newness but remain separable, to be recovered unchanged in a
later generation: clear-cut roundness and clear-cut wrinkledness.

" Mendel noticed another fact. The round-seeded parent had
yellow seed-color, while the wrinkled parent plant had green
seeds. Among the grandchildren four types appeared, with seeds
round yellow and round green, wrinkled yellow and wrinkled
green. Some of you will remember their proportions: 9:3:3:1.
But that is a minor matter. The lever for further insight is the
. . . fact that the parental traits, round and yellow, which came
from one parent, and wrinkled and green which came from the
other, had not always reappeared together in the combination
in which they had been introduced into the cross, but had also
appeared in the new combinations round green and wrinkled
yellow. This fact reveals that each parent does not transmit a
unified lump of hereditary matter, one whose joint consequences
are in one case roundness and yellowness and in the other wrin-
kledness and greenness. Rather it shows that the hereditary
matter of an individual is broken up not only into the two
contributions of his parents, but that each contribution itself
consists of separate and separable units. Thus the concept of the
hereditary make-up as an assembly of many independent units
was born." (p. 62 ).

The unit characters, or the substance that transmits them
from generation to generation, exist in the nuclei of cells as
genes, which are arranged in a linear manner in or on chromo-
somes, like beads on a string. Except in eggs and sperms, all of
the cells of our bodies have chromosomes present in pairs. Con-
sequently the genes for unit characters are present in pairs. The
members of a pair are called alleles. It has been estimated that
human cells contain many thousands of genes.

What, precisely, are chromosomes and genes? We have an
answer to that question from Dr. J. A. Fraser Roberts, ⁷ director,
Clinical Genetics Research Unit, Medical Research Council,
Great Britain: "Chromosomes may be regarded as nucleic acid

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