Linkage and crossing over

BY S.SRIVIDHYA

Linkage Genes far apart on the same assort independently

are not linkedThe position of the gene – locusOccurs in the prophase of meiosis 1 where

homologous chromosomes break at identical locations and rejoin with each other

Two genes are said to be under linkage, or linked, when they are located on the same chromosome.

Example: peas T=tall; t=short R=red; r=white

Used as a mapping tool and genomic studies

First identified in drosophila (Morgan)Linkage group = number of

chromosomes in one set (n)Human(2n=46) n=23; linkage

group =23Drosophila(2n=8) n=4

Gene mappingGene mapping : how genes are located in a chromosome (linear)

Units :map units – 10 map units apartRecombination frequency – 10%(chances of getting separated)

Types of linkagesComplete linkageDo not get separated very closedExample : drosophila 9:3:3:1 (not) ; 8:8

IncompleteCan separate because distance is more

Crossing Over  Crossing over is a recombination of genes due to exchange of

genetic material between two homologous chromosomes  It is the mutual exchange of segments of genetic material

between non-sister chromatids of two homologous chromosomes, so as to produce re-combinations or new combinations of genes.

It occurs in the pachytene stage, at four strand stage with the help of enzymes (endonuclease, exo-nuclease, R-protein or recombinase;

Stern and Hotta,(1969, 1978). There is breakage of chromatid segments, exchange of nonsister

chromatid segments and later their fusion in new places.

Types of Crossing Over: (i) Single Crossing Over: Crossing over occurs at one point between two non-sister chromatids of a homologous

chromosome pair. There are two parental types and two recombinants, (ii) Double Crossing Over: Crossing over occurs at two points in a homologous pair of chromosomes, (a) Reciprocal Double Crossing Over: Two points of crossing over occur between the same non-sister chromatids, (b) Complementary Crossing Over: The two crossing overs involve three or all the four chromatids so that the number of

cross overs is three or four with occurrenceof one or no parental type,

iii) Multiple Crossing Over: Three or more points of crossing over occur in the same homologous chromosome.

Double cross-overs and parental types may or may not occur.

Factors Influencing Crossing Over (and Linkage):

1. Distance: Nearer the genes lesser the recombination 2. Age: Increase in age decreases the degree of crossing over in

most of the cases. 3. Sex: Male Drosophila shows little crossing over. The

phenomenon of crossing over is quite common in the female fly. Negligible crossing over is also reported in one sex of some other heterogametic organisms.

4. X-Rays: Exposure to X-rays increases the incidence of crossing over. Whittinghill produced a number of cross-overs in male

Drosophila with the help of X-rays. 5. Temperature: Variations in temperature increase the frequency of crossing

over. 6. Chemicals: A number of chemicals present in the food have been found

to change the degree of crossing over in animals.

Is crossing over important for the diversity of biological evolution?

New species are producedMutation and EvolutionFrom single cell to humanVariationStruggle for exhistance

Importance: 1. Crossing over is a means of introducing new combinations of

genes and hence traits. 2. It increases variability which is useful for natural selection under

changed environment. 3. Since the frequency of crossing over depends upon the distance

between the two genes, the phenomenon is used for preparing linkage chromosome maps.

4. It has proved that genes lie in a linear fashion in the chromosome. 5. Useful re-combinations produced by crossing over are picked up

by breeders to develop useful new varieties of crop plants and animals. Green revolution has been achieved in India due to this selective picking up of useful re-combinations. Operation flood or white revolution is also being carried out on the similar lines.