Essential Cell Biology 5th edition

672 CHAPTER 19 Sexual Reproduction and Genetics
(A)
F
E
D
C
B
A
f
e
d
c
b
a
on average,
several
crossover events
will occur
between these
two genes
it is unlikely
that a crossover
event will occur
between these
two genes
(B)
f
e
D
C
B
A
F
E
d
c
b
a
Figure 19−29 Genes that lie far enough apart on the same
chromosome will segregate independently. (A) Because several
crossover events occur randomly along each chromosome during
prophase of meiosis I, two genes on the same chromosome will obey
Mendel’s law of independent assortment if they are far enough apart.
Thus, for example, there is a high probability of crossovers occurring in
the long region between C/c and F/f, meaning that a gamete carrying
the F allele will wind up with the c allele as often as it will the C allele.
In contrast, the A/a and B/b genes are close together, so there is only
a small chance of crossing-over between them: thus the A allele is
likely to be co-inherited with the B allele, and the a allele with the b
allele. From the frequency of recombination, one can estimate the
distances between the genes. (B) An example of a crossover that has
separated the C/c and F/f alleles, but not the A/a and B/b alleles.
different gametes (Figure 19−29). We now know, for example, that the
genes for pea shape and pod color that Mendel studied are located on
the same chromosome, but because they are far apart they segregate
independently.
ECB5 e19.29/19.29
Not all genes segregate independently as per Mendel’s second law. If
genes lie close together on a chromosome, they are likely to be inherited
as a unit. For example, human genes associated with red–green colorblindness
and hemophilia are typically inherited together for this reason.
By measuring how frequently genes are co-inherited, geneticists can
determine whether they reside on the same chromosome and, if so, how
far apart they are. These measurements of genetic linkage have been used
to map the relative positions of the genes on each chromosome of many
organisms. Such genetic maps have been crucial for isolating and characterizing
mutant genes responsible for human genetic diseases such as
cystic fibrosis.
Mutations in Genes Can Cause a Loss of Function or a
Gain of Function
Mutations produce heritable changes in DNA sequence. They can arise in
various ways (discussed in Chapter 6) and can be classified by the effect
they have on gene function. Mutations that reduce or eliminate the activity
of a gene are called loss-of-function mutations (Figure 19−30). An
organism in which both alleles of a gene bear loss-of-function mutations
will generally display an abnormal phenotype—one that differs from
the most commonly occurring phenotype (although the difference may
sometimes be subtle and hard to detect). By contrast, the heterozygote,
which possesses one mutant allele and one normal, “wild-type” allele,
generally makes enough active gene product to function normally and
retain a normal phenotype. Thus loss-of-function mutations are usually
recessive, because—for most genes—decreasing the normal amount of
gene product by 50% has little impact.
In the case of Mendel’s peas, the gene that dictates seed shape codes for
an enzyme that helps convert sugars into branched starch molecules. The
dominant, wild-type allele, R, produces an active enzyme; the recessive,
Figure 19−30 Mutations in protein-coding
genes can affect the protein product in
a variety of ways. (A) In this example, the
normal or “wild-type” protein has a specific
function, denoted by the red rays. (B)
Various loss-of-function mutations decrease
or eliminate this activity. (C) Gain-of-function
mutations boost this activity, as shown, or
lead to an increase in the amount of the
normal protein (not shown).
(A) (B) (C)
normal, wild-type
protein
loss-of-function mutations
point mutation truncation deletion
gain-of-function
mutation