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Chapter 18
Section 18.4
Other Patterns of Inheritance
Other Patterns of
Inheritance
• The traits that Mendel studied showed little
variability.
• Each trait had only two alleles, one that was clearly
dominant and one clearly recessive.
• However, many inherited traits have more than just
two alternative forms.
• There can be other patterns of inheritance.
• Examples:
o Pleiotropic genes
o Multiple alleles
o Incomplete dominance
o Codominance
o Dependence on Environment
Pleiotropic Genes
• Pleiotropic genes – are genes that affect
more than one characteristic.
• Ex. Sickle Cell Anemia
o Sickle cell anemia is a disease that affects the
shape of hemoglobin.
o Hemoglobin carries oxygen within our blood.
o The abnormal shape of the hemoglobin affects the
bodies ability to deliver oxygen.
o As a result, people with sickle cell anemia can
suffer from fatigue, weakness, an enlarged spleen*,
rheumatism*, and pneumonia.
* The spleen removes old red blood cells and holds
a reserve of blood in case of a hemorrhage (loss of
blood).
* Rheumatism is a non-specific term for medical
problems affecting the joints and connective
tissue.
Multiple Alleles
• Some traits have multiple alleles that can determine
the phenotype.
• The most common example is the fruit fly.
• The Drosophila melanogaster or fruit fly has four
possible eye colours.
• When multiple alleles occur, the most commonly
seen trait or dominant trait is called the wild type.
• The other alleles are known as mutants.
• When multiple alleles are present for the same
gene, upper case letters for the gene and
superscript numbers are used to express the
dominance relationships between the alleles.
• The fruit fly alleles are as follows:
Phenotype
Allele Symbol
Possible
Genotypes
Dominant Over
Wild Type (red)
E1
E1E1, E1E2, E1E3,
E1E4
Apricot, honey
and white.
Apricot
E2
E2E2, E2E3, E2E4
Honey and
white.
Honey
E3
E3E3, E3E4
White.
White
E4
E4E4
None.
Incomplete Dominance
• Incomplete dominance – the expression of both
forms of an allele in a heterozygous individual in the
cells of an organism, producing an intermediate
phenotype.
• The most common example are snapdragons
(flowers).
• Snapdragons have three possible genotypes and
phenotypes:
• CRCR = red
• CRCW = pink
• CWCW = white
Codominance
• Codominance – the expression of both forms of an
allele in a heterozygous individual in different cells
of the same organism.
• Both alleles are fully expressed in a heterozygous
individual, but not in the same cells.
• A common example is Shorthorn cattle:
o HrHr = red hair
o HrHw = red and white hair together
o HwHw = white hair
• Another example of codominance is blood type:
o
o
o
o
Type A
Type B
Type AB
Type O
Discuss
Environment and
Phenotype
• Some variation of a trait is determined by the
interaction of the genotype with the environment.
• Himalayan rabbits have black fur when they are
raised at low temperatures, but white fur when
raised at high temperatures.
• Pg. 609 #1
• Pg. 612 #1-3
• Diploma examples