5.3: Following Patterns of Inheritance in Humans
pg. 219
Pedigree: is a flowchart that uses symbols to show the inheritance
patterns of traits in a family over many generations.
Genetic studies can not be performed on humans, this limits the
experimentation and the accumulation of data when trying to study
crosses between males and female, and statistical reliability.
Geneticists collect data by studying past generations and their traits
as they are inherited. Symbols are used to study the movement of
these traits that are controlled by a single gene or more than one
gene.
Figure 5.11: A pedigree is used to illustrate the inheritance pattern of a particular trait in
a family. Standard symbols are used to indicate what is known about the trait being
studied. The term affected means an individual who has or shows a particular trait. These
individuals are indicated in a pedigree by darkened or coloured symbols.
Autosomal Inheritance
Autosomal Inheritance: is the inheritance of traits determined by
genes on the autosomal chromosomes.
Autosomal Dominant: is the inheritance of a dominant phenotype
whose gene is on an autosomal chromosome.
Autosomal Recessive: is the inheritance of a recessive phenotype
whose gene is on an autosomal chromosome.
Most traits studied so far come from autosomal genes, each gene
having a dominant or recessive allele. Some autosomal genes are
responsible for inherited genetic disorders, which are either
classified as autosomal dominant or recessive.
** See Table 5.3: Examples of Human Diseases with Autosomal Inheritance
Patterns
Autosomal Dominant Inheritance
Autosomal dominant disorder occurs when the disease-causing
allele is dominant and an individual has one or both copies of the
allele.
Figure 5.12: An unaffected child born of two affected parents indicates autosomal
dominant inheritance.
Huntington Disease, an Autosomal Dominant Disorder
Huntington disease is a genetic disorder. This autosomal dominant
disease, leads to the brain deteriorating over a period of fifteen
years, and death. Symptoms included decreased muscle
coordination, erratic body movements, personality changes, and a
decline in mental abilities.
Each affect individual has at least one allele for the disease. The
only individuals not affected by this disease must be homozygous
recessive, genotype aa.
Figure 5.13: This pedigree shows the inheritance pattern for an autosomal dominant
disorder. Notice that an affected child must have at least one affected parent to be
affected, and that once a trait is no longer in the family, it will not reappear.
Autosomal Recessive Inheritance
Autosomal recessive disorder occurs when the disease-causing
allele is recessive and an individual has both copies of the allele. In
this pattern of inheritance two unaffected parents can have an
affected child. Individuals who are carriers are heterozygous and
they carry the recessive allele, yet are not affected.
Figure 5.14: In autosomal recessive inheritance, if both parents are heterozygous for the
disorder, they will have an affected child.
Cystic Fibrosis, an Autosomal Recessive Disorder.
Cystic Fibrosis is the most common fatal genetic disorder affecting
young Canadians. The symptoms are caused by a defective protein
which disrupts the movement of chloride ions across cell
membranes. Since cystic fibrosis is autosomal recessive disorder, it
can only occur if the person is homozygous recessive.
Heterozygous individuals show no signs of the disease.
Figure 5.15: This pedigree shows the inheritance pattern for an autosomal recessive
disorder. Notice that the appearance of the recessive phenotype can skip generations, and
that two unaffected parents can have an affected child.
Learning Check, questions 17- 22, pg. 223
Genetic Tests
Table 5.4: Types of Genetic Tests
Genetic Counseling
Gene Therapy: A Cure for Genetic Disorders?
Gene Therapy: is a technique aimed at treating genetic disorders
by introducing the correct form of the defective gene into a
patient’s genome.
Review Questions, questions 1 – 13, pg. 227