Gene Mapping Questions and Answers

Answer: A

The true statement is there is a greater probability for a crossover to occur between 2 genes farther apart than between genes nearer to each other.

Crossovers occur at more or less random points along a chromosome, so the longer the stretch between two genes, the more likely a crossover falls within it. This is the basis of Sturtevant's linkage mapping: recombination frequency is used as a measure of distance, 1 percent recombination being one map unit (centimorgan).

The second and third options state the opposite relationship. The fourth is wrong because the maximum recombination frequency between two genes is 50 percent: even with multiple crossovers, at most half the gametes are recombinant, which is the value seen for unlinked genes.

Answer: C

The first linkage map was made by A.H. Sturtevant.

In 1913, as a student in Morgan's laboratory, Sturtevant reasoned that recombination frequencies between genes on the Drosophila X chromosome reflect the distances between them. He used them to place sex-linked genes in a single linear order, the first genetic map.

T.H. Morgan discovered sex linkage and proposed that crossing over separates linked genes, but did not build the map. S. Benzer mapped the fine structure of the rII locus of phage T4 decades later, and Mendel worked before chromosomes were linked to heredity.

Answer: C

Such a set of genes is a haplotype.

A haplotype is a combination of alleles at linked loci on the same chromosome, inherited together as a unit. The term is used especially for the MHC (HLA) genes, which each parent passes on as a block.

Allotype, isotype and idiotype describe antibody variation: isotypes are the different heavy-chain classes (IgG, IgM and so on), allotypes are allelic differences between individuals, and idiotypes are the unique antigen-binding regions of a particular antibody. None refers to genes grouped on a chromosome.