Human Inheritance
Review – What is Heredity?
• Heredity is the
passing of traits
from parents to
offspring
• Genes are passed
from parents to
offspring
(**Remember a
gene is the unit that
controls traits)
• Genes are located
on our
chromosomes
Autosomes vs Sex Chromosomes
• All human cells contain 46 chromosomes
or 23 pairs of chromosomes
• The first 44 chromosomes (or 22 pairs) are
called autosomes
• The 23rd pair of chromosomes are the sex
chromosomes
– In female the sex chromosomes are XX
– In males the sex chromosomes are XY
Karyotypes
• A karyotype is a photograph of chromosome
pairs.
• A normal human karyotype is the result of a
haploid sperm containing 23 chromosomes
fertilizing a haploid egg also carrying 23
chromosomes (23+23=46)
• Karyotypes are used to detect chromosomal
disorders and to determine the sex of an
unborn child
Normal Female Karyotype
This karyotype
would be labeled
as: 46,XX
Normal Male Karyotype
This
karyotype
would be
labeled
as: 46,XY
Pedigrees
• A pedigree is a diagram that follows the
inheritance of a single gene through
several generations of family
How to Read a Pedigree
Circle = female
Square = male
Horizontal line connect
siblings or spouses
Vertical lines connect
parents and children
Shaded figure = Individuals
who are affected by the trait
Non-shaded = individuals
who are unaffected
**Children are place in
birth order from left to
right
Practice
The following pedigree shows the
inheritance of a recessive trait.
1. How many generations are
shown?
2. How many children did
Parents 1-1 and 1-2 have?
How many were boys?
How many were girls?
3. How many children did
Parents II-1 and II-2 have?
How many were boys?
How many were girls?
Key: R = Non-affected; r = affected
4. Use the key to determine
the genotypes of each
person in this pedigree.
(Hint: label all homozygous
recessives first)
Chromosome Number Disorders
• Number disorders occur because of a failure
of the chromosomes to separate correctly
during meiosis.
• Nondisjunction – chromosomes fail to
separate
– Nondisjunction in anaphase I: homologous pairs
fail to separate
– Nondisjunction in anaphase II: sister chromatids
fail to separate
• In either case, gametes have an abnormal
number of chromosomes
Nondisjunction in Anaphase I
Chromosome Number Disorders
Number disorders are NOT inherited;
therefore they CANNOT be predicted
using Punnett squares
Autosomal Number Disorders
• Most autosomal number disorders are
lethal
• The only autosomal number disorder that
allows survival into adulthood is Down
syndrome
Down Syndrome
• Also known as Trisomy 21 because there
are 3 chromosomes in the 21st position
• Characteristics:
– Flattened nose and face; upward slanting eyes
– Growth, behavior and mental development are
all retarded
– Risk of heart defects
• Incidence of having a baby with Down
syndrome is higher in older mothers
Down Syndrome
Sex Chromosome Number Disorders
• Caused by nondisjunction of the
chromosomes in the 23rd position or the
sex chromosomes
• Turner Syndrome
• Klinefelter Syndrome
Turner Syndrome
• Also called 45,X0 because individuals lack
a 2nd sex chromosome.
• Characteristics:
– Female
– Short stature
– Underdeveloped sexually
– Sterile (unable to bear children)
– Normal life expectancy
Turner Syndrome
Klinefelter Syndrome
• Also called 47,XXY because individuals
inherit an extra sex chromosome.
• Characteristics:
– Symptoms appear at puberty
– Males shows poor sexual development
– Infertile
– Normal life expectancy
Klinefelter Syndrome
Sex-Linked Inheritance
• A gene is referred to as “sex-linked” if it is
located on a sex chromosome (either X or Y)
• In humans, sex-linked genes are almost
always located on the larger X chromosome.
• The Y chromosome is much smaller and
carries only a few genes related to sexual
development.
• Females have 2 X chromosomes; males have
1 X.
Sex-Linked Inheritance
• Females will only show recessive traits located
on the X chromosome if they are homozygous
recessive.
• Males will always show a recessive trait located
on the X chromosome because he only has 1 X.
• Females can be carriers for sex-linked recessive
disorders.
– A carrier someone who has the defective allele, but
doesn’t show the trait.
Sex-Linked Disorders
**Important – all of these
disorders are sex-linked
RECESSIVE**
Sex-Linked Disorders
1. Color Blindness: the inability to differentiate
and distinguish colors
Ex: red-green colorblindness
Sex-Linked Disorders
2. Hemophilia: Patients are missing an
enzyme required for normal blood clotting.
– Have uncontrolled bleeding episodes
– Treated with blood transfusions
Sex-Linked Disorders
3. Duchenne’s Muscular Dystrophy
– Symptoms develop at 3-6 years of age.
– Causes muscle weakening and the breakdown
of muscle fibers, leading to eventual death.
– There is no available treatment or cure.
Sex-Linked Disorders
• Genotypes for sex-linked traits are written
using the X and Y chromosomes to show
path of inheritance.
C = normal vision c = colorblindness
Colorblind male = XcY
Colorblind female = XcXc
Carried female = XCXc
**Remember males CANNOT be carriers**
Sex-Linked Punnett Squares
• In sex-linked traits, probabilities for male
and female offspring must be calculated
separately because traits are inherited
differently.
Sex-Linked Punnett Square Practice
1.
A colorblind female marries a man with normal vision.
What is the probability of them having colorblind
children?
Key: ________________________________________
Cross: _______________________________________
Probability of having a colorblind daughter = ___________
Probability of having a colorblind son = ____________
Sex-Linked Punnett Square Practice
2. A genetics counselor interviews a couple with a family history of
hemophilia to evaluate the possibility of having offspring with the
disorder. The wife does not have hemophilia, but states that her father
had the disorder. The husband is normal.
Key: ________________________________________
Cross: _______________________________________
Probability of having a colorblind daughter = ___________
Probability of having a colorblind son = ____________
Autosomal Disorders
• Most genes are carried
on the autosomes, the
44 chromosomes other
than the sex
chromosomes.
• Most genetic disorders
are autosomal
disorders.
• These disorders affect
males and females
equally.
Autosomal Disorders
• Autosomal disorders can be divided into
three groups based on the pattern of
inheritance.
– Autosomal Recessive: Person needs two
recessive alleles to have the disorder
– Autosomal Co-dominant: People with a
heterozygous genotype express similar
symptoms to the abnormal phenotype
– Autosomal Dominant: the dominant allele will
cause the disorder; only need one dominant
allele
Autosomal Recessive Disorders
1. Albinism: characterized by failure to produce
the pigment, melanin.
– Lack coloration in skin, hair and eyes
– At risk for skin cancer
– Symptoms appear at birth
Autosomal Recessive Disorders
2. Tay-Sachs Disease: characterized by the inability of
nerve cells to break down a specific type of lipid.
– Symptoms appear between 3-6 months of age
– Lipid build-up causes seizures, blindness, degeneration of
mental & motor skills
– Death usually occurs at or before age 5
– Historically associated with the Jewish population
Autosomal Recessive Disorders
3. Cystic Fibrosis: characterized by excess
mucus production in the lungs & respiratory
system
– Symptoms appear just after birth
– Suffer from frequent respiratory infections and
poor nutrition
– Treatment helps patients live to their 20’s & 30’s
Cystic Fibrosis
Autosomal Recessive Disorders
4. Phenylketonuria or PKU: characterized by
the inability to breakdown the amino acid,
phenylalanine.
– Build-up results in severe brain damage &
mental retardation
– All babies born in US hospitals are tested for
PKU because it is easily treated with a diet low in
phenylalanine
Phenylketonuria or PKU
Autosomal Co-Dominant Disorders
1. Sickle Cell Anemia affects hemoglobin
production.
– Hemoglobin is the protein that carries oxygen
in our red blood cells (RBCs)
– Individuals that are homozygous for the
abnormal allele (ss) produce an abnormal
hemoglobin which causes RBCs to “sickle”
during periods of low oxygen
•
Ex: stress, high altitudes
– Sickled RBCs block vessels causing severe
pain and brain damage
Sickle Cell Anemia
Autosomal Dominant Disorders
1. Marfan Syndrome:
affects the
connective tissue
of the skeletal
system, eyes and
circulatory system
Characteristics:
– Long limbs, vision
problem,
susceptible to
aortic rupture
Autosomal Dominant Disorders
2. Huntington’s Disease: characterized by
deterioration of the nervous system
Characteristics:
– Fatal disorder
– Symptoms arise in the 40’s-50’s
– Mental deterioration
– Uncontrollable movements