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
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.
show Punnett squares and pedigrees.
Inheritance of cystic F
brosis from carrier parents
illustrates autosomal recessive inheritance. (
) Each child has a 25%
chance of being una±
ected and not a carrier, a 50% chance of being
ected carrier, and a 25% chance of being a±
ected. Sexes are
ected with equal frequency. A Punnett square (
) and a pedigree (
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).
Inheritance of Huntington disease from a parent
who will be a±
ected in middle age illustrates autosomal dominant
) A person with just one HD allele develops the disease.
A Punnett square (
) and pedigree (
) 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.
= wild type
cf = cystic fibrosis
50% chance unaffected carrier
(cf allele inherited
from either parent)
For each child conceived:
= wild type
HD = Huntington