Download Gene-linkage and Karyotype

INTRODUCTION TO LINKED
GENES AND SEX LINKAGE
H. Biology/ Ms. Kim
Locating Genes on
Chromosomes
• Genes
– Are located on chromosomes
– Code for protein that determines
specific traits.
Figure
Gene Linkage
• Linked genes
– Usually inherited together because located
near each other on the SAME chromosome
• Genes closer together on the same chromosome are more often
inherited together
• Each chromosome
– Has 100’s or 1000’s of genes
• Sex-linked genes exhibit unique patterns
of inheritance; genes linked on the X or Y
chromosome
• Thomas Hunt Morgan (1910) noticed that eye
color in fruit flies and wing size always
seemed to be inherited together in offspring.
– This seemed to go against Mendel’s Law of
Independent Assortment
• Morgan discovered chromosomes are a group
of linked genes
• Law of Independent Assortment was still true,
but chromosomes assort independently, not
genes.
Gene Linkage
• Genes located on the same chromosome will NOT sort
independently
• They will be inherited together
– Gene Linkage:
• Tendency of 2 alleles (for different traits) to be inherited together
• The closer the genes are, the more likely they are to be inherited together
Mendel’s Principle of Independent
Assortment?
• Remember: this refers to genes on separate
chromosomes
Gene 1
Gene 2
• What would cause genes on the same chromosome
to be inherited separately?
– Crossing over!
• Location of Gene on a chromosome is it’s
LOCUS
KARYOTYPE AND GENETIC
DISORDERS
H. Biology
Chromosomes Determine Your Gender
• Humans have 23 pairs of chromosomes
• 22 pairs are autosomes (chromosomes that
are NOT involved in making gender)
– Also called homologous chromosome pairs
• 1 pair are sex chromosomes (NOT
HOMOLOGOUS; different genes)
– The sex chromosomes are “X” and “Y”
X
» “XX” is a girl
Egg cells ALWAYS
have “X” chromosome
» “XY” is a boy
Y
X
Y
A karyotype
– an ordered, visual
representation of
ALL chromosomes
in a human cell
– Arranged
according to
homologous pairs
Pair of homologous
chromosomes
Centromere
Sister
chromatids
5 µm
a
REAL
view…
wow!!!
What is a genetic disorder? What
causes a genetic disorder?
• Caused by an abnormality in an individual’s DNA
How do these abnormalities occur?
1) Non-disjunction
– Addition or subtraction of chromosome or set of chromosomes
2) Mutations
– Change to DNA sequence (gene)
Abnormal Chromosome
Number
• When nondisjunction occurs
– Pairs of homologous chromosomes do NOT
separate normally during meiosis
– Gametes contain 2 copies or NO copies of a
particular chromosome
• What are some disorders caused by non-disjunction?
• Downs Syndrome
– Trisomy 21
• Klinefelter’s Syndrome
– XXY
• Turner Syndrome
– Monosomy X
What happens when meiosis goes wrong? 
NONDISJUNCTION!!!
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n+1
n+1
n1
n–1
n+1
n –1
n
Number of chromosomes
(a)
Nondisjunction of homologous
chromosomes in meiosis I
(b)
Nondisjunction of sister
chromatids in meiosis II
n
Aneuploidy
Results from the fertilization of
gametes in which nondisjunction
occurred
Offspring have an abnormal
number of a particular
chromosome
What are some disorders caused by nondisjunction?
• Downs Syndrome
– Trisomy 21
• Kleinfelter’s Syndrome
– XXY
• Edwards Syndrome
– Trisomy 18
• Patau Syndrome
– Trisomy 13
• Turner Syndrome
– Monosomy X
1.) Trisomy 18-Edwards Syndrome
2.) Trisomy-13: Usually Fatal
3.) Klinefelter - XXY
• Klinefelter syndrome
– a result of an extra chromosome in a
male, producing XXY individuals
– 1/2000 live births
XYY individuals
Triple X Syndrome
XXX individuals
• Females
• Healthy
• Same
phenotype as
XX individuals
• 1/1000 live
births
4.) Turner Syndrome
• Turner syndrome
– Is the result of monosomy
X, producing an X0
karyotype
– 1/5000 live births
– Only known monosomy in
humans
– Normal intelligence
– Sterile
5.) Trisomy 21
Human Disorders Due to
Chromosomal Alterations
• Down syndrome
–Is usually the
result of an extra
chromosome 21
• trisomy 21
What are some disorders caused by
mutations?
•
•
•
•
•
Sickle cell anemia (recessive)
Cystic fibrosis (recessive)
Maple syrup urine disorder (recessive)
Congenital hypothyroidism (recessive)
Huntington’s Disease (dominant)
Cystic Fibrosis
• Example of recessive disorder
• Affect mostly people of European
descent
• Symptoms
– Mucus buildup in the some
internal organs
– Abnormal absorption of nutrients
in the small intestine
Sickle Cell Anemia
oAnother recessive disorder
oAffects one out of 400 African-Americans
oHemoglobin is the O2 carrying protein in
red blood cells
o SCA is an inherited blood disease where
hemoglobin clumps together 
causes red blood cells to stiffen and curl into
a sickle/crescent shape
o Because of this, the RBC cannot transport
O2 effectively
Symptoms
Physical weakness, pain, organ
damage, and even paralysis
Human Chromosomal Disorders
Recessive Disorders
• Cystic Fibrosis
– Excess mucus in lungs, digestive tract, liver
– More susceptible to pneumonia, infection
– Early death ~ average age of 37
• Tay-Sachs
– Lipid accumulation in brain cells, mental retardation
– Blindness, death in childhood
Dominant Disorder
• Huntington’s disease
– Deterioration of nervous system, uncontrollable
movements
– Develops in middle age, death soon after
32
Dominantly Inherited Disorders
• Some human disorders
– Are due to dominant alleles 
only need 1 dominant allele
• Example is achondroplasia
– Form of dwarfism lethal
when homozygous for the
dominant allele
Another Dominant Disorder
• Huntington’s disease (HD)
– degenerative disease of nervous system
– No obvious phenotypic effects until about 35 to 40
years of age
HD
Normal