Linkage, Recombination, and Gene Mapping Vocabulary

Vocabulary flashcards covering the principles of genetic linkage, recombination frequencies, mapping techniques, and human genetic association studies.

Mendel’s Law of Independent Assortment

The principle assuming that four possible allele combinations (e.g., $TB$, $Tb$, $tB$, $tb$) are all equally likely to occur in gametes.

Recombinant Chromosomes

Chromosomes that are different from what was inherited from the parents because of recombination during meiosis.

Nonrecombinant Chromosomes

Chromosomes that are the same as what was inherited from the parents, occurring when no recombination takes place.

Genetic Linkage

A condition where genes lie close to each other on the same chromosome and do not assort independently.

Recombination Frequency ($Rf$)

Calculated as the $\text{number of recombinant progeny}$ divided by the $\text{total number of progeny}$, typically expressed as a percentage.

Maximum Recombination Frequency

The highest possible observed value for $Rf$, which is $50\%$. Even if a breakpoint always falls between two genes, two of the four chromatids in the tetrad do not participate in recombination.

Coupling (Cis Orientation)

An arrangement where two wild-type alleles are on the same chromosome (e.g., \frac{p^+ ^+}{p }).

Repulsion (Trans Orientation)

An arrangement where two wild-type alleles are on different chromosomes (e.g., \frac{p^+ }{p ^+}).

Genetic Map Units ($m.u.$)

A measure of distance also known as centiMorgans ($cM$), where $1 \, m.u. = 1 \, cM = 1\%$ recombination.

Double Crossover

Two recombination events occurring between two genes; these are often undetected and cause the observed $Rf$ to underestimate the actual physical distance.

Three-Point Test Cross

A genetic cross used to map three linked genes by analyzing offspring phenotypes to determine gene order and distances.

Interference

The phenomenon where a crossover in one region of a chromosome reduces (or sometimes increases) the likelihood of another crossover occurring nearby.

Coefficient of Coincidence

The ratio of the $\text{observed double crossovers}$ to the $\text{expected double crossovers}$; used to calculate interference ($1 - \text{coefficient of coincidence}$).

Linkage Analysis

A method used to map human genes by tracing the inheritance of a disease-causing mutation along with polymorphic markers through several generations of a family pedigree.

Polymorphic Marker

A polymorphic DNA sequence whose specific location on a chromosome is known, used to track chromosomal inheritance.

LOD Score (Logarithm of Odds)

A statistical value comparing the likelihood of linkage at a specific $Rf$ to the likelihood of the data occurring by chance; a positive score favors linkage.

Association Study (Case-Control Study)

A study comparing a group of people with a disease (cases) to a group without it (controls) to determine if specific alleles appear more or less frequently in the affected group.

Genome-Wide Association Study (GWAS)

An analysis where subjects are genotyped for a very large number of markers from across the entire genome to identify genetic associations with a disease or trait.

Candidate Gene

A gene located within a region identified by mapping studies whose protein function is suspected to influence a specific disease risk or phenotypic trait.