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11.2 Complex Patterns
of Inheritance
Textbook pages 302-310
Main Idea
Complex inheritance of traits
does not follow inheritance
patterns described by
Mendel.
Incomplete Dominance
• Definition – the heterozygous phenotype is
an intermediate phenotype between the
two homozygous phenotypes
• Example:
– Red flowered snap dragons (RR) are crossed
with white flowered snap dragons (rr), the
heterozygous offspring is pink (Rr)
– F1 self-fertilized, the flowers were red, pink,
and white (ratio of 1:2:1)
Codominance
• Definition - both alleles are expressed in
the heterozygous condition.
• Recall: When an organism is
heterozygous for a trait, the dominant
phenotype is expressed.
• Example
– Sickle Cell Disease
Sickle Cell Disease
• Sickle cell allele common in people of
African descent
• About 9% of African Americans have one
form or another of the trait.
• Affects red blood cells and their ability to
transport oxygen
• Changes in hemoglobin cause a “C” shape
• Heterozygous - normal and sickle shaped
cells
Sickle Cell Disease and Malaria
• In Africa, those who are heterozygous for
the sickle cell trait have a higher
resistance to malaria.
• Malaria death rate is lower where sickle
cell is higher
• Lower malaria rate, allows for people with
sickle cell to live and reproduce, passing
the trait to their offspring.
• Consequence = sickle cell continues to
increase in Africa.
Multiple Alleles
• Definition – having more than 2 alleles for
a specific trait
• Examples:
– Blood groups in humans
– Coat color in rabbits
Blood Groups in Humans
• ABO blood groups have 3 different forms
of alleles, called AB markers
• IA is type A
• IB is type C
• i is type O (absence of AB markers)
• i is recessive to IA and IB
• Rh factors: Rh+ or Rh-, Rh+ is dominant
Coat Color of Rabbits
• In rabbits, 4 alleles code for coat color: C,
cch, ch, and c
• Allele C is dominant to other alleles,
results in full coat color
• Allele c is recessive, results in albino
phenotype when genotype is homozygous
recessive
• Allele cch is dominant to ch
• Allele ch is dominant to c
• Notation: C > cch > ch > c
• Meaning: full color is dominant over
chinchilla, which is dominant over
Himalayan, which is dominant over albino.
• 4 alleles for rabbit coat color produce 10
possible genotypes and 4 phenotypes
• More variation in coat color come from the
interaction of the color gene with other
genes, agouti or broken gene
Epistasis
• Definition – interaction between alleles in
which one allele hides the effects of another
allele.
• Example:
– Labrador coat color is controlled by 2 sets of
alleles
– Dominant allele E determines if coat will have
dark pigment
– Dominant allele B determines how dark pigment
will be
– EEbb or Eebb – chocolate brown coat
– Eebb, eeBb, eeBB – yellow coat (e masks the
dominant B allele)
Sex Determination
• Sex chromosomes – determine gender, X
and Y
• XX = female
• XY = male
• Autosomes – the other 22 pairs of
chromosomes
Dosage Compensation
• Females have 22 pairs of autosomes and
1 pair of X chromosomes
• Males have 22 pairs of autosomes and 1 X
and 1Y chromosome
• X chromosome carries variety of genes
necessary for development of males and
females
• Y chromosome has genes for male
characteristics
• In females, one of the X chromosomes
stops working called dosage
compensation or X-inactivation, occurs
randomly and in all mammals
• Due to the Human Genome Project, the
National Institute of Health (NIH) believe
some genes on the inactivated X
chromosome are more active than
previously thought.
Chromosome Inactivation
• Calico cat coat color caused by random
inactivation of a particular X chromosome
• Colors depend on active X chromosome
• Orange formed by inactivation of X
chromosome carrying black coat color
allele
• Black formed by activation
of X chromosome carrying
orange coat color allele
Barr Bodies
• Murray Bar (1949) observed inactivated X
chromosome in female calico cats
• Barr body – darkly stained, condensed
inactivated X chromosome in nucleus
• Only females have Barr bodies in their
nuclei
Sex-Linked Traits
• Definition – traits controlled by genes
located on X chromosome, also called Xlinked traits
• Males have 1 X, more affected by
recessive traits more than females
• Females have 2 Xs which mask the effect
of a recessive trait
Red-Green Color Blindness
• Recessive X-linked trait
• 8% of males in the U.S. have red-green
color blindness
• Very rare in females
Hemophilia
• Recessive sex-linked disorder – delayed
clotting of the blood
• More common in males than females
• Queen Victoria of England (1819-1901) –
famous pedigree of hemophilia
• Safer methods of blood transfusion
decreased transmittal of blood-borne
viruses
Polygenic Traits
• Definition – characteristic, such as eye
color, skin color, height, and fingerprint
pattern, that results from the interaction of
multiple gene pairs
• When frequency of dominant alleles is
graphed, result is a bell-shaped curve
Environmental Influences
• Heart disease can be inherited, but also
influenced by diet and exercise
• Environment can influence phenotype
• Examples:
– Sunlight
– Water
– Temperature
Sunlight and Water
• These can affect the phenotype
• Without sunlight, flowering plants don’t
produce flowers
• Without water, plants lose their leaves
• In extreme heat, leaves droop, flower bud
shrivel, chlorophyll disappears, roots stop
growing
Temperature
• Temperature also influences the
expression of genes
• Siamese cat’s tail, ears, nose are dark
because they are cooler than rest of body
• Genes that code for color production only
function under cooler conditions
• Cooler regions = darker fur
• Warmer regions = pigment production
inhibited resulting in lighter fur
Twin Studies
• Studying inheritance patterns in twins
helps scientists separate genetic
contributions from environmental
contributions
• Identical twins are genetically the same
and will both have the same trait
• If traits are expressed differently, can be
result of influence by environment
• Concordance rate - % of twins who
express a given trait
• A large difference between fraternal twins
and identical twins shows a strong genetic
influence.