The number of genes within a cell vastly surpasses the count
of chromosomes; in fact, each chromosome holds thousands of genes. Genes
situated closely together on the same chromosome, referred to as linked genes,
tend to be inherited as a group, a phenomenon known as linkage. For instance,
Thomas Hunt Morgan's research on fruit flies, which commenced in 1909 at
Columbia University in New York City, unveiled genes inherited together.
In one of his experiments, Morgan bred a purebred fruit fly
variety with a gray body (G) and normal-length wings (W) with another purebred
variety bearing a black body (g) and significantly shortened wings (w).
Essentially, he crossed GGWW flies with ggww flies, yielding offspring with a
genotype of GgWw in the F1 generation. He subsequently mated the GgWw flies
with ggww flies, as illustrated in the figure. Surprisingly, Morgan's results
deviated significantly from what the law of independent assortment would
predict. So, what occurred?
From his data, Morgan deduced that the genes governing body
color and wing length resided on the same chromosome, and thus, they did not
assort independently during meiosis. Instead, they were inherited together.
Genes positioned on the same chromosome that do not assort
independently are considered genetically linked. All genes residing on the same
chromosome collectively form a linkage group.
In humans, an estimated 100,000 genes are distributed across
23 pairs of homologous chromosomes. On average, each of our 23 chromosomes
contains approximately 4,348 genes. This arrangement results in 23 linkage
groups. For example, genes responsible for conditions such as color blindness,
hemophilia, and gout form a linkage group on the X-chromosome. On chromosome
number 11, genes associated with sickle cell anemia, leukemia, and albinism
constitute another linkage group. Genetic linkage serves to limit the
likelihood of genetic recombination and variation among offspring.
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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. |
