921
CHAPTER TWENTY-FOUR
Genetics and Genomics
In an autosomal recessive illness, an affected person’s
parents are usually carriers—they do not have the illness.
Or, if the phenotype is mild, a parent might be homozygous
recessive and affected. In an autosomal dominant condi-
tion, an affected person typically has an affected parent. He
or she need inherit only one copy of the mutant allele to
have the associated phenotype; in contrast, expression of
an autosomal recessive condition requires two copies of the
mutant allele.
A
pedigree
is a diagram that depicts family relationships
and known genotypes and phenotypes. Circles are females
and squares are males; shaded-in symbols represent people
who have a trait or condition; half-shaded symbols denote
carriers. Roman numerals indicate generations.
Figures
24.4
and
24.5
show Punnett squares and pedigrees.
FIGURE 24.4
Inheritance of cystic F
brosis from carrier parents
illustrates autosomal recessive inheritance. (
a
) Each child has a 25%
chance of being una±
ected and not a carrier, a 50% chance of being
an una±
ected carrier, and a 25% chance of being a±
ected. Sexes are
ected with equal frequency. A Punnett square (
b
) and a pedigree (
c
)
are other ways of depicting this information. Symbols in the pedigree
with both black and white indicate una±
ected carriers (heterozygotes).
The pedigree illustrates the makeup of one possible family. Each
child is an independent event. The three siblings could have any
combination of genotypes (allele combinations).
FIGURE 24.5
Inheritance of Huntington disease from a parent
who will be a±
ected in middle age illustrates autosomal dominant
inheritance. (
a
) A person with just one HD allele develops the disease.
A Punnett square (
b
) and pedigree (
c
) depict the inheritance of HD. The
pedigree symbols for HD are completely F
lled in to indicate that the
person is a±
ected. Autosomal dominant conditions a±
ect both sexes.
cf
cf
cf
cf
cf
cf
Punnett Square
Carrier parents
++
+
cf
+
cf
cf
cf
+
cf
+
cf
+
= wild type
allele
cf = cystic fibrosis
allele
50% chance unaffected carrier
(cf allele inherited
from either parent)
25% chance
unaffected
noncarrier
(a)
(b)
(c)
25% chance
affected
For each child conceived:
I
II
Mary
Joe
Bill
Sue
Tina
+
= wild type
allele
HD = Huntington
disease
allele
+
HD
+
HD
++
++
Affected parent
50% chance
affected
For each
individual
conceived:
50% chance
unaffected
Unaffected parent
(a)
(b)
(c)
I
II
Ann
Dan
Eric
Pam
HD
HD
HD
Punnett Square
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