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Topic: Chromosomes
Keyword: HX4042
How Chromosome Number and
Structure Affect Development
Each human somatic cell (any cell other than a sperm or egg cell)
normally has two copies of 23 different chromosomes, for a total of
46 chromosomes. The 23 chromosomes differ in size, shape, and
set of genes. Each chromosome contains thousands of genes that
play important roles in determining how a person’s body develops
and functions. For this reason, a complete set of all chromosomes
is essential to survival.
Sets of Chromosomes
Each of the 23 pairs of chromosomes consists of two homologous
(hoh MAHL uh gus)chromosomes, or homologues (HOH muh logs).
are chromosomes that are similar in
size, shape, and genetic content. Each homologue in a pair of ho-
mologous chromosomes comes from one of the two parents, as
shown in Figure 3.Thus, the 46 chromosomes in human somatic
cells are actually two sets of 23 chromosomes. One set comes from
the mother, and one set comes from the father. A human chromo-
some is shown in Figure 4.
Homologous chromosomes
120 CHAPTER 6Chromosomes and Cell Reproduction
When haploid gametes fuse, they produce a diploid zygote.
Figure 3 Fertilization
Fertilization
Sperm cell
n = 23
Egg cell
n = 23
Zygote
2 n = 46
