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Chapter 8
Section 4: Complex Patterns of Heredity
Grade 10 Biology
Spring 2011
Objectives
Identify five factors that influence
patterns of heredity
 Describe how mutations can cause
genetic disorders
 List two genetic disorders, and describe
their causes and symptoms
 Evaluate the benefits of genetic
counseling

Characters Influenced by Several
Genes

Polygenic Inheritance: when several
genes influence a character
◦ May be scattered along the same
chromosome or located on different
chromosomes
◦ Ex. Eye colour,
height, weight,
hair, skin colour
(have degree of
intermediate
conditions)
Intermediate Characters

Incomplete Dominance: individual
displays a phenotype that is intermediate
between two parents
Intermediate Characters

Incomplete Dominance:
◦ Ex: snapdragons
 Red flower x white flower = pink flowers
 Neither the red or white allele is completely
dominant over another
 Flowers appear pink because they have less red
pigment than red flowers
Three of More Alleles
Multiple Alleles: genes with three or
more alleles
 Ex. ABO blood groups (blood types)

◦ Determined by 3 alleles: IA, IB, i
◦ IA and IB are dominant over i, but not over
each other
◦ Therefore 4 different blood types: A, B, AB, O
Codominance

Codominance: two
dominant alleles are
expressed at the same
time, both forms of
character are displayed
◦ Blood type is an example
◦ Different than incomplete
dominance because both
traits are displayed
Characters Influenced by the
Environment
An individuals phenotype often depends on
conditions in the environment
 Hydrangea flowers display flowers from
blue to pink

◦ Acidic soil = blue flowers
◦ Neutral to basic soil = pink flowers

Ex. Siamese cat, arctic fox, humans
Genetic Disorders
Mutation: changes in genetic material
 Genetic Disorders: harmful effects
produced by inherited mutations

Human Genetic Disorders
Disorder
Dominant or
Recessive
Symptom
Defect
Abnormal hemoglobin
molecule
Frequency
Sickle Cell
Anemia
Recessive
Poor blood circulation
1 in 500
(African
Americans)
Hypercholester
olemia
Dominant
Excessive cholesterol levels Abnormal form of cell
in blood, leading to heart
surface receptor for
disease
cholesterol
1 in 500
Tay-Sachs
Disease
Recessive in
early childhood
Deterioration of central
nervous system; death
Defective form of a brain
enzyme
1 in 3,500
(Ashkenazi
Jews)
Cystic Fibrosis
Recessive
Mucus clogs organs (lungs,
liver, pancreas) Usually do
not survive to old age.
Defective chloride-ion
transport protein
1 in 2,500
(Caucasians)
Hemophilia A
(Classical)
Sex-linked
recessive
Failure of blood to clot
Defective form of a
blood clotting factor
1 in 10,000
males
Huntington’s
Disease
Dominant
Deterioration of brain
tissue in middle age;
shortened life expectancy
Inhibitor of brain-cell
metabolism is made
1 in 10,000
Treating Genetic Disorders

Most cannot be cured
◦ Progress being made
Genetic counseling: form of medical
guidance that informs people about
genetic problems that could affect them
or their offspring
 Some disorders if caught early enough can
be treated

◦ PKU in newborns
Gene Therapy

Gene Therapy: replacing defective genes
with copies of healthy ones
◦ First step is to isolate a copy of the gene
◦ Next step is to transfer a healthy gene into
the body and have it replace the defective
gene
Review
1.
2.
3.
What is the difference between
incomplete and codominance?
What are examples of some genetic
disorders that are recessive?
Why would one utilize genetic
counseling?
Review- Answers
Incomplete dominance is when an
individual displays a phenotype that is
intermediate between two parents.
Codominance is when two dominant
alleles are expressed at the same time,
both forms of character are displayed.
2. Examples of recessive genetic disorders
are: sickle cell anemia, tay-sachs, cystic
fibrosis, hemophilia A
3. One may utilize genetic counseling if they
believe they or their offspring is at risk for
a genetic disorder.
1.