How Do Linkage Groups Affect Genetic Inheritance?
Linkage can be defined as the tendency of certain genes to stay together during chromosomal inheritance. The genes that are inherited together and located on the same chromosome are called linked genes and the genes that are located on different chromosomes and show independent assortment are called non-linked genes. Linked genes do not show independent assortment at the time of gamete formation.
Linkage was first discovered by Bateson and Punnett in 1906 in sweet pea. Although, the term linkage was first coined by Morgan.
What is a Linkage Group?
In order to define linkage groups or specifically define linkage in biology, one has to understand that genes are located on the chromosomes. These genes can be specific markers that are located on the chromosomes. These also result in certain phenotypes i.e. physical characteristics such as long, short, round, rough, etc.
These genes, according to Mendel’s Laws of Inheritance, are typically known to assort independently of each other. But some of the phenotypes are known to be combined together as they appear in the species along with one another. Genes that are present on a homologous pair of chromosomes are known as linkage groups and they inherit together except for crossing over. The number of linkage groups present in an organism corresponds to the haploid number of chromosomes in the organism. For example, Drosophila melanogaster consists of 4 pairs of chromosomes and thus has 4 linkage groups. When two or more genetic markers are present physically near to each other on a chromosome and are highly unlikely to be separated on different chromatids when there is chromosomal crossover while the cell is undergoing meiosis cell division, they are said to be linked together with one another.
Linkage Group in Biology
The discovery of linkage groups clarified the reason why certain traits are usually seen to be inherited together. This work provided proof of the concept that genes are physical structures that are related by a unit of physical distance.
This unit of physical distance is centimorgans (cm). A distance of 1 cm is said to be the separation of two different markers per 100 meiotic product or 50 meiosis cycle. These linkage groups and linkage concepts are used to construct linkage maps that show the relative distances between two markers.
Linkage Maps and Linkage Analysis
A linkage map is also known as a genetic map. Such a map is a tabular representation of a species or experimental population which states that the position of the known genes or genetic markers is relative to each other in terms of frequency of recombination instead of a specific physical distance along each of the chromosomes. One of the first such linkage maps to be developed was prepared using the linkage group in Drosophila. A linkage map is prepared on the basis of the frequencies of the recombination event between two or more markers during the crossing over of the homologous chromosomes.
Based on the concepts that define linkage in biology, there is a method of linkage analysis that is used to search for the segments of chromosomes that usually segregate together with a specific phenotype through the generations of the same family. Linkage analysis can also be used to determine the linkage maps in cases of both binary and quantitative traits. But there are certain limitations to the method of linkage analysis.
Although the linkage analysis has been successful in identifying genetic variants in human beings, via the different number of linkage groups in humans, that are the cause of rare disorders like Huntington’s disease, it has failed itself when it is applied for more common disorders like heart disease and different forms of cancer. An explanation for this sort of occurrence is that the genetic mechanisms that play a part in common disorders are different from the mechanisms that play a role in rare disorders.
Common Example of Linkage Groups - Sex Linkage
Sexual phenotypes or sexual characteristics are one prominent example that can be used to state linkage and linkage groups. This concept of sex linkage can explain the linkage group in human male and female and provide explanations for the characteristics to be transferred and carried as linkage groups. Sex linkage is the concept in which certain characteristics or phenotypes can be linked to one sex. The complete set of genes of the X-chromosome is carried together in both human beings and Drosophila flies, while the Y-chromosomes carry only a few genes together. Hence, the linkage group in human males is relatively small as compared to the linkage group in human females.
It is well-established that the eggs of the female carry the X-chromosome and the sperm cells may carry either X-chromosome or Y-chromosome. When an egg carrying an X-chromosome is fertilised by a sperm carrying another X-chromosome a female is born, and when an egg is fertilised by the sperm carrying a Y-chromosome a male is born. Hence, in a child carrying an XY chromosome pair, any phenotype or trait that is carried by the X-chromosome will be expressed unless and until a corresponding allele is present on the Y-chromosome.
Examples of sex-linked traits in males that follow the linkage group in human males are red and green colour blindness and haemophilia. This is because the phenotypes are controlled by the genes present on the X-chromosome and have a higher frequency of occurrence in males than females because of the absence of a corresponding allele on the Y-chromosome.
Linkage and Crossing Over
Linkage can be described as the tendency of genes in a chromosome to stay together during chromosomal inheritance. Whereas crossing over can be defined as the exchange of genetic materials between the homologous chromosomes that results in a new combination of genes.
Linkage produces parental types and helps to maintain a new improved variety. On the contrary, crossing over results in recombinations and plays a crucial role in evolution.
Significance of Linkage
The significance of linkage is given below:
It helps to restore the parental genes for the coming future generations.
It is useful to maintain the good characters of a newly developed variety.
It plays a pivotal role in determining the hybridisation scope of a particular plant.
FAQs on Linkage Group in Genetics: Definition, Examples & Analysis
1. What is a linkage group in genetics?
A linkage group is the complete set of all genes located on a single chromosome. Because these genes are physically linked, they tend to be inherited together as one unit from parents to offspring. The total number of linkage groups in an organism corresponds to its haploid number of chromosomes (n).
2. How is a linkage group different from linked genes?
The main difference is scale. Linked genes refer to two or more specific genes on the same chromosome that are often inherited together. A linkage group is a broader term for the entire collection of all genes present on that one chromosome. For instance, while two genes for eye colour and wing shape might be linked, the linkage group includes all the thousands of genes on that particular chromosome.
3. How many linkage groups are present in humans?
The number of linkage groups in humans varies by sex due to the sex chromosomes:
- Human females (XX) have 23 linkage groups. This includes 22 autosomal groups and one X-chromosome group (since both X chromosomes are homologous).
- Human males (XY) have 24 linkage groups. This includes 22 autosomal groups, plus one for the X chromosome and one for the Y chromosome, as they are non-homologous and carry different sets of genes.
4. What is the rule or formula to determine the number of linkage groups in an organism?
The number of linkage groups in any organism is determined by a simple rule: it is equal to the number of chromosomes in one haploid set (n). For example, in pea plants (Pisum sativum), which have 14 chromosomes in a diploid cell, the haploid number (n) is 7, so they have 7 linkage groups.
5. Why is linkage considered an exception to Mendel's Law of Independent Assortment?
Mendel's Law of Independent Assortment applies to genes located on different chromosomes, stating they are inherited independently. Linkage is a direct exception because genes within the same linkage group (on the same chromosome) are physically tied together. Therefore, they do not assort independently and are inherited as a single unit, unless separated by the process of crossing over.
6. What is the difference between complete and incomplete linkage?
The key difference lies in the occurrence of crossing over between linked genes:
- Complete linkage is a rare scenario where two genes are so close on a chromosome that crossing over never occurs between them. Only parental combinations of traits appear in the offspring.
- Incomplete linkage is common. The genes are far enough apart for crossing over to sometimes occur, resulting in a mix of both parental and new recombinant combinations in the offspring.
7. How does crossing over affect genes within a linkage group?
Crossing over is the mechanism that can break up a linkage group during meiosis. When homologous chromosomes exchange segments, it can separate genes that were previously linked. The farther apart two genes are on a chromosome, the higher the probability that a crossing over event will occur between them, leading to new genetic combinations (recombinants). This frequency is used to create genetic maps.
8. What are linkage maps and what is their significance?
A linkage map (or genetic map) is a diagram that illustrates the linear order and relative distances between genes on a chromosome. These maps are not based on physical distance but on recombination frequencies. Their significance is immense as they help geneticists:
- Understand the genetic architecture of an organism.
- Predict inheritance patterns for multiple traits.
- Identify the locations of genes that cause genetic disorders.
9. How did studies on Drosophila and pea plants provide evidence for linkage groups?
Early genetic studies provided crucial evidence for linkage groups:
- In Drosophila melanogaster (fruit fly), Thomas Hunt Morgan observed that traits like body colour and wing size were often inherited together, deviating from Mendelian ratios. This showed they were on the same chromosome.
- In sweet pea (Lathyrus odoratus), Bateson and Punnett found that flower colour and pollen shape did not assort independently, which also supported the concept of genes being physically linked on chromosomes.
10. If two genes are on the same chromosome but at opposite ends, will they show genetic linkage?
Technically, genes on the same chromosome always belong to the same linkage group. However, if they are very far apart, the frequency of crossing over between them can be very high (approaching 50%). When recombination frequency is 50%, the genes assort as if they were on different chromosomes. In such cases, while they are physically linked (syntenic), they do not exhibit genetic linkage in inheritance patterns.
