4. Genetic Mapping and Linkage

In Drosophila, a cross was made between females—all expressing the three X-linked recessive traits scute bristles (sc), sable body (s), and vermilion eyes (v)—and wild-type males. In the F₁, all females were wild type, while all males expressed all three mutant traits. The cross was carried to the F₂ generation, and 1000 offspring were counted, with the results shown in the following table.Phenotype Offspringsc s v 314+ + + 280+ s v 150sc + + 156sc + v 46+ s + 30sc s + 10+ + v 14No determination of sex was made in the data.Calculate the coefficient of coincidence. Does it represent positive or negative interference?

In Drosophila, a cross was made between females—all expressing the three X-linked recessive traits scute bristles (sc), sable body (s), and vermilion eyes (v)—and wild-type males. In the F₁, all females were wild type, while all males expressed all three mutant traits. The cross was carried to the F₂ generation, and 1000 offspring were counted, with the results shown in the following table.Phenotype Offspringsc s v 314+ + + 280+ s v 150sc + + 156sc + v 46+ s + 30sc s + 10+ + v 14No determination of sex was made in the data.Are there more or fewer double crossovers than expected?

Another cross in Drosophila involved the recessive, X-linked genes yellow (y), white (w), and cut (ct). A yellow-bodied, white-eyed female with normal wings was crossed to a male whose eyes and body were normal but whose wings were cut. The F₁ females were wild type for all three traits, while the F₁ males expressed the yellow-body and white-eye traits. The cross was carried to an F₂ progeny, and only male offspring were tallied. On the basis of the data shown here, a genetic map was constructed.Phenotype Male Offspringy + ct 9+ w + 6y w ct 90+ + + 95+ + ct 424y w + 376y + + 0+ w ct 0Were any double-crossover offspring expected?

In a diploid plant species, an F₁ with the genotype Gg Ll Tt is test-crossed to a pure-breeding recessive plant with the genotype gg ll tt. The offspring genotypes are as follows:Genotype NumberGg Ll Tt 621Gg Ll tt 3Gg ll Tt 64Gg ll tt 109gg Ll Tt 103gg Ll tt 67gg ll Tt 7gg ll tt 6261600Explain the meaning of this I value.

In a diploid plant species, an F₁ with the genotype Gg Ll Tt is test-crossed to a pure-breeding recessive plant with the genotype gg ll tt. The offspring genotypes are as follows:Genotype NumberGg Ll Tt 621Gg Ll tt 3Gg ll Tt 64Gg ll tt 109gg Ll Tt 103gg Ll tt 67gg ll Tt 7gg ll tt 6261600What is the interference value for this data set?

In a diploid plant species, an F₁ with the genotype Gg Ll Tt is test-crossed to a pure-breeding recessive plant with the genotype gg ll tt. The offspring genotypes are as follows:Genotype NumberGg Ll Tt 621Gg Ll tt 3Gg ll Tt 64Gg ll tt 109gg Ll Tt 103gg Ll tt 67gg ll Tt 7gg ll tt 6261600Why is the recombination frequency for the outside pair of genes not equal to the sum of recombination frequencies between the adjacent gene pairs?

Because of the relatively high frequency of meiotic errors that lead to developmental abnormalities in humans, many research efforts have focused on identifying correlations between error frequency and chromosome morphology and behavior. Tease et al. (2002) studied human fetal oocytes of chromosomes 21, 18, and 13 using an immunocytological approach that allowed a direct estimate of the frequency and position of meiotic recombination. Below is a summary of information [modified from Tease et al. (2002)] that compares recombination frequency with the frequency of trisomy for chromosomes 21, 18, and 13. (Note: You may want to read appropriate portions of Chapter 8 for descriptions of these trisomic conditions.)Trisomic Mean Recombination Live-bornFrequency FrequencyChromosome 21 1.23 1/700Chromosome 18 2.36 1/3000–1/8000Chromosome 13 2.50 1/5000–1/19,000Other studies indicate that the number of crossovers per oocyte is somewhat constant, and it has been suggested that positive chromosomal interference acts to spread out a limited number of crossovers among as many chromosomes as possible. Considering information in part (a), speculate on the selective advantage positive chromosomal interference might confer.