Transcript Lesson 5. Dihybrid crosses, pedigrees and - Blyth-Biology11

Dihybrid crosses
and
Patterns of inheritance (pedigrees)
Lesson 5.
Learning Goals
Dihybrid cross genetic diagram
P
Phenotypes
Round
Yellow seed
Genotypes
RRYY
rryy
meiosis
meiosis
RY
ry
Gametes
X
Wrinkled
Green seed
(Pure bred)
fertilisation
F1
Phenotypes
RrYy
Genotypes
Round Yellow
Proportions
100%
(Selfed)
A dihybrid cross can be treated as two
separate monohybrid crosses
The expected probability of each type
of seed can be calculated:
•
•
•
•
Probability of an F2 seed being round = 75% or ¾
Probability of an F2 seed being wrinkled =
Probability of an F2 seed being yellow =
Probability of an F2 seed being green =
A dihybrid cross can be treated as
two separate monohybrid crosses
The expected probability of each type of seed can
be calculated:
•
•
•
•
Probability of an F2 seed being round = 75% or ¾
Probability of an F2 seed being wrinkled = 25% or ¼
Probability of an F2 seed being yellow = 75% or ¾
Probability of an F2 seed being green = 25% or ¼
THE LAW OF INDEPENDENT
ASSORTMENT
• It appears that the inheritance of seed shape
has no influence over the inheritance of seed
colour
• The two characters are inherited
INDEPENDENTLY
• The pairs of alleles that control these two
characters assort themselves independently
Mendel & Meiosis
• The pairs of chromosomes could
orientate in different ways at Anaphase 1
Patterns of Inheritance
Pedigrees
• A pedigree is a genetic
family tree that shows
how prevalent a trait is
in a family unit from
generation to
generation.
• They are often used to
track the expression of
genetic conditions and
disorders.
Pedigrees
• Squares represent males
and circles females.
• A coloured in shape
means that person has
the trait in question.
• A half coloured in shape
means that they are
carrying an allele for a
recessive trait.
Anatomy of a Pedigree
Male (left) Female (right)
Affected individuals
Carriers (Heterozygotes for
autosomal recessive)
Deceased individuals
Sex unspecified
Autosomal Dominant Inheritance
• Autosomal means not on the sex chromosomes.
• Refers to those situations in which a single copy
of an allele is sufficient to cause expression of a
trait.
Autosomal Dominant Inheritance
• 1. Every affected person should have at least one
affected parent.
• 2. Males and females should be equally often affected.
• 3. An affected person has at least a 50% chance of
transmitting the dominant allele to each offspring.
Characteristics of a Dominant Pedigree
• An affected individual has at least one affected
parent
• As a result, dominant traits show a vertical
pattern of inheritance
– the trait shows up every generation
• Two affected individuals may have unaffected
children
A
a
A AA Aa
a Aa aa
Autosomal Dominant Inheritance
examples
• Progeria (caused by a mutation) in which the person ages
very rapidly. They die before they can reproduce.
• Huntington’s Disease in which the central nervous system
starts to break down around the age of 30.
Autosomal Recessive Inheritance
• Refers to those situations where two recessive
alleles result in a trait being expressed.
Autosomal Recessive Inheritance
• 1. An affected person may not have affected parents.
Parents would be carriers.
• 2. Affects both sexes equally. Can appear to skip
generations.
• 3. Two affected parents will have affected children 100%
of the time.
Characteristics of Recessive Pedigrees
• In pedigrees involving rare traits, a horizontal pattern
of inheritance is observed
– the trait may not appear in every generation
• An individual who is affected may have parents who
are not affected, particularly as a result of
consagineous matings
• All the children of two affected individuals are
affected
a a
aa aa
a aa aa
a
Autosomal Recessive Examples
• Albinism is a genetic condition which is the loss of pigment in hair, skin
and eyes.
• Tay Sachs is a genetic disorder which is a build up of fatty deposits in
the brain, eventually proving to be fatal.
• Cystic Fibrosis is the most common fatal genetic disorder. Mutation in
chloride transport protein that causes thick mucus to build up in lungs
Pedigree of a family with some members showing
Huntington disease
Huntington disease is
Huntington disease is
Genetic Tests
• Karyotype
• Fluorescence in situ hybridization (FISH)
– Details chromosomal abnormalities through
fluorescent tags on chromosomes
• Gene testing
– Analyzes specific sequence of gene. i.e. breast
cancer susceptibility gene BRCA1 and BRCA2
• Biochemical testing
– Analyzes abnormal enzymes and proteins (Tay
Sachs)
FISH
FISH
Genetic Test for Cystic Fibrosis
• In 1989 researchers at Sick Kids identified the gene for
cystic fibrosis
• Gene was on chromosome 7 and named CFTR (cystic
fibrosis transmembrane conductance regulator)
• Over 1600 possible mutations in CFTR!
• Genetic tests can identify mutations 85-90% of the time
• 1 in every 3600 children in Canada are born with CF
Genetic test for Huntington disease
• In 1983 the gene for Huntington disease called
huntingtin was the first human disease-associated
gene to be mapped to a chromosome
• In 1993 huntingtin gene was sequenced
– CAG repeats
• Current genetic test looks for CAG repeats (36-39
repeats necessary for disease)
Genetic Counselling
• If there is a family history of a disease,
couples may consult a genetic counsellor
• Use pedigrees to determine genotypes of
the family members
• Explains probability on passing on
disease-causing allele to children
Issues with genetic screening
• Why carry out genetic screening at all?
• When is a test accurate and comprehensive
enough to be used as the basis for screening?
• Once an accurate test becomes available at
reasonable cost, should screening become
required or optional?
• If a screening program is established, who should
be tested?
• Should private companies and insurance
companies have access to employees and client
test results?
• What education needs to be provided regarding
test results?
Gene Therapy: A future cure?
• Technique aimed at treating genetic disorders
by introducing the correct form of the defective
gene into a patient’s genome
• Copy of “normal” gene is inserted into a vector
(usually viral DNA)
• Virus infects cells and delivers gene into
chromosomes
Future of Gene Therapy
• Still in experimental stages due to two
obstacles:
– immune response to viral vector and poor
integration into target chromosome
• In 2000 St. Michael’s Hospital became
performed Canada’s first gene therapy for
treatment of heart disease; gene produced
protein for stimulation of new blood vessels