Linkage
Linkage is a genetic phenomenon where genes that
are located on the same chromosome are often inherited together. This occurs
because chromosomes, rather than individual genes, are segregated independently
during meiosis, so genes on the same chromosome tend to follow each other
during the formation of gametes. The concept of linkage explains why certain
traits are frequently passed on as groups, and it contrasts with Mendel’s
principle of independent assortment, which applies to genes on different
chromosomes.
Key Concepts of Linkage
- Linked Genes: Genes located on the same
chromosome are termed "linked genes." They are physically
connected on the chromosome and thus are usually inherited as a unit,
rather than assorting independently. The closer two genes are on a
chromosome, the less likely they are to be separated by a crossover event,
making them more tightly linked.
- Linkage Groups:
Each chromosome acts as a "linkage group," carrying numerous
genes. In organisms with diploid cells, the number of linkage groups
corresponds to the haploid number of chromosomes. For example, humans have
23 pairs of chromosomes, so there are 23 linkage groups.
- Complete
vs. Incomplete Linkage:
- Complete Linkage:
Occurs when genes are so close to each other on a chromosome that they
are inherited together 100% of the time. This means no crossover occurs
between these genes during meiosis. Complete linkage is rare in nature
and often found in genes that are directly adjacent on the chromosome.
- Incomplete Linkage:
This is more common and occurs when linked genes are separated by some
distance on the chromosome. In this case, crossing over can occur between
the genes during meiosis, allowing for the exchange of genetic material
and the production of recombinant offspring. The probability of crossing
over increases with the distance between the genes.
Types of Linkage
- Coupling (Cis) Configuration: In
this type of linkage, both dominant alleles (A and B) are on one
homologous chromosome, and both recessive alleles (a and b) are on the
other. The arrangement is AB/ab. During inheritance, the dominant and
recessive alleles tend to be passed together.
- Repulsion (Trans) Configuration:
Here, one homologous chromosome has one dominant and one recessive allele
(Ab), while the other chromosome has the opposite configuration (aB). The
arrangement is Ab/aB. This configuration reduces the probability of dominant
and recessive alleles being passed together.
Mechanism of Linkage and Its
Effect on Inheritance
Since linked genes reside on the same chromosome,
they tend to be inherited as a group. For example, if genes A and B are linked,
they are likely to appear together in the offspring, as they don’t assort
independently like genes on different chromosomes. The extent of linkage
depends on the physical distance between genes on the chromosome. Genes that
are very close together have a higher chance of being inherited together, while
genes farther apart have a greater chance of being separated by crossing over.
Linkage Mapping and Genetic
Distance
The study of linkage has enabled geneticists to
create linkage maps (or genetic maps) that represent the positions of genes on
chromosomes. The distances between genes are measured in centimorgans
(cM), a unit that reflects the frequency of recombination between genes. One
centimorgan represents a 1% chance of a crossover occurring between two genes
during meiosis.
High Recombination Frequency: Genes
that are far apart on a chromosome have a higher likelihood of recombination
and, therefore, a higher recombination frequency. These genes will have a
larger distance on a genetic map.
Low Recombination Frequency: Genes
that are close together have a lower chance of recombination, resulting in a
smaller genetic distance.
Linkage maps are important tools in genetics and
are used to locate genes associated with specific traits or diseases.
Historical
Background and Discovery of Linkage
The concept of linkage was first discovered by
British geneticists William Bateson, Edith Rebecca Saunders, and Reginald
Punnett in the early 20th century. While experimenting with sweet peas, they
observed that certain combinations of traits did not assort independently, as
Mendelian genetics predicted. Later, Thomas Hunt Morgan and his students
expanded on this work using Drosophila melanogaster (fruit flies), a
model organism in genetics. Morgan’s work led to the discovery of crossing
over, a process that could break linkage, providing a basis for genetic
mapping.
Importance
of Linkage in Genetics
Ø Predicting
Inheritance Patterns: Linkage helps predict how certain traits may be
inherited together, especially in selective breeding and studying genetic
diseases.
Ø Linkage
and Disease Genes: Linkage studies have been used to identify genes
associated with genetic diseases. By examining how frequently a disease is
inherited along with certain genetic markers, researchers can identify the
probable location of the disease gene.
Ø Evolutionary
Insights: Linkage affects how traits are inherited in populations, influencing
genetic diversity and adaptation over generations.
