In genetics, linkage refers to the tendency of genes located close to each other on the same chromosome to be inherited together. This principle was first observed by Bateson and Punnett (1905) during their experiments with sweet peas and later explained by Thomas Hunt Morgan (1910) in Drosophila melanogaster.
Linkage is a deviation from Mendel’s Law of Independent Assortment, which states that genes on different chromosomes assort independently. When genes are on the same chromosome, they may not assort independently, leading to linkage.
Types of Linkage
- Complete Linkage
When two or more genes are so close together on the same chromosome that no crossing over occurs between them. They are always inherited together.
Example: Drosophila males show complete linkage as crossing over is absent. - Incomplete Linkage
When genes are close but not inseparable, crossing over occurs occasionally, producing new (recombinant) gene combinations along with parental ones.
Example: Linkage between genes for body color and wing shape in Drosophila.
Linkage Groups
A linkage group is the group of all genes present on the same chromosome that tend to be inherited together. The number of linkage groups in an organism is equal to its haploid chromosome number.
- Pea plant → 7 linkage groups (n=7).
- Human → 23 linkage groups (n=23).
Significance of Linkage
- Helps in the construction of chromosome maps.
- Provides evidence that genes are located on chromosomes.
- Useful in plant and animal breeding to maintain desirable traits.
- Explains deviation from Mendelian ratios.
Estimation of Linkage
The strength of linkage between two genes can be measured by the frequency of recombination (resulting from crossing over).
1. Recombinant Frequency (RF)
Formula:
RF = (Number of recombinant progeny ÷ Total progeny) × 100
- If RF = 50% → Genes are unlinked (assorting independently).
- If RF < 50% → Genes are linked.
- The smaller the RF, the closer the genes are on the chromosome.
2. Coupling and Repulsion Phases
Coupling (cis arrangement): Dominant alleles are on one chromosome and recessive alleles on the homologous one (AB / ab).
Repulsion (trans arrangement): Each chromosome has one dominant and one recessive allele (Ab / aB).
Recognizing these helps in calculating recombination frequencies.
3. Linkage Map and Map Units
A linkage map is a linear representation of gene positions on a chromosome based on recombination frequencies.
1% recombination = 1 map unit (m.u.) or 1 centimorgan (cM).
Example: If two genes show 20% recombination, they are said to be 20 cM apart on the chromosome.
4. Example of Estimation
Suppose a test cross is performed between heterozygous individuals (AB/ab × ab/ab):
- Parental (AB, ab) = 800
- Recombinant (Ab, aB) = 200
- Total = 1000
RF = (200 ÷ 1000) × 100 = 20%
So, the genes A and B are linked and are 20 map units apart.
Linkage is a key concept in genetics that explains why certain traits are inherited together. Its estimation through recombination frequency allows scientists to create genetic maps, which are crucial for understanding chromosome structure, gene positions, and assisting in breeding programs.
