Complex Patterns of
Heredity
Chapter 12
Sometimes Heredity Follows
Different Rules
• Dominant allele patterns
• Incomplete Dominance: Appearance of a
third phenotype
• Codominance: Expression of both alleles
• Multiple phenotypes from multiple alleles
• Sex determination
• Sex-linked inheritance
• Polygenic Inheritance
• Environmental Effects
Use the textbook to find out information
about the following genetic patterns
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•
•
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Dominant Genetic disorders (pg 298)
Incomplete Dominance (pg 302)
Codominance (pg 302)
Multiple Alleles (pg 304)
• Use two columns – fold paper in half lengthwise
right side – notes
left side - examples
Incomplete Dominance
• With incomplete dominance, a
cross between organisms with
two different phenotypes
produces offspring with a third
phenotype that is a blending of
the parental traits.
– RED Flower x WHITE Flower --> PINK Flower
– The allele for red is not completely
dominant over the allele for white
Codominance
• The genetic gist to codominance is pretty
much the same as incomplete dominance. A
hybrid organism shows a third phenotype --not the usual "dominant" one & not the
"recessive" one ... but a third, different
phenotype.
• In COdominance, there are basically two
“dominant” traits which appear together in the
phenotype of hybrid organisms.
Codominance Cont
• R = allele for red flowers
W = allele for white flowers
red x white ---> red and white
spotted
• RR x WW ---> 100% RW
Codominance
• Example
red x white ---> red & white spotted
With codominance, a cross between
organisms with two different phenotypes
produces offspring with a third phenotype
in which both of the parental traits appear
together.
Examples of Codominance
• A very common phenotype used in
questions about codominance is roan fur in
cattle.
• Cattle can be red (RR = all red hairs), white
(WW = all white hairs), or roan (RW = red
& white hairs together).
• Another example of codominance is human
blood type AB, in which two types of
protein ("A" & "B") appear together on the
surface of blood cells.
Multiple Alleles
• When a trait is controlled by Multiple Alleles
– Controlled by 3 or more alleles of the same gene
that code for a single trait
– Ex. Blood types
• A & B are codominant – both are expressed when
together; and both are dominant to O.
• A person can only have type O blood if they receive
the “O” allele from both parents.
Multiple Phenotypes from Multiple Alleles
• Although each trait that we have studied so far
only has two alleles, it is common for more that
two alleles to control a trait in a population
• For instance, Pigeons – three colors possible (red,
blue, chocolate)
• However, each pigeon can have only two of these
alleles
Blood Types
Codominance and Multiple Alleles
• Human blood is separated into
different classifications because of the
varying proteins on the surface of
blood cells.
• These proteins are there to identify
whether or not the blood in the
individual's body is it's own and not
something the immunity system should
destroy.
ABO Blood type and genetics
Codominance and Multiple Alleles
• The proteins (antigens) that are present on the surface of the
cells are controlled by three alleles
A, B, o
• The individual's blood type is determined by which combination
of alleles he/she has.
Type A – AA or Ao
Type B – BB or Bo
Type AB – AB
Type O - oo
• Blood types A and B are codominant alleles.
AB – both antigens present
• O is the recessive allele
No antigens present
Human Blood Types
Human Blood Types
Blood type practice
Use a Punnett Square!
1. A woman has type
A blood. Her father
has type O blood.
The woman marries
a man with type O
blood. What is the
chance that they
will have a child
with type A blood?
2. What is the chance
that the couple
from question 1 will
have a child with
type AB blood?
*Show me your
answers when you
are finished. Keep
these in your notes!
Simple Dominant Heredity
• Traits controlled by a Single Allele
– More than 200 human traits are determined by a
single dominant allele
• Ex. Huntington’s Disease
– More than 250 other traits are determined by
homozygous recessive alleles
• Both parents must have this allele in order for their
offspring to have the disease.
• Ex. Cystic Fibrosis & Sickle cell anemia
Comparison Inheritance Patterns
• Recessive Allele Inheritance Patterns
– Unaffected parents can have affected offspring
– The phenotype can skip a generation
– Carriers - Individuals with no signs of the trait but
carry the allele (Tt)
• Dominant Allele Inheritance Patterns
– Affected offspring must have at least one affected
parent, there are no carriers for the trait.
– The phenotype appears in every generation without
skipping
– Two unaffected parents have no affected offspring
Polygenic traits
• A trait that is controlled by a groups of genes.
Polygenic traits are controlled by two or more genes
at different locations on different chromosomes.
• Examples are height, skin color and weight.
Polygenes allow a wide range of physical traits.
• For instance, height is regulated by several genes so
that there will be a wide range of heights in a
population.
Eye Color Activity
• http://www.athro.com/evo/gen/genefr2.html
Sex determination
• Remember that in humans the diploid number of
chromosomes is 46, or 23 pairs.
• There are 22 matching pairs of homologous
chromosomes called autosomes.
• The 23rd pair differs in males and females, they
determine the sex of an individual (sex
chromosomes)
– X females (XX)
– Y males (XY)
Complete a punnett square to determine the expected
ratio of males to females produced given their
possible gamete contribution
Sex-linked inheritance
• Traits controlled by genes
located on sex chromosomes
are called sex-linked traits
• Thomas Hunt Morgan
discovered this pattern of
inheritance
Patterns of Inheritance
• Sex-Linked Traits
– Are found only on the X chromosome
– Ex. Colorblindness (recessive)
– Ex. Hemophilia (recessive)
Pedigrees
• Pedigree – a family record that shows how
a trait is inherited over several generations.
• Scientists often study disease causing genes
because they can easily be traced
Patterns of Inheritance
• Sex-Influenced Traits
• Influenced by male or female sex hormones
• Ex. Patterned Baldness
– Homozygous baldness-both will lose hair
– Heterozygous-men will lose hair but women
will not
Pedigrees Cont.
Square = Male
Circle = Female
No shading = normal
Shaded = displays trait
Half/Half = Carrier
• Carriers – Heterozygous;
they do not express the
recessive allele, but they pass
it along to their offspring.
Pedigree
A Pedigree of Hemophilia in the Royal Families of Europe
Human Karyotype
Things to look for in pedigrees
• Autosomal recessive disorders (rr)
males and females have the disorder
equally and can be carriers
can skip generations
• Autosomal dominant disorders (Hh)
males and females could have the disorder
equally, no carriers
will appear in every generation
• Sex-linked recessive disorders (X CXc XCY)
males are more likely to have disorder
only female carriers
Environmental Effects
• Genes are inherited from parents, but
sometimes their expression is modified by
environmental factors.
• An example is the snowshoe hare we
discussed earlier in the year-these hares
have dark fur in the summer and white fur
in the winter.
Epigenetics
• Watch the video and record a few notes on
what epigenetics is.
Snowshoe Hare
What causes the change in coat color in these rabbits?
Nondisjunction
• Nondisjunction is the failure of
homologous chromosome pairs to separate
properly during meiosis. The result of this
error is a cell with an abnormal (too few or
too many) number of chromosomes.
Nondisjunction
Patterns of Inheritance
• Disorders due to Nondisjunction
– Monosomy (45 Chromosomes)
– Trisomy (47 Chromosomes)
• Trisomy-21 (Down’s Syndrome)
– Klinefelter’s (XXY)-male w/ some female traits
– Turner’s (XO)-female appearance
– Single Y chromosome do not survive
Detecting Human
Genetic Disorders
• Genetic Screening – examination of
genetic makeup
– Karyotype: a picture of chromosomes
grouped in pairs and arranged in
sequence.
– Screening of Blood: look for certain
proteins
• Genetic Counseling-medical guidance
informing of problems that could affect their
offspring.
Human Karyotype
Detecting Human
Genetic Disorders
– Amniocentesis: removal of small amount
of amnionic fluid surrounding the fetus
– Chorionic Villi Sampling: tissue that
grows between the mother’s uterus and
the placenta (between the 8th and 10th
week)
– Screening Immediately after Birth:
• Ex PKU (Phenylketonuria)-body cannot
metabolize the amino acid phenylalanine
– Special diet lacking phenylalanine
Karyotyping
• http://www.biology.arizona.edu/human_bio/
activities/karyotyping/karyotyping2.html