All of the cells in the body, other than gametes, are somatic cells.
When a cell, such as a somatic cell, contains two sets of chromo-
somes, it is said to be (DIHP loyd). Unlike somatic cells,
human gametes contain only one set of chromosomes (23 total).
When a cell, such as a gamete, contains one set of chromosomes, it
is said to be (HAP loyd). Biologists use the symbol nto rep-
resent one set of chromosomes. The haploid number in a human
gamete can be written as n23. The diploid number in a somatic
cell can be written as 2n46. The fusion of two haploid gametes—
a process called fertilization—forms a diploid zygote, as shown in
Figure 3. A (ZY goht)is a fertilized egg cell, the first cell of
a new individual.
As seen in Table 1,each organism has a characteristic number of
chromosomes. The number of chromosomes in cells is constant
within a species. Fruit flies, for example, have only eight chromo-
somes in each cell. Although most species have different numbers
of chromosomes, some species by chance have the same number.
For example, potatoes, plums, and chimpanzees all have 48
chromosomes in each cell. Many plants have far more chromo-
somes. Some ferns have more than 500. A few kinds of organisms—
such as the Australian ant Myrmecia, the plant Haplopappus
(a desert relative of the sunflower), and the fungus Penicillium
(from which the antibiotic penicillin is obtained)—have only one
pair of chromosomes.
zygote
haploid
diploid
SECTION 1Chromosomes 121
Figure 4 Human
chromosome.As many as
500 chromosomes lined up
end to end would fit in a
0.2 cm space—about the
thickness of a nickel. The
chromosome above has
replicated and consists of
two identical chromatids.
Table 1Chromosome Number of Various Organisms
Organism Number of chromosomes
Penicillium 1–4
Saccharomyces (yeast) 18
Mosquito 6
Housefly 12
Garden pea 14
Corn 20
Adder’s tongue fern 1,262
Frog 26
Human 46
Orangutan 48
Dog 78
Magnification: 12,542