Understanding Linkage

In linkage, two or more genes are inherited together for more than two generations, but in recombination, genetic material exchange between different organisms creates offspring with the combination of traits.

The chromosomal theory of inheritance came after the rejection of Mendelian genetics due to a lack of advanced technology and physical proof. But further experiments verified Mendel’s results.

A biologist invented the chromosomal theory of inheritance. T.H. Morgan carried out the research and proved the results of the work. As a result, scientists came up with the concept of gene linkage and recombination.

Types of linkage 

Complete linkage

When two or more characteristics pass down from one generation to another in their original or parentage-specific combinations, is complete linkage. These genes do not produce non-parental combinations, and the genes that show these linkages are on the same chromosome.

Incomplete linkage

Genes that produce non-parental combinations display incomplete linkage. The location of these genes on the chromosomes contribute to the deconstruction of chromosomal segments while crossing over.

What is Glycosidic linkage

A glycosidic linkage is a covalent bond that joins one carbohydrate (sugar) molecule to another carbohydrate group.

A glycosidic bond forms between a saccharide’s hemiacetal or hemiketal group and the hydroxyl group of another compound, such as alcohol. A glycoside is a substance with a glycosidic bond.

Compounds having hemiacetal sugar groups and chemical groups other than hydroxyls, such as -SeR (selena glycosides), NR1R2 (N-glycosides), thioglycosides, or even CR1R2 R3 (C-glycosides) are glycosides.

The aglycone is the ROH compound from which the carbohydrate residue removes, while the carbohydrate residue itself is the ‘glycine,’ particularly in naturally occurring glycosides.

What is genetic linkage?

During meiosis, inherited DNA sequences on a chromosome together cause genetic linkage. Two close genetic markers near each other are less likely to separate during the chromosomal crossover, making them more linked. The less likely two genes are to recombine, the closer they are on a chromosome, and different chromosomes’ markers unlink.

Genetic linkage is an exception to Mendel’s Law of Independent Assortment. Scientists conducted the first experiment to show linkage in 1905. At the time, no one knew why certain traits run in the family. Later research revealed genes are physical structures connected by Physical distance.

A centimorgan is a common unit of genetic linkage. One cM between two markers means that the markers separated into different chromosomes on average every one cM between two markers indicates that the markers separated into different chromosomes on average once every 100 meiotic products or once per 50 meiosis.

Linkage and crossing over

Crossing over is the exchange of chromosomal sections that fractures existing linkages and produces new ones, whereas linkage is the tendency of genes to remain intact and passed down through generations.

As linkages increase, they generate parental types and ages. A decrease in the frequency of crossing over between two genes occurs when they are located close together, whereas an increase in the linkage strength between two genes occurs when a chromosome is close.

On the other hand, crossing over serves as a source of variation to create new varieties. Crossing over, or the exchange of segments between non-sister chromatids in homologous chromosomes, happens at the pachytene stage of prophase I during meiosis and always occurs inside linked genes. Crossing over causes the recombination of linked genes, vital in evolution.

Recombination

Recombination of DNA molecules creates new allele combinations. We call it also genetic recombination because it involves the exchange of DNA between two chromosomes or regions of the same chromosome. It occurs in eukaryotes and prokaryotes, increasing genetic diversity by introducing new genes into sexually reproducing organisms. 

Linkage map

A linkage map is a table that shows the location of a species’ or experimental population’s known genes or genetic markers in terms of recombination frequency rather than a particular physical distance along each chromosome. Alfred Sturtevant, a former student of Thomas Hunt Morgan, developed the use of linkage maps.

The frequency of recombination between markers during homologous chromosomal crossing forms a linkage map. Greater recombination (segregation) between two genetic markers implies greater distance—the lower the recombination frequency, the smaller the physical distance between markers. Historically, we use as markers detectable phenotypes produced from coding DNA sequences; later, noncoding DNA sequences such as microsatellites or those causing restriction fragment length polymorphisms (RFLPs) are used as markers.

By looking for genetic linkage between known markers, linkage maps help researchers find new markers, such as genes. Data build linkage groups, a set of linked genes, early in the linkage map development stage. It’s possible to increase the number of markers in a group to cover the entire chromosome. The linkage groups and chromosomes correspond one-to-one in well-studied organisms.

What is Linkage analysis?

Linkage analysis is a genetic technique for searching chromosomal segments associated with disease phenotypes. This method maps binary and quantitative traits to genes. Linkage analysis can be parametric or nonparametric. The LOD score indicates whether a co-segregating disease and marker are due to linkage or chance in a given pedigree. Nonparametric linkage analysis examines the likelihood of an allele being identical by descent to itself.

Conclusion

The close co-location of genes or DNA variations on chromosomes enables genetic techniques to identify them transmitted within families is linkage. The closer genes are to one another on a chromosome, the better chance you can detect this linkage since there is less recombination at meiosis between these genes. So two genes that are likely to transmit together link.

Linkage determines the nature and scope of hybridisation and selection programmes. Linkage reduces the likelihood of gene recombination, which helps preserve the parents’ characteristics. It helps the organism retain parental, racial, and other characteristics. Because of this, it is difficult for plant and animal breeders to combine different characteristics.