeny if one of the parents of the dihybrid had the genotypes AABBCC?
Genes for body colour (B black dominant to b yellow) and wing shape (C straight dominant to c curved) are located on the same chromosome in flies. If single mutants for each of these traits are crossed (i.e. a yellow fly crossed to a curved-wing fly), and their progeny is test crossed, the following phenotypic ratios are observed among their progeny.
Body Colour and Wing Shape Phenotypic Ratios black, straight 17 yellow, curved 12 black, curved 337 yellow, straight 364
- Calculate the map distance between B and C.
- Why are the frequencies of the two smallest classes not exactly the same?
- Given the map distance you calculated between B-C in question 5, if you crossed a double mutant (i.e. yellow body and curved wing) with a wild-type fly, and test crossed the progeny, what phenotypes in what proportions would you expect to observe among the F2 generation?
- Wild-type mice have brown fur and short tails. Loss of function of a particular gene produces white fur, while loss of function of another gene produces long tails, and loss of function at a third locus produces agitated behaviour. Each of these loss of function alleles is recessive. If a wild-type mouse is crossed with a triple mutant, and their F1 progeny is test-crossed, the following recombination frequencies are observed among their progeny. Produce a genetic map for these loci.
Fur Tail Behaviour Frequency white short normal 16 brown short agitated 0 brown short normal 955 white short agitated 36 white long normal 0 brown long agitated 14 brown long normal 46 white long agitated 933