The number of genes within a cell significantly exceeds the
number of chromosomes. Each chromosome contains thousands of genes, many of
which are inherited together due to their close proximity. This phenomenon,
known as genetic linkage, was first identified by Thomas Hunt Morgan
through groundbreaking experiments on fruit flies.
Thomas Hunt Morgan’s Fruit Fly Experiments
In 1909, Thomas Hunt Morgan, working at Columbia University
in New York City, conducted a series of experiments that revealed genetic linkage.
One of his key experiments involved crossing two distinct Drosophila
melanogaster (fruit fly) varieties:
- A
purebred fly with a gray body (G) and normal-length wings (W)
- A
purebred fly with a black body (g) and significantly shortened wings
(w)
By crossing GGWW flies with ggww flies, he
produced F1 offspring with a GgWw genotype. Morgan then performed a test
cross by mating GgWw flies with ggww flies.
Breaking the Law of Independent Assortment
Morgan's results deviated significantly from predictions based
on Mendel’s Law of Independent Assortment. Instead of an even
distribution of traits, certain gene combinations were inherited together more
frequently than expected.
This led Morgan to a crucial conclusion:
- The
genes controlling body color and wing length were located on
the same chromosome and did not assort independently.
- Instead,
they were inherited as a unit due to genetic linkage.
What Are Linked Genes?
Genes that are positioned on the same chromosome and
inherited together are called linked genes. All genes residing on a
chromosome collectively form a linkage group.
Genetic Linkage in Humans
In humans, approximately 100,000 genes are spread
across 23 pairs of homologous chromosomes, forming 23 linkage groups.
On average, each chromosome contains around 4,348 genes.
Examples of Human Linkage Groups
- X-Chromosome
Linkage Group: Genes responsible for color
blindness, hemophilia, and gout are linked together on the X
chromosome.
- Chromosome
11 Linkage Group: Genes associated with sickle
cell anemia, leukemia, and albinism are located on chromosome 11.
The Role of Genetic Linkage in Evolution and Variation
Genetic linkage plays a crucial role in limiting the
likelihood of genetic recombination during meiosis. As a result:
- Some
traits tend to be inherited together, reducing genetic variation in
offspring.
- Linkage
patterns influence evolutionary processes by maintaining beneficial gene
combinations.
Final Thoughts
Thomas Hunt Morgan’s discovery of genetic linkage
revolutionized our understanding of inheritance. By demonstrating that genes on
the same chromosome do not always assort independently, his research laid the
foundation for modern genetic mapping and disease gene identification.
Today, genetic linkage studies continue to provide valuable insights into
hereditary conditions and evolutionary biology.
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| Thomas Hunt Morgan found that the gene for body colour (dominant allele G for gray, recessive allele g for balck) was linked to the gene for wing length (dominant allele W for normal length, recessive allele w for greatly reduced length). This linkage occurred because the two genes were on the same chromosome. |
