HUMAN BIOLOGY

iNtroDUCtioN to GeNetiCs 375

wHy does a gamete get only one copy of

eacH gene?

• The two copies of each chromosome in a diploid organism
  separate from each other (segregate) during meiosis in germ
  cells, so each copy ends up in a different gamete.


• Because a gamete has only one copy of each chromosome,
  it contains only one copy of the genes on chromosomes.

taKe-Home messaGe

n We inherit pairs of gene alleles on pairs of chromosomes,

but a gamete receives only one gene from each pair.

n Links to Stages of meiosis 18.7, 18.8

As you’ve just read, parents may have the same or different
alleles for a given gene. In their children, traits that result
from the particular combination of parental alleles provide
evidence of basic patterns of inheritance.
Consider, for example, the “dimple” in actor Viggo
Mortensen’s chin, a trait called a chin fissure (Figure 19.3).
The configuration of a person’s chin is governed by a gene
that has two allele forms. One allele, which calls for a chin
fissure (actually an indentation in the skin), is dominant
when it is present. We can use C to represent this allele. The
recessive allele, which codes for a smooth chin, is c. The
diagram below shows the genotypes gametes can have
when one parent is homo zygous for the C form of the gene
and has a chin fissure, and the other is homozygous for the
c form and has a smooth chin. The CC parent’s gonads
make only C gametes while the cc parent makes only
c gametes:

Parents:

CCc c

CC cc

Gametes:

(meiosis) (meiosis)

© Cengage Learning

one Chromosome, one Copy of a Gene

Figure 19.4 Each pair of gene alleles is separated and the two

alleles end up in different gametes. Due to this segregation,

two parents that are each homozygous for a different version of

a trait will have only offspring who are heterozygous for that trait.

(© Cengage Learning)

meiosis

I

homozygous

dominant parent

C

C

C

C C

C

C

C

C

C

CCCC

meiosis

II

C

c

homozygous

recessive parent

c

c

c

c

c

c

c

c

c

c

c ccc

(chromosomes

duplicated

before meiosis)

(gametes) (gametes)

fertilization

produces

heterozygous

offspring

Figure 19.3 The trait called a chin fissure arises from

one allele of a gene. A Actor Viggo Mortensen received

a gene that influences this trait from each of his parents.

At least one of those genes was dominant. B What

Mr. Mortensen’s chin might have looked like if he had

inherited identical alleles for “no chin fissure” instead.

A B

Lisa O’Connor/ZUMA/Corbis

19.2

segregation The separation

of pairs of gametes during

meiosis.

chromosomes—as it must do to reduce the diploid num-

ber of chromosomes to the haploid number—the pairs

of alleles on those chromosomes

also are separated and each one

ends up in a different gamete. This

separation of gamete pairs is called

segregation.

Each gamete gets only one allele for the trait because
each gamete has only one copy of each chromosome.
This process was described in Section 18.7, and Fig-
ure 19.4 will remind you how it works. To summarize what
the diagram shows, when meiosis separates homologous

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