Chapter 7: Linkage, Recombination, and Eukaryotic Gene Mapping

Vocabulary flashcards covering linked genes, recombination frequency, coupling and repulsion configurations, two-point and three-point testcrosses, interference, double crossovers, and deletion mapping.

Linked genes

Genes located on the same chromosome that do not assort independently.

Nonrecombinant gametes

Gametes that contain only original combinations of alleles present in the parent organism, also called parental gametes.

Recombinant gametes

Gametes with new combinations of alleles produced by crossing over between homologous chromosomes.

Crossing over

The physical exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis.

Recombination frequency

The percentage of recombinant progeny produced in a cross, calculated as $\left(\frac{\text{number of recombinant progeny}}{\text{total number of progeny}}\right) \times 100\%$.

Coupling configuration

The arrangement in which dominant alleles at two loci are on one homologous chromosome and recessive alleles are on the other ($A\,B / a\,b$), also known as cis configuration.

Repulsion configuration

The arrangement in which each chromosome of a homologous pair carries one dominant and one recessive allele ($A\,b / a\,B$), also known as trans configuration.

Centimorgan

A unit of measure for genetic map distance (also called a map unit or $\text{m.u.}$) that corresponds to a $1\%$ recombination frequency.

Two-point testcross

A testcross between an individual heterozygous at two loci and an individual homozygous recessive at both loci, used to measure recombination frequency and distance between two genes.

Three-point testcross

A testcross involving three linked loci, used to determine gene order and map distances in a single set of progeny.

Double crossover

Two crossover events occurring between two outer linked genes, which reverses the effects of the first crossover on the outer loci while swapping the middle locus.

Coefficient of coincidence

The ratio of observed double crossovers to expected double crossovers, calculated as $\frac{\text{number of observed double crossovers}}{\text{number of expected double crossovers}}$.

Interference

The phenomenon in which one crossover event in a chromosomal region reduces the probability of a second crossover event nearby, calculated as $1 - cc$ where $cc$ is the coefficient of coincidence.

Linkage group

A collection of genes located on the same chromosome that tend to be inherited together.

Deletion mapping

A technique used to map the physical position of a gene on a chromosome by crossing an individual homozygous for a recessive mutation with an individual heterozygous for a chromosomal deletion.

Polytene chromosomes

Giant chromosomes formed by repeated rounds of DNA replication without cell division, found in certain tissues of Drosophila, which display visible loops when homologous deletion regions fail to pair.

McClintock and Creighton experiment

A landmark 1931 study using maize chromosome 9 with physical landmarks (a knob and an extra piece) that provided visual evidence that genetic recombination is accompanied by physical exchange between homologous chromosomes.