linkage & gene mapping

Linkage group

A group of genes located on the same chromosome that are physically part of the same DNA molecule

genetic linkage

The phenomenon where genes close together on a chromosome are more likely to be inherited as a single unit

syntenic alleles

Linked alleles located on the same chromosome

independent assortment (linked genes)

Genes far apart on the same chromosome may assort independently due to crossing over

crossing over

Exchange of DNA segments between non-sister chromatids of homologous chromosomes during Prophase I

bivalents

paired homologous chromosomes during prophase I of meiosis

recombinant chromosomes

Chromosomes that contain new allele combinations due to crossing over

effect of distance on crossing over

Genes farther apart are more likely to be separated by crossing over than genes close together

parental chromosomes

Chromosomes whose alleles have not been reshuffled by crossing over

recombinant chromosomes

Chromosomes whose alleles have been reshuffled by crossing over

parental gametes

Gametes containing original (non-recombined) allele combinations

recombinant gametes

Gametes containing new allele combinations due to crossing over

parental offspring

offspring produced from parental chromosomes

recombinant offspring

offspring produced from recombinant chromosomes

evidence of linkage

Deviation from expected Mendelian ratios (e.g., not 1:1:1:1 in a dihybrid test cross)

another sign of linkage

Most frequent offspring are parental types, least frequent are recombinant types

test cross

A cross with a homozygous recessive individual that reveals the genotype of another organism

purpose of test cross in linkage

Allows the dihybrid parent to determine offspring phenotypes

expected ratio without linkage

1:1:1:1 in a dihybrid test cross

recombination frequency

The percentage of recombinant offspring produced in a cross

Maximum recombination frequency

50%

Why recombination frequency ≤ 50%

At 50%, genes behave as if independently assorting

Parental gamete frequency

Always greater than or equal to 50% when genes are linked

Relationship between distance and recombination

Greater distance = higher recombination frequency

Relationship between distance and recombination (short distance)

Smaller distance = lower recombination frequency

gene map

A representation of the relative positions of genes on a chromosome

locus

The specific location of a gene on a chromosome

Map unit (m.u.)

A unit of distance between genes based on recombination frequency

CentiMorgan (cM)

Another term for map unit

Map unit definition

1% recombination = 1 map unit (1 cM)

Two-point test cross

A cross used to determine recombination frequency and distance between two genes

Map distance formula

(Number of recombinant offspring / Total offspring) × 100

Purpose of two-point test cross

Determines distance between genes but NOT gene order

Three-point test cross

A cross involving three genes used for more accurate mapping and determining gene order

Double crossover (double recombinant)

A rare recombination event involving two crossovers

Frequency of double recombinants

Lowest frequency class of offspring

Parental classes (3-point cross)

Highest frequency offspring

gene order determination

The gene that differs between parental and double crossover classes is in the middle

double crossover importance

used to determine gene order