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Complex Inheritance
and Human Heredity
Chapter 11
Let’s Review!
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We inherit traits from
our parents, we get half
of our genes from mom,
and half from dad
Alleles are either
recessive (lowercase) or
dominant (uppercase)
Homozygous recessive,
homozygous dominant,
heterozygous
Genetic Disorders
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An abnormal condition that a person
inherits through their genes
Can be caused by either changes in DNA, or
an extra or missing chromosome
Genetic disorders can be recessive or
dominant
Recessive Disorders
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Two recessive alleles, one
from each parent
If an individual is
heterozygous, they are a
carrier (this means they
could pass it on even if they
don’t have it!)
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BB – Normal, Bb – Normal, but
a carrier, bb – Disease
Often skips a generation
Recessive Disorders
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Cystic Fibrosis: Affects cell
membrane protein, water
cannot diffuse from cells
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body produces an
abnormally thick mucus in
the lungs
Chromosome 7
Medication, strict diet,
enzyme replacement
Death is usually caused by
lung complications
Recessive Disorders
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Tay-Sachs: Absence of enzyme
that breaks down fatty
substances
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Chromosome 15
Buildup of fatty acids in brain,
cherry red spot on back of eye
Causes deafness, blindness,
severe retardation, and death
usually by age 4
Most carriers choose to avoid
birth because there is no cure
Recessive Disorders
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Albinism: altered
genes resulting in
the absence of the
skin pigment
melanin in hair and
eyes
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White hair
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Very pale skin
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Pink pupils
Recessive Disorders
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Galactosemia:
characterized by the
inability of the body
to digest galactose.
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People with this
disorder should
avoid milk products
Dominant Disorders
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Caused by dominant
alleles
Those who do not have
the disorder are
homozygous recessive.
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BB – Disease, Bb – Disease, bb
– Normal
THERE ARE NO CARRIERS OF
DOMINANT DISORDERS! ONE
DOMINANT ALLELE MEANS
YOU HAVE THE DISORDER
Dominant Disorders
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Huntington’s Disease: Affects
the nervous system,
deterioration of brain tissue,
usually begins between age 30
and 40
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Chromosome 4
No cure, but have
medications to cope with
symptoms
People usually die 15-20 years
after onset of degeneration
Dominant Disorders
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Achondroplasia: Causes
small body size and limbs
that are comparatively
short, most common type
of dwarfism
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Chromosome 4
75% born to parents of
average size
When this happens it is due
to a gene mutation
Complex Inheritance
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Sometimes the laws and
principles of genetics
established by Gregor
Mendel do not always explain
the inheritance patterns seen
in an organism.
Codominance: Both alleles
completely expressed in
heterozygous condition.
Complex Inheritance
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Sickle Cell Anemia: Abnormal hemoglobin
causes a person’s red blood cells to be sickle
shaped, causing clots and blockages, they
also carry less oxygen
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Alleles for the disease are codominant
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rr – all sickle shaped cells, Rr – mixture of
both, RR – normal shaped cells
People who are heterozygous for sickle cell
also have a higher resistance to malaria
Complex Inheritance
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Incomplete Dominance:
Neither allele is completely
dominant nor completely
recessive
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Heterozygous phenotype is
a blend of the two
homozygous phenotypes
a mix in physical appearance
between dominant
and recessive
Codominant or Incomplete?
Complex Inheritance
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Multiple Alleles: When there are 3 or 4
alleles that code for a single trait
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Blood Groups
This doesn’t mean a person has more than 2
alleles for the trait, just that more than 2
exist in the population
1 gene, more than 2 alleles
Complex Inheritance
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3 forms of the allele
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IA – type A, IB – type
B, i – type O
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i is recessive to IA and IB
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IA and IB are codominant
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IAIB – type AB
Complex Inheritance
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Heterozygous Genotype
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IAi – Type A
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IBi – Type B
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IAIB – type AB
Homozygous Genotype
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IAIA – Type A
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IBIB – Type B
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ii – Type O
Complex Inheritance
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Epistasis: When the one allele hides the effects of
another
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A dog’s coat color
Polygenic Traits: traits that result from the
interaction of multiple gene pairs
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Skin color, height, eye color, possibly intelligence
Sex-Linked Traits
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Genes that are found on
the X chromosome – Xlinked
Typically recessive, IF
dominant, an affected
father will pass it to all of
his daughters
Recessive expressed more
in males because they
only have one X (XY)
compared to females (XX)
Sex-linked Traits
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Hemophilia
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Recessive sex-linked
disorder
Takes a long time for
blood to clot (body
lacks proteins involved
in clotting)
Very rare in females
because she would
need both X’s with
recessive alleles.
Patricia is a healthy carrier of hemophilia
and Sam is completely healthy. Complete
the Punnett square below.
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Mom’s genotype:
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Dad’s genotype:
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What is the probability of getting:
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Daughter with hemophilia?
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Son with hemophilia?
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Carrier?
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Healthy child?
Sex-linked Traits
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Red-Green Colorblindness
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Recessive sex-inked
Male needs 1 copy, female needs
two copies, so it is very rare to find
a color blind female.
Judy is homozygous recessive for
colorblindness and Dennis is healthy.
Complete their Punnett square below.
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Mom’s genotype:
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Dad’s genotype:
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What is the probability of getting:
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Colorblind daughter?
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Color blind son?
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Carrier?
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Heterozygous child?
Sex-linked Traits
Baldness is X-linked Recessive
Grandpa = XbY
Grandma =
XBXB
daughter = XBXb marries Male = XBY
Sons?
50% XB (normal)
50% Xb (bald)
This is why men should look to their
mom’s dad for probability of baldness.
But beware grandma could be a
carrier too!
Environmental Factors
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The environment can have an
influence on phenotypes
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Toxic agents, Diet and exercise,
Sunlight and water,
Temperature, Medications
Conditions can cause a gene to
shut down or turn on
Twins have identical genes,
scientists conclude that twins
with different phenotypes are
influenced by the environment
Pedigrees
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Pedigree: Diagram that
traces the inheritance of a
particular trait through
several generations of the
same family
Allows us to use family
trees and affected
individuals to predict the
risk of disease in future
offspring
Pedigrees
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Individuals are in birth order from left to right (oldest at
left, youngest on right)
Individuals are numbered, Generations are numbered with
roman numerals
When referring to an individual, use the generation
number, then the individual number
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II-4
Example of a Pedigree
I
Grandparents
Grandparents
Parents
II
Aunts, Uncles
Aunts, Uncles
III
Brother
Yo
u in this family??
Do any disorders run
Pedigrees
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From a pedigree, you can tell whether a family
carries an autosomal dominant/recessive
disorder, or sex-linked disorder
Autosomal or Sex-linked:
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Autosomal will show in both sexes equally
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Sex-linked will show mostly in males
Dominant or Recessive:
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Dominant – every individual with trait has parent with
trait
Recessive – individual with trait has parent without
trait
Karyotype
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Karyotype: an image
that shows
homologous
chromosomes
arranged in
decreasing size
The staining bands
mark identical places
on the homologous
chromosomes
What’s
different?
Nondisjunction
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Cell division during which sister chromatids fail
to separate properly
If this occurs during Meiosis I or II then the
resulting gametes will not have the correct
number of chromosomes
Nondisjunction occurs in both autosomes (body
cells) and in gametes
Trisomic: one extra chromosome (only 3
trisomies that result in survival after birth)
Monosomic: one missing chromosome (usually
lethal, except in Turner’s Syndrome)
Down Syndrome
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Chromosome 21
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Trisomy
Can be moderate or severe
Short stature, heart defects, and
mental disability
Correlated with mother’s age, can
be from nondisjunction of father’s
chromosome 21
Nondisjunction
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Autosomal
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Edwards’s Syndrome - Trisomy 18: almost every
organ system affected 1:10,000 live births.
Children with full Trisomy 18 generally do not live
more than a few months.
Patau Syndrome - Trisomy 13: serious eye, brain,
circulatory defects as well as cleft palate. 1:5000
live births. Children rarely live more than a few
months Click above for Choosing Thomas.
Nondisjunction
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Sex Chromosomes
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Klinefelter Syndrome – XXY: Male sex organs; unusually
small testes, sterile. Breast enlargement and other
feminine body characteristics. Normal intelligence.
Trisomy X – XXX: 1:1000 live births - healthy and fertile usually cannot be distinguished from normal female
Monosomy X – Turner’s Syndrome: 1:5000 live births; the
only viable monosomy in humans - women with Turner's
have only 45 chromosomes!!! XO individuals are
genetically female, however, they do not mature sexually
during puberty and are sterile. Short stature and normal
intelligence. (98% of these fetuses die before birth)