Chapter 7
Extending Mendelian Genetics
Autosomal Recessive Genetic Disorders
 A recessive trait is expressed when the
individual is homozygous recessive for the
trait.
Basic Patterns of Human Inheritance
Affects the mucus-producing glands,
digestive enzymes, and sweat glands
 Chloride ions are not absorbed into the
cells of a person with cystic fibrosis but
are excreted in the sweat.
 Without sufficient chloride ions in the
cells, a thick mucus is secreted.
 Video

Cystic Fibrosis

Caused by altered genes, resulting in the
absence of the skin pigment melanin in
hair and eyes
Albinism
Caused by the absence of the enzymes
responsible for breaking down fatty acids
called gangliosides
 Gangliosides accumulate in the brain,
inflating brain nerve cells and causing
mental deterioration.

Tay-Sachs Disease
Cystic Fibrosis Punnett Square
Autosomal Dominant Genetic Disorders
Huntington’s disease affects the nervous
system.
 Achondroplasia is a genetic condition that
causes small body size and limbs that are
comparatively short.

Basic Patterns of
Human Inheritance
Huntington’s Disease
Punnett Square

A diagram that traces the inheritance of a
particular trait through several
generations
Pedigrees
Inferring Genotypes
 Knowing physical traits can determine
what genes an individual is most likely to
have.
Predicting Disorders
 Record keeping helps scientists use
pedigree analysis to study inheritance
patterns, determine phenotypes, and
ascertain genotypes.

Sex chromosomes determine an
individual’s gender
Sex Determination
Genes located on the X chromosome
 Red-green color blindness
 Hemophilia
 Punnett Square

Sex-Linked Traits
Dosage Compensation
 The X chromosome carries a variety of
genes that are necessary for the
development of both females and males.
 The Y chromosome mainly has genes that
relate to the development of male
characteristics.
 In females, one X chromosome is
inactivated in each cell. The inactivated X
chromosome is visible in stained cells as a
Barr body.

Tutorial
X Chromosome Inactivation
Variety is the result of one allele hiding
the effects of another allele.
 An epistatic gene can interfere with the
expression of other genes.
 Albinism in mammals is caused by an
epistatic gene that blocks the production
of pigments.

Epistasis
Phenotype can depend on interactions of
alleles.
Complex Patterns of Inheritance

The heterozygous phenotype is an
intermediate phenotype between the two
homozygous phenotypes.
Incomplete Dominance
Both alleles are expressed in the
heterozygous condition.
 Sickle-cell disease is one example.
 People who are heterozygous for the trait
have both normal and sickle-shaped cells.

Codominance
Blood groups in
humans
 ABO blood groups
have three forms of
alleles.
 Sometimes said to be
codominant.
 Multiple alleles can
demonstrate a
hierarchy of
dominance.

Multiple Alleles
Polygenic traits arise from the interaction
of multiple pairs of genes.
 Read pg. 206 in textbook.

Polygenic Traits
Environmental factors influence an
organism’s phenotype
 Diet and exercise
 Sunlight and water
 Temperature

Environmental Influences

Read pgs.209-211
Gene Linkage
Karyotype—micrograph in which the pairs
of homologous chromosomes are
arranged in decreasing size
 Images of chromosomes stained during
metaphase
 Chromosomes are arranged in decreasing
size to produce a micrograph.

Karyotype Studies
Is this a
normal
karyotype?
Male or
female?
Is this a normal
karyotype?
Male or
Female?
Is this a normal
karyotype?
Male or
Female?
Cell division during which sister
chromatids fail to separate properly
 Down syndrome, also called trisomy 21

Nondisjunction