Chapter 11
(Part 4)
Human Genetics
Honors Genetics
Ms. Gaynor
REVIEW…Nature and Nurture:
The Environmental Impact on
Phenotype

Departs from simple Mendelian genetics
 phenotype depends on environment
as well as on genotype
Called multifactorial inheritance
 Ex: human fingerprints
hydrangea flowers
Al in soil; need
LOW pH
Add H+ to
soil; need
HIGHER pH
Many human traits follow
Mendelian patterns of
inheritance
 Humans
are not
convenient subjects for
genetic research
 However, the study of
human genetics
continues to advance
 We use pedigrees!
Pedigree Analysis
pedigree
Is a family tree that
describes the
interrelationships of
parents and children
across generations
A
Inheritance patterns of particular traits
can be traced and described using
pedigrees
Ww
ww
Ww ww ww Ww
WW
or
Ww
ww
Ww
First generation
(grandparents)
Ww
ww
Second generation
(parents plus aunts
and uncles)
FF or Ff
Ff
Ff
Third
generation
(two sisters)
ww
Widow’s peak
Ff
No Widow’s peak
Attached earlobe
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
Free earlobe
Figure 14.14 A, B
(a) Dominant trait (widow’s peak)
(b) Recessive trait (attached earlobe)
Pedigrees
 Can
also be used to
make predictions
about future offspring
Recessively Inherited
Disorders
Many genetic disorders are
inherited in recessive manner
 Show up only in individuals
homozygous for the alleles
 Carriers (only for
DOMINANT TRAITS)
 Are heterozygous individuals,
who carry recessive allele but
are show “normal”
phenotype

Cystic Fibrosis
Example of recessive
disorder
 Affect mostly people of
European descent
 Symptoms
 Mucus buildup in the
some internal organs
 Abnormal absorption
of nutrients in the
small intestine

Sickle-Cell Disease


Another recessive disorder
 Affects one out of 400
African-Americans
 Is caused by the
substitution of a single
amino acid in the
hemoglobin protein in
red blood cells
Symptoms
 Physical weakness, pain,
organ damage, and even
paralysis
Dominantly Inherited
Disorders
Some human disorders
 Are due to dominant
alleles
 Example is
achondroplasia
 Form of dwarfism
lethal when
homozygous for the
dominant allele

Another Dominant
Disorder

Huntington’s disease (HD)
 degenerative disease of nervous
system
 No obvious phenotypic effects until
about 35 to 40 years of age
HD
Normal
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n+1
n+1
n1
n–1
n+1
n –1
n
Number of chromosomes
(a)
Nondisjunction of homologous
chromosomes in meiosis I
(b)
Nondisjunction of sister
chromatids in meiosis II
n
Down Syndrome
 Down
syndrome
 Is usually the
result of an
extra
chromosome
21
trisomy
21
Genetic Testing and
Counseling
 Genetic
counselors
 Can provide
information to
prospective parents
concerned about a
family history for a
specific disease
Tests for
Identifying Carriers
 For
a growing number of
diseases
 Tests are available that
identify carriers and
help define the odds
more accurately
 Examples
Tay Sachs & CF
Fetal Testing
 In
amniocentesis
 The liquid that bathes
fetus is removed & tested
 In chorionic villus
sampling (CVS)
 A sample of the placenta
is removed and tested
Can make karyotypes, too!
Newborn Screening
 Some
genetic disorders can
be detected at birth
 Simple tests are now
routinely performed in
most hospitals in the
United States
 Example- PKU test