BIOLOGY 1111
CHAPTER 12
Instructor Terry Wiseth
Northland Community & Technical College
CHROMOSOMAL THEORY OF INHERITANCE
1) Genes
Units of instruction for heritable traits are arranged in succession on chromosome
2) Sexual reproduction
assures that each new individual will have same number of chromosomes as its parents
3) Diploid cells (2N)
have homologous chromosomes which line up with each other during meiosis
4) Independent Assortment
Genes located on different chromosomes are inherited independently of each other
5) Genetic re-combinations
occur during crossing over
6) Chromosomal abnormalities
deletions, duplications, inversions, or misplacements occur during meiosis
7) Variation
Independent assortment, crossing over and chromosomal abnormalities lead to variation upon which selection agents can act leading to evolution
AUTOSOMES & SEX CHROMOSOMES
A distinctive pair of chromosomes which are different in males and females
Carry hereditary instructions about gender
Human, mammals
XX Female
XY Male
birds, moths
XX is male
XY is female
Autosomes
similar shaped chromosomes which do not govern sex of an individual
KARYOTYPE
Chromosomes can be characterized at metaphase by
length
banding patterns
centromere location
Visual representation of arranged chromosomes
Karyotypes can be used to identify chromosomal abnormalities
SEX DETERMINATION
Y-chromosomes carries "male determining gene"
causes new individual to develop testes
SEX DETERMINATION
SRY gene
Sex determining Region on Y chromosome
master gene on Y chromosome for male sex determination
becomes active 4 weeks after conception
SEX DETERMINATION
individuals lacking the Y chromosome will result in females
X chromosome have genes that deal with non-sexual traits
GENE LINKAGE
When inheritance of a chromosome is coupled to that of a particular gene the condition is called linkage
If the chromosome involved is a sex chromosome, the trait is said to be sex-linked
Genes on the same chromosome belong to the same linkage group
these genes do not separate independently during meiosis
Linkage is the tendency of genes located on the same chromosome to be transmitted together in inheritance
Linkage can be disrupted by crossing over
CROSSING OVER
an exchange of parts of homologous chromosomes
Crossing over between homologous chromosomes disrupts gene linkages
results in the production of non-parental combinations of genes in chromosomes
LINKAGE MAPPING
a method of measuring the relative linear distances between genes on the same chromosome
the farther apart two genes are on a chromosome
the greater the frequency of crossing over and therefore of genetic recombination
CHROMOSOME MAPPING
Chromosome map
the puffed region indicates an active area of the chromosome
Comparative scale of mapping chromosomes
HUMAN GENOME PROJECT
Attempt to map every gene in the human genome
Expected to be complete by 2005
LINKAGE EXAMPLE
each germ cell contains two of each chromosome
one from each parent
the yellow chromosome was inherited from the father
the green from the mother
When genes are very close together on a chromosome, crossing over does not occur
Fifty percent of the gametes receive one parental chromosome and the other fifty percent receive the other
Linked genes that are some distance apart may be recombined by crossing over, creating new genotypes in the gametes
The farther apart two genes are on a chromosome, the greater will be the frequency of crossing over
SEX-LINKED CHARACTERISTICS
Genes located on X and Y chromosomes are called sex-linked
SEX-LINKED INHERITANCE
X- linked
mutated gene on X (not Y) sex chromosome
Ex: Hemophilia
Y-linked
mutated gene on Y sex chromosome
no direct evidence of any Y-linked traits
PEDIGREES
Pedigrees-Charts of genetic relationships of individuals
ROYAL PEDIGREE CHART
Queen Alexandra of Russia (Hemophilia carrier)
GENETIC DISORDERS
Over 1,000 known genetic disorders
most are recessive
1) Sex-linked
2) Autosomal mutations
3) Chromosomal structural abnormalities
4) Chromosome number variations
SEX-LINKED
Sex linked genes are carried on the X or the Y chromosome
usually recessive
Hemophilia
Color-blindness
Duchenne muscular dystrophy
Eye color in fruit flies
Calico coat color in cats
Anhydrotic ectodermal dysplasia
Y-linked (baldness ?)
Faulty enamel
HEMOPHILIA
19th century European Royal Family – Queen Victoria
X chromosome carries genes necessary to produce enzymes essential for causing blood to clot
Factor VIII
1-7000
COLOR BLINDNESS
red-green color blindness
normal individuals can see over 150 colors
color blind can see less than 25
Ex: Color-blind male x normal female
X Y x X X
daughters = carriers
sons = normal
grandsons may show trait
Color-blind carrier X normal male
XX x XY
50% chance color-blind son
50% chance color-blind carrier
DUCHENNE MUSCULAR DYSTROPY
X-linked recessive
Progressive weakening and loss of muscle tissue
missing protein dystrophin
1-3500 male infants
death by age 20
EYE-COLOR IN FRUIT FLIES
Wild-type = red eyes (XR)
Muted gene = white eyes (Xr)
CALICO COAT COLOR IN CATS
Calico coat coloration in cats is a sex-linked characteristic
each of the X sex chromosomes carry a gene for coat color
one for black and the other for yellow
during embryonic development one of the two chromosomes was inactivated at random in each of the cells that had formed at that time
all of the cellular descendents of each cell will have the same gene inactivated
leaving only one functional gene for coat color
the different patches of color are produced depending on which allele is functional in the cells making up that tissue in any given region
the white patches result from interactions with other genes that determine if there is any deposition of pigment and thus color at all
ANHIDROTIC ECTODERMAL DYSPLASIA
Similar inheritance to calico coat color in cats
heterozygous for recessive allele on X chromosome that prevents sweat glands from forming
mosaic patches of tissues where maternal and paternal genes of X sex chromosome are expressed
MALE PATTERN BALDNESS
May be a sex-influenced recessive trait
influences are probably carried on the Y chromosome
mutated gene on the Y chromosome
Trait seems to follow paternal lines of inheritance
Adams Family
Y-INFLUENCED TRAITS
Feather development in poultry
Horn development in Dorset sheep
Hairy ears in humans
FAULTY ENAMEL
Dominant sex-linked trait
enamel coating that protects the teeth does not develop properly
allele is expressed in heterozygous females
can a son inherit this disease from their father?
AUTOSOMAL MUTATIONS
Mutations on genes other than the sex chromosomes
usually recessive but can be dominant
Dominant
Hypercholesterolemia
Huntington’s Disease
Alzheimer’s Disease
Retinoblastoma
Achondroplasia
Progeria
ALS
HYPERCHOLESTEROLEMIA
Caused by excess cholesterol in the blood
leads to heart disease and hardening of the arteries
1-500 are heterozygous for the condition
HH = normal
Hh = mild disease
hh = severe disease, usually die in childhood
HUNTINGTON’S DISEASE
degeneration of the nervous system
always fatal
disease which killed folk singer Woody Guthrie
"This Land is Your Land"
Homozygous and heterozygous individuals will experience symptoms of the disease
ALZHEIMER’S DISEASE
Mental deterioration
may include uncontrollable movements
disease usually manifests itself after the age of 60 years old
1-25,000
RETINOBLASTOMA
Cancer in the eyes of infants
mutated gene which normally prevents development of cancer
afflicted individuals have only one copy of the cancer suppressor gene
ACHONDROPLASIA
Dwarfism
Early-aging
homozygous state is lethal in the embryo
head and trunk are normal size
1-25,000
PROGERIA
accelerated aging
1 in 8,000,000
no cure
thin skin, soft bone, hair loss, hardening of arteries
probably caused by spontaneous mutation as a dominant gene
dominant lethal gene
ALS
Amyotrophic Lateral Sclerosis
Lou Gerhig’s Disease
1 in 1000 individuals
Defective EAAT2 gene
specifies a membrane transport protein
protein sponges up excess glutamate in the CNS
glutamate serves as a communication signal between neurons and muscle cells
in excess amounts glutamate kills cells
AUTOSOMAL MUTATIONS
Recessive
Tay-Sachs disease
Sickle cell anemia
Cystic Fibrosis
Galactosemia
Albinism
PKU
TAY - SACHS DISEASE
1-3,500
1 in 30 Jews of Russian origin is carrier
lack of enzyme
N-acetylhexosaminidase
Disease results from lipid accumulation in brain cells
mental deficiency, blindness
death at ages 3-6
able to be diagnosed prenatally
SICKLE CELL ANEMIA
1-500 African Americans
gene defect in normal function of the hemoglobin molecule
hemoglobin binds oxygen at normal concentrations but at low oxygen concentrations (under exercise stress)
molecules become insoluble, clump, cell assumes sickle shape, poor at carrying oxygen
Genetics
HbA = normal allele
HbB = sickle allele
HbA HbA = normal
HbA HbB = normal carrier
HbB HbB = anemic, often lethal
in Africa, find up to 40-45% of native population are carriers
Why?
CYSTIC FIBROSIS
most common genetic disease among Caucasians
1-1800
1 in 20 carriers (Caucasians)
6000 children born each year with disease
with treatment afflicted are able to survive to adulthood
individuals afflicted with CF experience
lung failure (suffocation)
secretes thick mucus
liver failure
blocks ducts of liver & pancreas
Sweat is salty
most people have Cl- channel in membrane
CF people don't pass Cl-
sweat does not taste salty
Water is prevented from passing from bloodstream into lung passages
mucus becomes too thick
GALACTOSEMIA
1 in 100,000 newborns
galactose not able to be metabolized
accumulations of galactose in cells
eye, liver damage
treatment includes eliminate galactose from the diet
ALBINISM
1-22,000
involves a defective gene for proper melanin production
individuals lack pigmentation in skin and hair
PKU
Phenylketonuria
1 in 10,000
1 in 50 is a carrier
inability to break down a toxic product of protein metabolism (Phenylalanine)
brain damage, mental retardation in infant
if detected early, can control diet, prevent symptoms
reduce intake of amino acid phenylalanine
CHROMOSOMAL STRUCTURAL ABNORMALITIES
1) deletions
2) duplications
3) inversions
4) translocation
DELETION
Loss of chromosome segment
Cri-du-chat syndrome
Deletion of arm of chromosome 5
DUPLICATION
Important in evolution as duplicate free to mutate with no harmful consequence
Fragile X Syndrome
INVERSION
A reversal of position and relative order of genes on chromosomes
TRANSLOCATION
Transfer of part of one chromosome to a non-homologous chromosome
May change chromosome number
CRI-DU-CHAT
deletion of part of chromosome #5
Mental retardation
abnormally formed larynx
FRAGILE X SYNDROME
multiple copies of FMR-1 (700+) gene
Mental retardation
1-1500 males
1-2500 females
CHROMOSOMAL NUMBER VARIATION
Sometimes, meiosis makes mistakes
chromosome nondisjunctions
Result is egg or sperm with one too many or one too few chromosomes
CHROMOSOMAL NUMBER VARIATION
Usually causes spontaneous abortion
for smallest chromosomes, development can occur with one extra chromosome
NON-DISJUNCTION
Mechanism by which chromosome number changes
Trisomy Monosomy
1 extra 1 missing
Normal Meiosis
Non-disjunction meiosis
ANEUPLOIDY
Extra, or missing chromosome
Occurs one of every two newly fertilized eggs
Miscarried (usually)
POLYPLOIDY
Three or more extra, missing chromosomes
Lethal to humans
Common in plants
DOWN SYNDROME
extra chromosome #21 (trisomy 21)
Maturation of bones slowed (short stature)
mental development affected
always mentally retarded
occurs in 1 in 750 children
also in higher primates
incidence of Down increases with mother's age
if mother is young, risk is very low
By age 45 risk is as high as 1 in 16
extra copy of the Gart gene leads to high levels of purines
may account for the mental retardation associated with the syndrome
tests are available to detect substances in the blood that would identify mothers who are carrying a Down syndrome child
NONDISJUNCTION OF SEX CHROMOSOMES
occur just as often as autosomal nondisjunctions
but not as lethal
KLINEFELTER’S SYNDROME
TURNER’S SYNDROME
XYY SYNDROME
XXX or XXXX SYNDROMES
KLINEFELTER’S SYNDROME
XX Y male
47 chromosomes
1/2000 births
mild mental and sexual retardation
sterility
TURNER’S SYNDROME
1-5000 births
Missing Y or X sex chromosome
45 chromosome
Sterile, premature aging
XYY SYNDROME
1-2000 births
usually tall
Mild mental retardation
Predisposition to violent behavior?
Controversy about prison inmates
2% prisoners have XYY, vs. 0.01% in general population
but note that 96% lead normal lives
XXX or XXXX SYNDROMES
no apparent harmful consequences
Only one X chromosome is active per cell
Other one stays condensed as Barr body
Detection of Barr body used to bar certain contestants from Olympics
"women" with no Barr Body are actually XY individuals with female genitals and secondary sex
GENETIC COUNSELING
Frequency of birth defects
250,000 children born each year with birth defects
3% of all pregnancies
Sometimes possible to identify carriers
ex: Sickle cell anemia
PRENATEL DIAGNOSIS
Amniocentesis
14th-16th week
about two week diagnosis
Chorionic villi sampling (CVS)
8th week
2 day diagnosis
Family Pedigree
CONQUERING GENETIC DISEASE
What faulty gene causes the disease?
What protein does this gene normally produce?
Can the defective protein or gene be fixed?
END CHAPTER 12 HUMAN GENETICS