The key difference between genetic linkage and linkage disequilibrium is that genetic linkage is the tendency of two or more genes of the same chromosome to remain together in the process of inheritance while linkage disequilibrium is the non-random association of alleles at different loci in a population.
Genetic linkage and linkage disequilibrium are two concepts in genetics that follow non-Mendelian inheritance. Genetic linkage refers to the physical state of being linked in a chromosome. In simple words, it is the process in which genes are closely located in a chromosome so that they are always inherited together into gametes. On the other hand, in population genetics, linkage disequilibrium refers to the non-random association of alleles at different loci in a population. It is the measure of the difference between the observed frequency of a particular combination of alleles at two loci and the frequency expected for the random association.
CONTENTS
1. Overview and Key Difference
2. What is Genetic Linkage
3. What is Linkage Disequilibrium
4. Similarities Between Genetic Linkage and Linkage Disequilibrium
5. Side by Side Comparison – Genetic Linkage vs Linkage Disequilibrium in Tabular Form
6. Summary
What is Genetic Linkage?
Genetic linkage is the phenomenon in which DNA sequences, especially genes, are closely located on a chromosome, always showing their inheritance together during the meiosis phase of sexual reproduction. They are linked genes. Linked genes are defined as two or more genes located in the same chromosome and inherited together during the homologous chromosome separation. These, these linked genes are always inherited together by offspring. Linked genes do not tend to separate during the anaphase 1 and 2 of meiosis during sexual reproduction.
Figure 01: Genetic Linkage
Genetic linkage of these genes can be identified by the test crosses, and it is measured by centimorgan (cM). They are always expressed together in the offspring because linked genes cannot be assorted independently during cell division. In a normal dihybrid cross, when two heterozygotes are crossed with each other, the expected phenotypic ratio is 9:3:3:1. However, if the genes are linked, this expected ratio changes due to the failure of the independent assortment of alleles. If a normal dihybrid cross results in an unexpected ratio, it indicates the genetic linkage.
Furthermore, linked genes show a lower chance for recombination. These genes also do not follow Mendel’s’ law of independent assortment. Hence, it results in different products than usual phenotypes. However, linked genes can become unlinked during meiosis in the process of homologous recombination. When homologous recombination occurs, they exchange segments of chromosomes. This causes the separation of linked genes, allowing them to be inherited independently. If the genes are linked perfectly, it has a zero recombination frequency.
What is Linkage Disequilibrium?
Linkage disequilibrium (LD) is the non-random association of alleles of different loci in a population. In other words, linkage disequilibrium refers to the dependence of gene frequencies at two or more loci. Allelic association and gametic disequilibrium are two synonyms of linkage disequilibrium. It is calculated by the difference between the observed frequency of a particular combination of alleles at two loci and the frequency expected for the random association. Therefore, LD measures the deviation from the expectation of non-association along the genome. When there is linkage disequilibrium, the frequency of association of the different alleles is higher or lower than the expected frequency (when they are independent and associated randomly).
Figure 02: Linkage Disequilibrium
There are many factors affecting the linkage disequilibrium. Some are selection, rate of genetic recombination, mutation rate, genetic drift, the system of mating, population structure, and genetic linkage. In genetic studies, linkage disequilibrium is an important concept since it is useful for identifying or localizing genes related to disease susceptibility. Moreover, LD is a powerful signal of the population genetic processes, especially in the history of human populations. LD is also useful in mapping complex disease or trait-associated genes.
What are the Similarities Between Genetic Linkage and Linkage Disequilibrium?
- Genetic linkage and linkage disequilibrium are two concepts that aid in genetic studies in order to study genes that are responsible for diseases.
- Both concepts do not follow Mendel’s laws.
- These concepts are related to alleles at loci in the chromosomes.
- Moreover, they are important in order to study gamete formation during sexual reproduction.
What is the Difference Between Genetic Linkage and Linkage Disequilibrium?
Genetic linkage is the process in which genes or DNA sequences are situated very closely in a chromosome. Meanwhile, linkage disequilibrium is the difference between the observed frequency of a particular combination of alleles at two loci and the frequency expected for the random association. So, this is the difference between genetic linkage and linkage disequilibrium.
Moreover, genetic linkages are important in determining the nature of the scope of hybridization and selection programs and maintaining parental lines. Meanwhile, linkage disequilibrium is useful for identifying or localizing genes related to disease susceptibility, evaluating population genetic processes, especially in the history of human populations and in mapping the complex disease or trait-associated genes. Thus, this is also a difference between genetic linkage and linkage disequilibrium.
Summary – Genetic Linkage vs Linkage Disequilibrium
Genetic linkage is the tendency of two or more genes of the same chromosome to remain together in the process of inheritance while linkage disequilibrium refers to the non-random association of alleles at two (or more) loci. Therefore, this is the key difference between genetic linkage and linkage disequilibrium.
Reference:
1. “Linkage Disequilibrium.” Linkage Disequilibrium – an Overview | ScienceDirect Topics, Available here.
2. Khan, Razib. “Basic Concepts – Linkage Disequilibrium.” Discover Magazine, Discover Magazine, 21 May 2019, Available here.
Image Courtesy:
1. “Genetic linkage” Von Myself – Eigenes Werk (Gemeinfrei) via Commons Wikimedia
2. “Linkage and Linkage Disequilibrium” By William S. Bush, Jason H. Moore – (CC BY 3.0) via Commons Wikimedia