920
UNIT SIX
often than females, discussed in the section “Genes on
the Sex Chromosomes.”
2. A person most likely inherits a recessive condition
from two parents who are heterozygotes (carriers). The
parents are usually healthy. For this reason, recessive
conditions can “skip” generations.
3. A person who inherits a dominant condition has
at least one affected parent. Therefore, dominant
conditions do not skip generations. (An exception is if
the dominant allele arises, as a new mutation, in the
sperm or egg.) If, by chance, a dominant trait does not
appear in a generation in a particular family, it does
not reappear in subsequent generations, as a recessive
trait might.
Cystic ± brosis (CF) is an example of an autosomal reces-
sive disorder. The wild type allele for the gene, which is
dominant over the disease-causing allele, speci± es formation
of chloride channels built of protein in the cell membrane of
cells lining the pancreas, respiratory tract, intestine, testes,
and other structures (see ± g. 24.2). Certain recessive mutant
alleles disrupt the structure and/or function of chloride chan-
nels. An individual who inherits two such mutant alleles has
cystic ± brosis and is homozygous recessive. A person inherit-
ing only one recessive mutant allele plus a dominant wild type
allele is a carrier and transmits the disease-causing allele in
half of the gametes. A person who has two wild type alleles
is homozygous dominant and does not have or carry CF. The
three possible genotypes are associated with only two pheno-
types, because carriers and homozygous dominant individuals
do not have the illness.
Using logic, understanding how chromosomes and
genes are apportioned into gametes in meiosis, and know-
ing that mutant alleles that cause CF are autosomal reces-
sive, we can predict genotypes and phenotypes of the next
generation.
Figure 24.4
illustrates two carriers of CF. Half
of the man’s sperm contain the mutant allele, as do half of
the woman’s eggs. Sperm and eggs combine at random, so
each offspring has a
25% chance of inheriting two wild type alleles (homozy-
gous dominant, healthy, and not a carrier)
50% chance of inheriting a mutant allele from either
parent (heterozygous and a carrier, but healthy)
25% chance of inheriting a mutant allele from each par-
ent (homozygous recessive, has CF)
Genetic counselors use two tools to explain inheritance to
families: Punnett squares and pedigrees. A Punnett square is
a table that symbolizes the logic used to deduce the probabili-
ties of particular genotypes in offspring. The mother’s alleles
(for a particular gene) are listed atop the four boxes compris-
ing the square, and the father’s alleles are listed along the left
side. Each box records an allele combination at fertilization.
(When more than one gene is considered, the Punnett square
has more boxes.)
The particular combination of gene variants (alleles) in
a person’s genome constitutes the
genotype
(je
no-tıˉp). The
appearance or health condition of the individual that develops as
a result of the ways the genes are expressed is termed the
phe-
notype
(fe
no-tıˉp). An allele is
wild type
if its associated pheno-
type is either normal function or the most common expression
in a particular population. Wild type is indicated with a + sign.
An allele that is a change from wild type, perhaps producing an
uncommon phenotype, is
mutant.
Disease-causing alleles are
mutant. Some mutations, however, are advantageous, as the
vignette in chapter 3 on page 76 describes.
Dominant and Recessive Inheritance
For many genes, in heterozygotes, one allele determines
the phenotype. Such an allele whose action masks that of
another allele is termed
dominant.
The allele whose expres-
sion is masked is
recessive.
For genes with only two alleles,
the dominant ones are usually indicated with a capital letter.
An allele that causes a disease can be recessive or domi-
nant. It may also be
autosomal
(carried on a nonsex chromo-
some) or
X-linked
(carried on the X chromosome) or
Y-linked
(carried on the Y chromosome). The mode of inheritance
refers to whether a trait is dominant or recessive, autosomal
or carried on a sex chromosome. This designation has impor-
tant consequences in predicting the chance that offspring
will inherit an illness or trait. The following rules emerge:
1. An autosomal condition is equally likely to affect either
sex. X-linked characteristics affect males much more
123
4
5
678
1
0
1
1
91
2
13
14
15
16
17
18
19
20
21
22
Sex chromosomes
(female)
FIGURE 24.3
A normal human karyotype has the 22 pairs of
autosomes aligned in size order, plus the sex chromosomes. The
individual represented by this karyotype is female, with two
X
chromosomes.
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