father and half from the mother. For homologous chromosomes, one set comes from
the father and one set from the mother.
The genetic activity conceived by Mendel can therefore be seen to correspond
fully with the chromosomal activity in meiosis. As shown in Fig. 2.5, overlaying
the gene process on the chromosomal process shows no inconsistencies whatsoever
in the genetic explanation. It is for this reason that many early in the 20th century
began conceiving of the gene as existing on the chromosome. In the 1930s, the U.S.
geneticist Morgan used fruit flies (Drosophila melanogaster) to show the location
of many genes on the four (n) chromosomes, including those responsible for eye
color—thus creating a chromosome map of genes. Through this research, it was
learned that genes occupy a defined position on the chromosome (Morgan et al.
1925).
Chromosome maps have been studied from a number of different organisms,
including corn and human beings. Artificial mutations have played an important
role in such research. Studies of the genes responsible for eye color in fruit flies, for
Fig. 2.4 Mitosis (a) and meiosis (b) for somatic cells, where 2n = 4
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2 Introduction to Molecular Biology