HUMAN BIOLOGY

wHat is gene linKage?

• Linkage refers to the physical distance between genes on a
  chromosome.


• Because chromosomes exchange segments during meiosis
  (the process of crossing over), the distance between genes is
  an important factor in determining whether particular genes
  end up in the same or different gametes.

taKe-home messaGe

n Chapters 18 and 19 provided general information about

chromosomes and what happens to them during meiosis.

We can relate this information to some common patterns

of heredity.

n Links to Chromosome structure 18.1, Meiosis 18.6–18.8,

Concepts of heredity 19.1

understanding inheritance starts

with gene–chromosome connections

Chapters 18 and 19 discussed how chromosomes carry

genes and outlined basic “rules” for how genes are passed

from one generation to another. To recap some of what you

have learned up to this point:

1. Each gene has a particular location (its locus) on a
   specific chromosome.


2. A diploid cell (2n) has pairs of homologous chromo-
   somes, one from the mother and one from the father.
   Except for the sex chromosomes (X and Y), the
   chromosomes of each pair are alike in length, shape,
   and the genes they include.


3. During meiosis in germ cells, homologous chromo-
   somes line up together, then later separate from each
   other. While they are lined up, they may exchange
   corresponding segments. This exchange of segments
   and their genes is called crossing over.


4. There may be two or more versions (alleles) of a
   given gene, but a diploid cell can have only two of
   them, one on each member of a pair of homologous
   chromosomes.
   5. In general, each gene on a chromosome is sorted into
   gametes indepen dently of the chromosome’s other
   genes. This is the process geneticists call independent
   assortment.
   Independent assortment helps explain why even close
   relatives have such a varied mix of genetic traits. Even so,
   however, there are exceptions to the “rule” of independent
   assortment, and they result in some traits often being
   inherited together.

some traits often are inherited together

because their genes are physically linked

Although most genes on a

chromosome do sort into gam-

etes independently, others are

physically connected. When the

distance between two genes

is short, we say there is close

linkage between them. Closely

linked genes nearly always end

up in the same gamete. On the

other hand, when two genes

on a chromosome are far apart,

it is more likely that crossing

over will break up the linkage.

Those genes are less likely to

stay together as gametes form

(Figure 20.1). The patterns in

which genes are distributed

into gametes are so regular

they can be used to map

the positions of the genes

on a chromosome. The

simplified map of human

chromosome 7 in Fig-

ure 20.2 shows the loca-

tion of the cystic fibrosis

gene (CFTR).

a review of Genes and Chromosomes

F i g u r e 20.1 Closely linked genes tend to stay together when

meiosis sorts genes into gametes. (© Cengage Learning)

meiosis, gametes form

AC ac

Most gametes have

parental genotypes

Some gametes have

new gene combinations

C

C

A

A c

c

a

× a

C cc

A A

C

a a

Genes

far apart

in parents

Gametes

Figure 20.2 The gene for cystic

fibrosis has been mapped to

human chromosome 7.

(© Cengage Learning)

location of

CFTR gene

Human chromosome 7

1.120.1

386 Chapter 20

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