gene linkage and genetic mapping

Gene Linkage and Genetic MappingJan Kevin FernandezDanriel DospueblosMeryl Marie Chua

Trishia Aminah DilleraHarim Quadrillero

Definition:– Genetic mapping: determining the relative

position of genes along a chromosome– Gene linkage: the tendency of genes located in the

same chromosome to be associated or inherited together during meiosis.

• Genetic mapping is important in human genetics

Thomas Hunt Morgan– Found out that the two

alleles (white eyes mutation & miniature wings mutation) present in the X chromosome of female fly tend remain together in inheritance

• Recombination: the exchange of parts between DNA molecules or chromosome. The term used for “crossing over"

• The use of slash or virgule (/) denotes which alleles are present in the parental chromosomes

AA BB x aa bbA B / a b

• Coupling or cis configuration• Repulsion or trans configuration

• Frequency of recombination: the proportion of gametes carrying a combination of alleles that are not present in the parental chromosomes– Genes with recombination frequency smaller than

50% are linked.– It is same in either coupling or repulsion; also

either in homologous or in heterozygous

Recombination results from crossing-over between linked

alleles

By: Dospueblos

• Recombination – a chromosome that result from crossing over and that

carriers a combination of alleles differing from that of either chromosome participating in the crossover

• Genetic map – A diagram of the order of genes in a chromosome in

which the distance between adjacent genes is proportional to the rate of recombination between them

– Also called linkage map or Chromosome map

• Alfred H. Sturtevant – A student of Morgan, who discovered the concept of

genetic map in 1913.

• The Early geneticist– understood that recombination between genes takes

place by an exchange of segments between homologous chromosomes in process now called crossing over

• Map unit– Unit of distance in a genetic map – “One map unit is equal to 1 percent

recombination”• For example:

– Two genes that recombine with a frequency of 3.1 percent are said to be located 3.1 map unit

– Also called a centimorgan abbreviated cM in honor of T. H. Morgan.

– a distance of 3.1 map units therefore equals 3.1 cM and indicates 3.1percent recombination between genes.

• As the frequency of recombination- in this case 0.0031

• As the percent recombination or 3.1 percent • As the distance in map units- in this example,

3.1 map units• As the distance in centimorgans or 3.1

centimorgans (3.1 cM)

Physically, one map unit corresponds to a length of the chromosome in which, on the average, one crossover is formed in every 50 cells

In situations in which there are genetic markers along the chromosome, such as the A, a and B, b pairs of alleles in Fig. 4.6 recombination between the marker genes takes place only when a crossover between genes.

Fig. 4 Illustrates a case in which a crossover takes place between the gene A and the centromere takes place between the genes A and B.

The result is that there is a physical exchange between the marker genes, but the double crossover remains undetected because the markers themselves are not recombined.

• The map distance between two genes equals one-half of the average number of crossovers that take place in the region per meiotic cell; it is a measure of crossing over.

• The recombination frequency between two genes indicates how much recombination is actually observed in a particular experiment; it is a measure of recombination.

Physical Distance is often- but not always- correlated with map distance

• The greater the physical separation between genes along a chromosome, the greater the map distance between them.

• Physical distance and genetic map distance are usually correlated, because the greater distance between genetic markers affords a greater chance for crossover to take place.; crossing over is a physical exchange between the chromatids of paired homologous chromosomes.

One crossover can undo the effects of another

• When two genes are located far apart along a chromosome, more than one crossover can be formed between them.

• The probability of multiple crossover increases with the distance between genes.

• “ no matter how far apart two genes may be, the maximum frequency of recombination between any two genes is 50 percent”

Recombination is initiated by a double-stranded break in DNA.

By: Chua

Recombination

• It is a complex process that involves alignment of two homologous DNA strands, precise breakage of each strand, equal exchange of DNA segments between the two strands, and sealing of the resultant recombined DNA molecules through the action of enzymes called ligases.

• It occurs when two molecules of DNA exchange pieces of their genetic material with each other.

• Chiasma - point where two homologous non-sister chromatids exchange genetic material during crossover

• Double strand breaks (DBS) – when this happens, a homologous chromosome can serve as the template for synthesis of whatever portion of the genetic material has been lost as a result of the break

• In homologous recombination, two double helices align and are nicked. Then RecA catalyzes the invasion of each double helix by one strand of the other. This forms a crossed structure called a Holliday junction, and was named after Robin Holliday.

• The breakage and rejoining is an enzymatic function carried out by an enzyme called Holliday junction-resolving enzyme.