Redefining Reality - The Intellectual Implications of Modern Science

Third, for sperm and egg cells, the cells contained only one
strand, so that when they combined to form a zygote, the new
potential offspring received one from each of the parents. Thus,
it seemed reasonable to hypothesize that the chromosomes
carried the units of Mendelian inheritance.
Not everyone accepted this chromosomal theory of inheritance. One
of the most vocal and stubborn critics was the American biologist
Thomas Hunt Morgan. If the chromosome theory was correct, Morgan
argued, then the environment had little to do with the determination
of traits. But there seemed to be cases in which the environment had
effects on the properties of the organisms conceived.
Morgan set out to disprove the chromosome theory using fruit
kept track of all the traits of the parents and offspring.
After years of work, however, Morgan realized that no matter
what environmental factors he toyed with, the only thing that
seemed statistically relevant were the properties of the parents,
and that seemed to point to chromosomes.
Morgan and his students examined chromosomes more closely
and found that they had a structure—parts that were termed genes.
A later researcher incorporated that term into two new coinages:
genotype, which refers to the difference in chromosomal makeup,
and phenotype, which refers to the difference in observable
properties of an organism. The question now was: How does a
difference in genotype create a difference in phenotype? In other
words, how does a difference in your chromosomes make a
difference in your body?
Morgan studied this question and found that genes that were close
together on a chromosome often were passed along together; this
of a chromosome, showing that the distance between genes was
important in understanding heritability and birth defects.
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