Transcript Conclude Mendelian Genetics - March 30

Mendelian Genetics
Genes and The Environment
• The product of a genotype is generally not a rigidly
defined phenotype, but a range of phenotypic
possibilities, the norm of reaction, that are
determined by the environment.
– In some cases the norm of reaction has no breadth
(for example, blood type).
• Norms of reactions are broadest for polygenic
characters.
• For these multifactorial characters, the environment
contributes to their quantitative nature.
Sun (smaller) and shade leaves in a hosta
Young Arnold Schwarzenegger
Environment and intelligence tests
Environmental influence – flower color in hydrangea –
Blue color occurs in more acid soil
Phenotype depends on the interaction of environment and genes –
even identical twins accumulate phenotypic differences
Mendelian Inheritance in Human
Pedigree Analysis
Key to reading
Genetic pedigrees
Key
Male
1st
generation
Affected
male
Female
Affected
female
Mating
1st
generation
Ww
ww
Ww
ww
2nd
generation
Ww
ww
3rd
generation
WW
or
Ww
Widow’s
peak
ff
ff
(a) Is a widow’s peak a dominant or
recessive trait?
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd
generation
ww
No widow’s
peak
ff
Ff
2nd
generation
FF or Ff
Ww ww ww Ww
Ff
Offspring
Attached
earlobe
Free
earlobe
b) Is an attached earlobe a dominant
or recessive trait?
Sex and Inheritance
• Sex-linked genes are carried on the sex
chromosomes – like hemophilia and color blindness
that are carried on the X and thus show up more often
in males or hairy ears that is carried on the Y and
only shows up in males
• Sex-influenced genes are carried on regular
autosomal chromosomes but the expression is
influenced by the sex of the individual - The sex
influence appears to be related to levels of male sex
hormones – in particular testosterone – more
testosterone leads to greater expression of the trait
Sex-linked - hemophilia
Sex-linked - hemophilia
Queen Victoria and hemophilia
Pattern baldness – sex-influenced
Method of inheritance
Genotype
Phenotype
Phenotype
Male
Female
BB
Bald
Bald
Bb
Bald
Not bald
bb
Not bald
Not bald
Pattern baldness in the Adams Family
Albinism
Parents
Normal
Aa
Normal
Aa
Sperm
A
a
A
AA
Normal
Aa
Normal
(carrier)
a
Aa
Normal
(carrier)
aa
Albino
Eggs
Cystic Fibrosis
Cystic fibrosis
• One in 25 whites of European ancestry is a carrier, 1 in
2500 is affected.
• The normal allele codes for a membrane protein that
transports Cl- between cells and the environment.
• If these channels are defective or absent, there are
abnormally high extracellular levels of chloride that causes
the mucus coats of certain cells to become thicker and
stickier than normal.
• This mucus build-up in the pancreas, lungs, digestive tract,
and elsewhere favors bacterial infections.
• Without treatment, affected children die before five, but
with treatment can live past their late 20’s or even longer.
Sickle-cell anemia
• It affects one of 400 African Americans, 1 in 12
African Americans carries the trait.
• It is caused by the substitution of a single amino
acid in hemoglobin.
• When oxygen levels in the blood of an affected
individual are low, sickle-cell hemoglobin
crystallizes into long rods.
• This deforms red blood cells into a sickle shape.
Sickle-cell anemia – pleiotropy
Distribution of Sickle-Cell Anemia
Distribution of malaria
Achondroplasia – a dominant trait
Effects 1 in 10,000 people,
99.99% of population are
homozygous recessive for trait
Huntington’s disease –
lethal autosomal dominant
Huntington’s Disease
• The dominant lethal allele has no obvious phenotypic effect
until an individual is about 35 to 45 years old.
• The deterioration of the nervous system is irreversible and
inevitably fatal.
• Any child born to a parent who has the allele for Huntington’s
disease has a 50% chance of inheriting the disease and the
disorder.
• Recently, molecular geneticists have used pedigree analysis of
affected families to track down the Huntington’s allele to a
locus near the tip of chromosome 4.
Woody Guthrie
Huntington’s Disease Sufferer
Genetic Counseling
• Consider a hypothetical couple, John and
Carol, who are planning to have their first
child.
• In both of their families’ histories cystic
fybrosis, a recessive lethal disorder is present,
and both John and Carol had brothers who died
of the disease.
Genetic Counseling
• While neither John and Carol nor their parents have the
disease, their parents must have been carriers (Aa x Aa).
• John and Carol each have a 2/3 chance of being carriers and a
1/3 chance of being homozygous dominant.
• The probability that their first child will have the disease = 2/3
(chance that John is a carrier) x 2/3 (chance that Carol is a
carrier) x 1/4 (chance that the offspring of two carriers is
homozygous recessive) = 1/9.
Genetic Counseling
• If their first child is born with the disease, we
know that John and Carol’s genotype must be
Aa and they both are carriers.
• The chance that their next child will also have
the disease is 1/4.
• Mendel’s laws are simply the rules of
probability applied to heredity.
(a) Amniocentesis
1
(b) Chorionic villus sampling (CVS)
Ultrasound monitor
Amniotic
fluid
withdrawn
Ultrasound
monitor
Fetus
1
Placenta
Chorionic villi
Fetus
Placenta
Uterus
Cervix
Cervix
Uterus
Suction
tube
inserted
through
cervix
Centrifugation
Fluid
Fetal
cells
Several hours
2
Several
weeks
Biochemical
and genetic
tests
Several
hours
Fetal cells
2
Several hours
Several weeks
3
Karyotyping
Figure 14.UN07
George
Sandra
Tom
Sam
Arlene
Wilma
Ann
Michael
Carla
Daniel
Alan
Tina
Christopher