Genes responsible for different characters may be located either on the same chromosome or on different chromosomes.
When genes are located on different chromosomes, the characters they control may appear together or separately in the next generation purely by chance.
Such genes follow Mendel’s law of independent assortment, meaning they assort independently during gamete formation.
When genes are located on the same chromosome and are positioned fairly close to one another, they tend to be inherited together.
This tendency of two or more genes present on the same chromosome to be transmitted together is called linkage.
Associated Terms
Associated terms refer to concepts related to genes that occur on the same chromosome and therefore show non-random assortment during meiosis.
Linkage group means all of the genes that are physically located on one particular chromosome and tend to be inherited together
Linkage map is a chromosome map, which is an abstract representation of the positions (loci) of genes on a chromosome, constructed on the basis of recombination (recombinant) frequencies between genes.
Characteristic of Linked Genes
Genes that are present on different chromosomes assort independently, producing a 1 : 1 : 1 : 1 ratio in a test cross.
Linked genes, which are located on the same chromosome, do not assort independently.
Instead, linked genes tend to remain together and are inherited in the same combination as they were present in the parents.
Chromosome Theory of Linkage
Morgan along with Castle proposed the chromosome theory of linkage.
Genes that exhibit linkage are located on the same chromosome.
These linked genes usually remain bound together by the chromosomal material, so they are not easily separated during the process of inheritance.
The distance between linked genes determines the strength of linkage.
Closely located genes show strong linkage, whereas widely separated genes show weak linkage.
Genes are arranged in a linear order along the length of the chromosome.
Types of Linkage
Morgan and his co-workers, through their studies on Drosophila, discovered that there are two types of linkage.
These two types are complete linkage and incomplete linkage.
Complete Linkage
Complete linkage is the phenomenon in which parental combinations of characters appear together for two or more generations in a continuous and regular manner.
In this type of linkage, genes are very closely associated on the same chromosome and therefore tend to be transmitted together without separation.
An example of complete linkage is seen in Drosophila melanogaster, where the genes for bent wings (bt) and shaven bristles (svn) located on the fourth chromosome show complete linkage.
In 1919, T. H. Morgan crossed gray-bodied, vestigial-winged fruit flies (b⁺vg / b⁺vg) with flies having black bodies and normal wings (bvg⁺ / bvg⁺).
The F₁ progeny had gray bodies and normal long wings (b⁺vg / bvg⁺), showing that gray body (b⁺) and normal wings (vg⁺) are dominant characters.
When the F₁ males (b⁺vg / bvg⁺) were backcrossed (test crossed) with double recessive females (bvg / bvg), which had black bodies and vestigial wings, the result was not the expected four types of offspring.
Instead, only two types of progeny were produced:
flies with gray bodies and vestigial wings (b⁺vg / bvg), and
flies with black bodies and normal wings (bvg⁺ / bvg).
The absence of recombinant types and the appearance of only parental combinations demonstrated complete linkage between these genes.
Incomplete Linkage
Incomplete linkage refers to the condition in which linked genes are located far apart on the same chromosome and therefore have a chance of being separated by crossing over; the pattern of inheritance produced is called incomplete linkage.
In incompletely linked genes, the genes do not always remain together because homologous non-sister chromatids may exchange segments of different lengths during meiotic prophase.
This exchange of chromosomal segments between homologous chromosomes is known as crossing over, and it produces new gene combinations.
Incomplete linkage has been reported in female Drosophila and in many other organisms, including tomato, maize, pea, mice, poultry, and humans.
In maize (Zea mays), Hutchison reported a case of incomplete linkage between the alleles controlling seed colour and seed shape.
A maize plant having coloured and full seeds (CS/CS) was crossed with another plant having colourless and shrunken seeds (cs/cs).
The F₁ hybrids showed the coloured and full phenotype and had the genotype CS/cs, indicating that CS is dominant over cs.
When the F₁ hybrid (CS/cs) was test crossed with the double recessive parent (cs/cs), four types of offspring were obtained instead of only two.
The parental combinations (CS/cs and cs/cs), which are expected in complete linkage, appeared in 96% of the progeny.
The recombinant types (Cs/cs and cS/cs), which represent new allele combinations, appeared in 4% of the progeny.
This shows that crossing over occurred in 4% of cases between the linked genes, proving that the genes are incompletely linked.
Significance of Linkage
The phenomenon of linkage is highly significant in living organisms because it reduces the possibility of variability in gametes, except when crossing over takes place, which can create new gene combinations.
References
Verma, P. S., and Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution and Ecology (1st edition). S. Chand & Company Ltd.
Gardner, E. J., Simmons, M. J., and Snustad, D. P. (1991). Principles of Genetics. New York: John Wiley & Sons.
ScienceDirect. Complete Linkage. Topic page in Biochemistry, Genetics, and Molecular Biology.
