HUMAN BIOLOGY

CeLL reprOduCtIOn 355

What are the tWo types of chromosomes?

• A cell’s DNA is organized into autosomes and sex chromosomes.


• In humans there are twenty-two kinds of autosomes and two
  possible types of sex chromosomes, X and Y.


• Body (somatic) cells have a diploid number of chromosomes—
  that is, they have paired sets of 23 chromosomes, making
  46 chromosomes in all. The chromosomes in each paired set are
  called homologous chromosomes. Gametes have half the diploid
  chromosome number—23—which is called the haploid number
  of chromosomes.


• In a diploid cell the paired set of sex chromosomes may be
  either two Xs or an X and a Y. Females have an XX pair. Males
  have an XY pair.

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material in each new cell. As a result, each cell ends up with
the diploid number of chromosomes.

Gametes have only one set of chromosomes

Unlike a “regular” body cell, a gamete has just one set of
23 chromosomes, instead of 46 paired chromosomes. This
is because the cell division process that creates gametes,
meiosis, halves the diploid num-
ber of chromosomes (2n) to a
haploid number (n). And not
just any half. Each haploid gam-
ete ends up with one partner
from each pair of homologous
parent chromosomes.
In Figure 18.2, each type of
human chromosome has been
lined up with its partner from
the other parent. Pairs 1 through
22 are autosomes: the two
members of each pair are about
the same length and both carry
hereditary instructions for the
same traits. Pair 23 consists of the
sex chromosomes, which determine a person’s biological
sex. The two types of sex chromosomes are denoted X and Y.
A female has two X chromosomes, while a male is XY.

The paired corresponding chromosomes, one from

each parent, are called homologous chromosomes

(Figure 18.2B), or simply homologues (from a Greek word

meaning “to agree”). The X and Y sex chromosomes are

considered to be homologues, even though they are of

different size and shape and for the most part they carry

genetic information for different traits. We will return to

the topic of sex chromosomes in Chapter 20. The rest of

this chapter deals with how cells divide, beginning with an

overview of the cell cycle, mitosis, and meiosis.

B

A

1 2 3 4 5

(^6789101112)
13 14 15
16 17 18
19 20 21 22
23 (XX)
L. Willatt/East Anglian Regional Genetics Service/Science Source
Figure 18.2 Human somatic cells have
46 chromosomes. A In this image, the
presence of pairs of chromosomes (two
of each type) tells you that they came from
a diploid cell. One member of each pair
contains genetic instructions inherited from
the father. The other member contains
instructions from the mother. B Close-up
of a pair of homologous chromosomes
from an animal cell.
Courtesy of Carl Zeiss MicroImaging, Thornwood, NY
autosomes Chromo-
somes other than the sex
chromosomes.
homologous chromosomes
Chromosomes with the
same length, shape, and
set of genes.
haploid Having one of each
type of chromosome occur-
ing in a species; gametes
are haploid.
sex chromosomes
Chromosomes that deter-
mine a person’s biological
sex.
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