Standards:
Gen 2.7 Use Punnett squares to explain
Mendel’s three genetic laws and to predict
offspring in monohybrid and dihybrid
crosses of common genetic traits and
disorders.
Essential Questions:
How did Gregor Mendel establish the basics of
genetics?
• Nearly all disorders and traits once
considered to be caused by single genes
are actually influenced y environmental
factors and usually by the actions of other
genes, too.
Human Alleles
•
Many human genes have become known through the
study of genetic disorders.
• Genetic Disorders can be caused by various “Modes of
Inheritance”
– Autosomal recessive alleles
– Autosomal dominant alleles
– Codominant alleles
• What makes an allele dominant, recessive, or
codominant?
– It all depends on the nature of a gene’s protein product and
its role in the cell.
Concept Map
Section 14-1
Some Single
Gene
Disorders in
Humans
caused by
Autosomal
Recessive
Autosomal
Dominant
include
Galactosemia
Albinism
Cystic
fibrosis
Go to
Section:
include
Phenylketonuria
Huntington’s
disease
Tay-Sachs
disease
Achondroplasia
Hypercholesterolemia
Section 14-1
Figure 14-8 The Cause of Cystic Fibrosis
Chromosome
#7
CFTR
gene
Go to
Section:
The most common
allele that causes
cystic fibrosis is
missing 3 DNA
bases. As a result,
the amino acid
phenylalanine is
missing from the
CFTR protein.
Normal CFTR is a
chloride ion
channel in cell
membranes.
Abnormal CFTR
cannot be
transported to the
cell membrane.
The cells in the
person’s airways are
unable to transport
chloride ions. As a
result, the airways
become clogged with
a thick mucus.
Comparing Dominance & Recessiveness
• Determining whether an allele is dominant
or recessive is critical in medical genetics
because it helps predict which individuals
are at high risk of inheriting a particular
condition (phenotype).
Comparing Dominance & Recessiveness
DOMINANT:
RECESSIVE:
• Can appear in either sex
because an autosome
carries the gene.
•
Can appear in either sex & can skip
generations.
•
Affected individuals have a
homozygous recessive genotype,
whereas in heterozygotes (carriers) the
wild type allele masks expression of
the mutant allele.
•
Parents of an affected individual are
heterozygous or have the trait.
•
Most occur unexpectedly
•
Incest increases the risk of having a
child with an autosomal recessive trait
• If the child has it, then at
least one parent has it.
• Do not skip generations
• If no offspring inherit the
trait in one generation, its
transmission stops
because the offspring can
pass on only the recessive
form of the gene.
Active Reading Strategy:
• Visit the ARIS website at
www.mhhe.com/lewisgenetics8
• Select “Self Study, chapter 4” and “Web
Activities” to find the website links needed to
complete the following activity:
– Go to the website for the National Organization for
Rare Disorders.
– Identify an autosomal recessive disorder and an
autosomal dominant disorder.
– Create a family for each one, and describe
transmission of the disease over three generations.