Linkage and Chromosome Mapping Practice Flashcards

A set of vocabulary flashcards covering the principles of genetic linkage, recombination frequency, the usage of chi-square analysis for linkage, and the calculation of map distances through two-point and three-point testcrosses.

Independent Assortment

The principle where an individual (e.g., $RrYy$) produces four gamete types in equal frequency due to genes being on different chromosomes or far apart.

Linked Genes

Genes located on the same chromosome where alleles at different loci tend to travel together and do not assort independently.

Crossing Over

A process occurring during meiosis that allows for the production of recombinant types by physically exchanging segments between homologous chromosomes.

Recombinant types

Gametes or progeny that result from a crossover event, possessing different allele combinations than the original pure-breeding parents.

Parental (nonrecombinant) types

Gametes or progeny that retain the same allele arrangement as the original pure-breeding parents.

Coupling Phase

Also known as the cis arrangement, where parental allele types are $AB$ and $ab$.

Repulsion Phase

Also known as the trans arrangement, where parental allele types are $Ab$ and $aB$.

Map Distance

A measurement numerically equal to the percentage of recombination between loci, calculated as $(\text{recombination frequency}) \times (100)$.

Centimorgan (cM)

The unit of map distance, where $1 \text{ cM}$ equals $1\%$ recombination.

Two-Point Mapping

An experiment used to determine the distance between two linked loci by testcrossing an $F_1$ heterozygote and analyzing the frequency of recombinant progeny.

Chi-square ($\chi^2$) test for linkage

A statistical analysis used to test the null hypothesis that two loci assort independently; for two classes, the critical value at the $5\%$ significance level ($df = 1$) is $3.84$.

Creighton and McClintock

The scientists (Harriet Creighton and Barbara McClintock) who provided cytological evidence that crossing over involves a physical exchange between homologous chromosomes.

Double Crossover (DCO)

Two crossover events between three loci; in these rare events, only the middle gene is altered relative to the noncrossover (NCO) configuration.

Three-Point Mapping

A method used to determine the gene order and map distances between three linked loci by analyzing the progeny of a trihybrid testcross.

Noncrossovers (NCO)

The most frequent progeny classes in a three-point testcross, representing the original chromosomal arrangement of the heterozygous parent.

Coefficient of Coincidence (C)

The ratio of observed double crossovers to expected double crossovers, calculated as $C = \frac{\text{Observed \# DCO}}{\text{Expected \# DCO}}$.

Interference (I)

The phenomenon where a crossover in one region affects the probability of another crossover in an adjacent region, calculated as $I = 1 - C$.

50% Recombination Limit

The maximum observable recombination frequency; loci that assort independently or are very far apart will appear to have a map distance of $50 \text{ cM}$.