Biology (Holt)

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