Document 6583147

Sex Chromosomes
In most animal species, chromosomes can be categorized
as two types:
a.Autosomes – non sex chromosomes
b.Sex chromosomes – determine if the individual is male
or female.
Sex chromosomes in the human female are XX, male XY.
Males produce X- containing and Y-containing gametes;
therefore males determine the sex of the offspring.
Besides genes that determine sex, sex chromosomes carry
many genes for traits unrelated to sex.
X- linked gene is any gene located on X chromosome.
Autosomes and Sex Chromosomes
Disorders
Down’s Syndrome (chrom 21)
Alzheimer’s (chrom 1, 10, 14, 19, 21)
Huntington’s (chrom 4)
X- Linked Alleles
Work with fruit flies by Thomas Hunt Morgan confirmed that
Genes were on chromosomes.
X- linked alleles are designated as superscripts to X
Chromosome.
Heterozygous females are carriers but do not show the trait
But can pass it on.
Males are never carriers but express the one allele on the X
Chromosome.
One form of colour blindness is X- linked recessive.
Importance of genetics
• Understanding hereditary diseases and to
develop new treatments
• Donor matches
• Paternity
• Forensics
• Evolution
Genetic Testing
Would you want to know?
• Ethical concerns
• Cost
• Insurance companies
see GATTACA
Genes- genetic material on a
chromosome that codes for a
specific trait
Genotype- the genetic makeup of
the organism
Phenotype- the expressed trait
Allele- an alternative form of a
gene
Dominance Mechanism
• Two alleles are carried for each trait
• In true-breeding individuals, both
alleles are the same (homozygous).
• Hybrids, on the other hand, have one
of each kind of allele (heterozygous).
• One trait is dominant, the other trait is
recessive
Allele Example
Gene = “eye color”
Alleles
brown
blue
green
lavender
Allele Examples
appearance
eye color:
homozygous
Allele Examples
appearance
eye color:
heterozygous,
brown dominant
over blue
Genotype vs Phenotype
genotype
phenotype
homozygous
(dominant)
heterozygous
homozygous
(recessive)
appearance
Punnett Square
If male & female are
heterozygous for eye
color
X
male
female
brown:
blue:
3/4 offspring
1/4 offspring
Red-Green Color Blindness
Sex-linked trait
XC
Y
Normal male
XC
X
XC
Xc
Normal female
recessive gene
Possible outcomes:
Xc
XC
Y
XC
XC
XC
Y
XC
Xc
Xc
Y
XCXC XCXc XCY
Normal Normal Normal
female Female male
(carrier)
XcY
Color-blind
male
Dominance
Most traits show complete
dominance
Blending unexpected
allele
E
e
gene
unconnected earlobe
connected earlobe
P
unconnected
EE x ee
connected
gametes
F1
E
e
Ee
F1
Ee x Ee
gametes
1/2 E 1/2 e
1/2 E 1/2 e
E
Punnett
Square
F2
e
E EE
Ee
e
ee
Ee
1 EE 2 Ee 1 ee
generation
genotypes
unconnected
E:e
P
EE, ee
50%
1:1
F1
Ee
100%
1:1
F2
EE, 2 Ee, ee
75%
1:1
phenotypes
ratio of alleles in the population
Basis of the Castle-Hardy-Weinberg Law
Tongue Roller
R = Tongue Roller
r = Unable to Roll Tongue
Widow’s Peak
W = Widows Peak
w = Lack of Widow’s Peak
Free Ear Lobe
Attached Ear Lobe
E = Free Ear Lobe
e = Attached Ear Lobe
Hitchhiker’s Thumb
Hi = Straight Thumb
hi = Hitchhiker’s Thumb
Bent Little Finger
Bf = Bent Little Finger
bf = Straight Little Finger
Mid-digital Hair
M = Mid-Digital Hair
m = Absence of Mid-Digital Hair
Dimples
D = Dimples
d = Absence of Dimples
Short Hallux
Ha = Short Hallux
ha = Long Hallux
Now complete Human Traits
Assignment
Predicting Offspring
Genetic disorders are medical conditions caused by alleles
inherited from parents.
These disorders are mapped on a Pedigree.
A carrier is a heterozygous individual who has no apparent
abnormality but can pass on an allele for a recessive
inherited genetic disorder.
Autosomal Dominant and
Autosomal Recessive
Characteristics of autosomal dominant disorders:
1. Affected children usually have an affected parent.
2. Heterozygotes are affected. Two affected parents can
produce unaffected child; two unaffected parents will
not have affected children.
Characteristics of autosomal recessive disorders:
1. Most affected children will have normal parents since
heterozygotes have a normal phenotype.
2. Two affected parents always produce an affected child.
3. Close relatives who reproduce together are more likely
to have affected children.
Chance has no memory!
Each child born to heterozygous parents has a 25%
chance of having a disorder regardless of prior sibling’s
conditions.
Tracking Human Traits
A pedigree is a diagram of family relationships that
symbols to represent people and lines to represent
genetic relationships. These diagrams make it easier to
visualize relationships within families, particularly
large extended families. Pedigrees are often used to
determine the mode of inheritance (dominant, recessive, etc.)
of genetic diseases.
.
In a pedigree, squares represent males and circles
represent females. Horizontal lines connecting a male and
female represent mating. Vertical lines extending downward
from a couple represent their children. Subsequent generations
are therefore written underneath the parental generations and
the oldest individuals are found at the top of the pedigree.
If the purpose of a pedigree is to analyze the pattern of
inheritance of a particular trait, it is customary to shade in the
symbol of all individuals that possess this trait.
In the pedigree , the grandparents had two children,
a son and a daughter. The son had the trait in question.
One of his four children also had the trait.
Autosomal Dominant Disorders
Achondroplasia - Achondroplasia is inherited as an autosomal dominant disorder. It is diagnosed in
the first years of life and affects 1 in 26,000 births; the majority of cases are sporadic.
This is the classic circus dwarf with disproportionate shortness of the limbs and large head, but apparently normal trunk.
Huntington's disease - Huntington's disease is a disorder passed down through families in which
nerve cells in certain parts of the brain waste away, or degenerate.
Marfan's syndrome - Marfan syndrome is a genetic disorder that affects the body's connective
tissue - the tissue that makes up our tendons, ligaments, joints, and muscles, including the heart, blood vessels,
and eyes.
People with this condition are generally very tall and slim with long arms and fingers.
It's quite rare - roughly 1 in 5,000 people have it.
Neurofibromatosis - is a disorder characterized by the growth of noncancerous tumors called
neurofibromas. They usually form on or just underneath the skin, as well as in the brain and peripheral nervous system.
But they can also develop in other parts of the body, such as the eye.
Autosomal Recessive Disorders
Cystic fibrosis (CF)
Cystic fibrosis is one of the most common inherited single gene disorders in Caucasians.
About one in 2500 Caucasian babies is born with CF and about one in 25 Caucasians of northern European
descent carries the gene for CF. People with CF secrete abnormal body fluids, including unusual sweat
and a thick mucus which prevents the body from properly cleansing the lungs. The mucus interrupts the
function of vital organs and leads to chronic infections. Classic CF also involves the pancreas and causes
decreased absorption of essential nutrients.
Life expectancy has improved, but, ultimately, death most often occurs from respiratory failure.
Other people with variants of CF may have only lung involvement, sinusitis, or infertility.
Tay Sachs Disease
Tay Sachs disease is a fatal disorder in children (usually by age 5) that causes a progressive degeneration
of the central nervous system. It is caused by the absence of an enzyme called hexosaminidase A (or hex A).
Without hex A, a fatty substance builds up on the nerve cells in the body, particularly the brain. The process
begins early in pregnancy when the baby is developing, but is not apparent until several months after the birth.
To date, there is no cure for Tay Sachs. Dr. Tay and Dr. Sachs, who originally described this condition, noted
that most Tay Sachs babies were usually of eastern European Jewish origin.
About one in 30 persons of Ashkenazi Jewish ancestry carries the Tay Sachs gene.
Sickle cell anemia (SC)
Sickle cell anemia is one of the most common, inherited
single gene disorders in African-Americans.
About one in 600 African-American babies is born with SC,
and about one in 12 African-American people carries
the gene for SC. SC involves the red blood cells, or hemoglobin, and their ability
to carry oxygen. Normal hemoglobin cells are smooth, round, and flexible,
like the letter "O", so they can move through the vessels in our bodies easily.
Sickle cell hemoglobin cells are stiff and sticky, and form into the shape of a sickle,
or the letter "C" when they lose their oxygen. These sickle cells tend to cluster together
and cannot easily move through the blood vessels. The cluster causes a blockage and
stops the movement of healthy, normal oxygen carrying blood. This blockage is what
causes the painful and damaging complications of sickle cell disease.
phenylketonuria (PKU)
Phenylketonuria (commonly known as PKU) is an inherited disorder that increases
the levels of a substance called phenylalanine in the blood. Phenylalanine is a
building block of proteins (an amino acid) that is obtained through the diet. It is found in
all proteins and in some artificial sweeteners.
If PKU is not treated, phenylalanine can build up to harmful levels in the body, causing
intellectual disability and other serious health problems.
Complexities
Multiple genes for one trait
Example: eye color
Blended traits (“incomplete
dominance”)
Influence of the environment
Many human traits are controlled by single genes.
True for disorders, too. Cystic fibrosis, deafness,
Tay Sachs. Hitchhiker thumb.
Pedigrees used to predict potential disorders in children,
help make a reproductive strategy. Some people may chose to
not have children but adopt instead.

Family pedigrees are used to determine
patterns of inheritance and individual genotypes
Dd
Joshua
Lambert
Dd
Abigail
Linnell
D_?
Abigail
Lambert
D_?
John
Eddy
dd
Jonathan
Lambert
Dd
Dd
dd
D_?
Hepzibah
Daggett
Dd
Elizabeth
Eddy
Dd
Dd
Dd
dd
Female Male
Deaf
Figure 9.8B
Hearing
Inherited Genetic Disorders:
 Most mutations usually involve
recessive alleles
 Phenylketonuria, PKU
 Tay-Sachs disease
 Cystic fibrosis
• A few are caused by dominant
alleles:
Achondroplasia,
Huntington’s disease
Sex-linked disorders affect mostly males
 Most sex-linked human
disorders are due to
recessive alleles
 Ex: hemophilia,
red-green color blindness
These traits appear mostly in males. Why?
Figure 9.23A
 If a male receives a single X-linked recessive allele from
his mother, he will have the disorder; while a female has
to receive the allele from both parents to be affected
Pedigree Chart: Inheritance Pattern for
an X-linked Recessive Disease
Figure 19.12
 A high incidence of hemophilia has plagued the royal
families of Europe
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Alexis inherited hemophilia from his mother
Alexandra, a condition that could be traced back to
her maternal grandmother
Figure 9.23B
Queen Victoria.
Hemophilia in European royalty
 Hemophilia figured prominently in the history of
European royalty in the 19th and 20th centuries.
Britain's Queen Victoria, through two of her five
daughters including Princess Alice and Princess
Beatrice, passed the mutation to various royal
houses across the continent, including the royal
families of Spain, Germany and Russia.
General Pedigree
Autosomal Dominant Pedigree
Look for:
 Trait in every generation
 Once leaves the pedigree does not return
 Every person with the trait must have a
parent with the trait
 Males and females equally affected
Autosomal Recessive Pedigree
Look for:
 Skips in generation
 Unaffected parents can have affected children
 Affected person must be homozygous
 Males and females affected equally
Sex linked Dominant pedigrees
Look for:
 More males being affected
 Affected males passing onto all daughter
(dominant) and none of his sons
 Every affected person must have an affected parent
Sex linked recessive pedigrees
 More males being affected
 Affected female will pass onto all her sons
 Affected male will pass to daughters who will be a
carrier (unless mother also affected)
 Unaffected father and carrier mother can produce
affected sons
Sex linked recessive
25
AA
If one of the parents is homozygous
for a dominant allele, all the children
will be affected
Aa
Aa
aa
Aa
If one parent is heterozygous for a
dominant allele and the other is
homozygous recessive, there is
a chance that half their children will
be affected
If both parents are heterozygous for
a recessive allele, there is a chance
that one in four of their children
will be affected