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GENE LINKAGE
Every organism possesses numerous characters controlled by thousands of genes,
but the number of chromosomes is limited. .Therefore, each chromosome must carry
many genes on it. All the genes located on the same chromosome are linked to each
other. This phenomenon of staying together of all the genes of a chromosome is called
linkage. Gene linkage is a physical relationship between genes. A chromosome carries
its linked genes en bloc in the form of a linkage group. The number of linkage groups
corresponds to the number of homologous pairs of chromosomes. Man has 23 linkage
groups. Genes for colour blindness, haemophilia, gout etc form one linkage group on
human X - chromosome. Similarly, gene for sickle cell anaemia, leukemia and albinism
make another linkage group on human chromosome 11. Linked genes whose loci are
close to each other do not obey Mendel’s law of independent assortment, because
these cannot assort independently during meiosis. Gene linkage also minimizes the
chances of genetic recombination and variations among ofspring.