What is the meaning of the term linkage?
When the progeny inherits DNA sequences that are very close together during meiosis, that process is called genetic linkage. When the two genes are very close together, they will not likely be separated during chromosome crossover and so are linked to each other and do not recombine with other genes.
Discovery of linkages:
Linkages are the exception to Mendel’s Law of Independent Assortment, which state that every trait is independently inherited and each trait that is inherited has no connection or linkage with each other. It was found out that this law had exceptions when in 1905, the British geneticists William Bateson, Edith Rebecca Saunders and Reginald Punnett cross-bred pea plants.
They were interested in trait inheritance in the sweet pea and studied the gene for flower colour (P, purple, and p, red) and the gene affecting the shape of pollen grains (L, long, and l, round). They crossed the pure lines PPLL and ppll and then self-crossed the resulting PpLl lines. According to the law Independent Assortment, the result of the crossing should have been in the ratio of 9:3:3:1. Still, it was found that there was a linkage between the combination of PL and pl and these combinations had a greater probability of forming as compared to other combinations because of the process of linkage, and hence the results of the experiment were justified.
Linkage maps:
The idea of linkage maps was first put forward by Alfred Sturtevant, a student of Thomas Hunt Morgan. They are basically tables that show positions of genes of a species which are relative to their recombination frequency which means the more recombination frequency two genes have, the further apart they are going to be on this map and vice-versa.
Scientists initially used detectable phenotypes (enzyme production, eye colour) derived from coding DNA sequences; eventually, confirmed or assumed noncoding DNA sequences such as microsatellites or those generating restriction fragment length polymorphisms (RFLPs) had been used. Linkage maps are also called genetic maps, and they are used to find new genes with linkages so scientists can learn new things about them and expand their knowledge.
What is meant by crossing over?
The exchange of genetic material during sexual reproduction between two homologous chromosomes’ non-sister chromatids results in recombinant chromosomes. It occurs in the pachytene stage of prophase I of meiosis during a process called synapsis. Synapsis begins before the synaptonemal complex develops and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome. The process of crossing over is also called chromosome crossover.
Who discovered the process of crossing over?
The physical basis of crossing over was first demonstrated by Harriet Creighton and Barbara McClintock in 1931. But the concept of crossing over was described by Thomas Hunt Morgan in 1909.
Non-homologous crossover:
During DNA replication, each strand of DNA is used as a template for creating new strands. This process results in two identical, paired chromosomes, often called sisters. Sister chromatid crossover involves an exchange of equal amounts of genetic information, but unequal interactions may occur due to sequence mismatch. These are referred to by various names, including non-homologous crossover and result in an insertion or deletion of genetic information into the chromosome. While rare compared to homologous crossover events, these mutations are drastic, affecting many loci simultaneously. The reasons for this event is still unknown and is still being researched by scientists, but so far from what scientists already know, they say one of the most significant factors influencing these events could be the repair of double-strand breaks (DSBs). The presence of transposable elements is another influential element of non-homologous crossover.
Another factor that plays a massive role in influencing the occurrence of non-homologous crossover can be ed. For instance, mismatch repair (MMR) proteins are a well-known regulatory family of proteins responsible for regulating mismatched sequences of DNA during replication and escape regulation. The operative goal of MMRs is the restoration of the parental genotype. As these proteins are responsible for preventing DNA mismatching, any malfunctions or any breakdown in the pathway of such proteins may lead to such a rare and undesirable event.
Difference between crossing over and linkage:
| Crossing over |
Linkage |
- It involves separating the genes
|
- It involves connecting the genes
|
- It involves exchange of segments between non-sister chromatids of homologous chromosomes.
|
- It involves individual chromosomes.
|
- It provides equal frequency of parental
and recombinant types in test cross
progeny |
- Provides higher frequency of parental types than recombinant types in test cross progeny
|
- It increases variability by forming new gene combinations
|
|
The tendency for genes to inherit in the same chromosome is known as linkage. The degree of linkage and the distance between the genes are inversely proportional.Crossing over, on the other hand, occurs when genes on the same chromosome split into multiple gametes.Genetic recombination occurs as a result, resulting in genetic variety.The influence of each method on genes on the same chromosome is the primary difference between linkage and crossing over.
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