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20.Chromosomes And DNA eLearn.Punjab
THE CHROMOSOMAL THEORY OF INHERITANCE

A central role for chromosomes in heredity was first suggested in 1900 by the German
geneticist Karl Correns, in one of the papers announcing the rediscovery of Mendel’s work.
Soon after, observations that similar chromosomes paired with one another during meiosis
led directly to the chromosomal theory of inheritance, first formulated by the American
Walter Sutton in 1902.
Several pieces of evidence supported Sutton’s theory. One was that reproduction involves
the initial union of only two cells, egg and sperm. If Mendel’s model was correct, then these
two gametes must make equal hereditary contributions. Sperm, however, contain little
cytoplasm, suggesting that the hereditary material must reside within the nuclei of the
gametes. Furthermore, while diploid individuals have two copies of each pair of homologous
chromosomes, gametes have only one. This observation was consistent with Mendel’s model,
in which diploid individuals have two copies of each heritable gene and gametes have one.
Finally, chromosomes segregate during meiosis, and each pair of homologue orients on the
metaphase plate independently of every other pair.
There is however one problem with this theory. If Mendelian characters are determined by
genes located on the chromosomes, and if the independent assortment of Mendelian traits
reflects the independent assortment of chromosomes in meiosis, why does the number of
characters that assort independently in a given kind of organism often greatly exceed the
number of chromosome pairs the organism possesses? This has led many early researchers
to have serious reservations about Sutton’s theory.

In 1910 Thomas Hunt Morgan, studying the fruit fly, Drosphila melanogaster, detected a
mutant male fly, one that differed strikingly from normal flies of the same species its eyes
were white instead of red.
Morgan crossed mutant male to a normal female. All F1 progeny had red eyes. He
then crossed red eyed flies from F1 generation with each other. Of the 4252 F2 progeny
Morgan examined, 782 (18%) had white eyes. Although the ratio of red eyes to white
eyes in the F2 progeny was greater than 3:1, the results of the cross nevertheless
provided clear evidence that eye colour segregates.

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