Download Inheritance The passing of traits from parents to offspring Genetics

Inheritance
The passing of traits from parents to offspring
Genetics
The scientific study of the inheritance
Gregor Mendel
-Father of modern genetics
-Used peas to successfully identify the laws of
heredity
Why use Peas as an Experimental Organism?
-Short life span
-Bisexual
-Cross- and self- pollinating so can have purebred
(homozygous) and hybrid (heterozygous)
-Many traits known
Mendel’s Work
-Studied crosses of seven characters, each with
two expressions or traits
Ex.:
Character- Height
Traits- Tall or short
Monohybrid or Mendelian Crosses
-Crosses that work with a single character at a
time
Ex.:
Tall or short
P Generation
The Parental generation or the first two
individuals (true breeding= homozygous parents)
used in a cross
Offspring
-F1: first filial (brother) generation ( offspring of
the P generation)
-F2: second filial generation, bred by crossing
two F1 plants together or allowing a F1 to selfpollinate
Another Sample Cross
P1- Tall x short (TT x tt)
F1- All tall (Tt)
F2- Three tall to one short (1TT:2Tt:1tt)
Result Summary:
-In all crosses, the F1 generation showed only
one of the traits (Dominant) regardless of which
male or female
-The other trait (Recessive) reappeared in the F2
at 25% (3:1 rate)
Mendel’s Hypothesis explaining the 3:1
inheritance pattern in the F2 offspring
1.Genes can have alternate versions called
alleles
2.Each offspring inherits two alleles, one from
each parent
3.If the two alleles differ, the dominant allele is
expressed. The recessive allele remains hidden
unless the dominant allele is absent
4.The two alleles for each trait separate during
gamete formation. This is called: Mandel’s Law
of Segregation
Homozygous
When organisms have two of the same alleles for
a particular trait (dominant traits are usually
capitalized (TT) and recessive traits are usually
lower case (tt).
Heterozygous
When the two alleles for that trait are different
(Tt). In complete dominance, the dominant allele
will be expressed.
Phenotype vs Genotype
-The physical appearance of the organism:
Phenotype
-The genetic makeup of the organism, usually
shown in a code: Genotype
Ex.:
-P allele= Purple
-p allele= White
Monohybrid Cross
-A cross involving only one character
Ex.:
What happens when you cross two pea plants
heterozygous for flower color?
-Genotype of parents? Pp xPp
-Results:
Genotype ratio: 1:2:1
Phenotype ratio: 3:1
6 Mendelian Crosses are Possible
Test Cross
Cross
TT x tt
Tt xTt
TT x TT
ttxtt
TT x Tt
Tt x tt
Genotype
All Tt
1TT:2Tt:1tt
All TT
All tt
1TT:1Tt
1Tt:1 tt
Phenotype
All Don
3 Dom: 1 Res
All Dom
All Res
All Dom
1 Dom: 1 Res
-Purpose: Done to determine if an individual
showing a dominant phenotype has a
homozygous or heterozygous genotype
-How to: Cross of an unknown dominant parent
with a homozygous recessive parent
Ex.:
T_ x tt
If TT- all dominant offspring
If Tt- 1 dominant: I recessive offspring
Dihybrid Cross
-Cross intended to study two genetic traits
Ex.: Tall (T) or Short (t) and Red (R) or White (r)
-Need four letters to code for the cross
Ex.: TtRr
-Each gamete must get one letter for each trait
-Use the FOIL method to figure out the possible
gamete combinations from a parent
Ex.: A parent TtRr can produce TR, Tr, tR, tr
offspring due to independent assortment
Dihybrid Cross: TtRr x TtRr
-Each parent can produce 4 types of gametes
-Cross is a 4x4 with 16 possible offspring
-Parent genotype: TtRr
-Possible offspring: TR, Tr, tR, tr
-Results:
Genotype ratio: Tall red=9, Tall white=3, Short
red=3, Short white=1
Phenotype ratio: 9:3:3:1
Law of Independent Assortment
-The inheritance of 1st genetic traits is NOT
dependent on the inheritance of the 2nd trait
-Inheritance of height is independent of the
inheritance of flower color
Probability
-Genetics is a specific application of the rules of
probability
-The chance that an event will occur out of the
total number of possible events
Genetic Ratios
-The monohybrid “ratios” are actually the
“probabilities” of the results of random
fertilization
Ex.: 3:1
75% chance of the dominant ¾
25% chance of the recessive ¼
Rule of Multiplication
-The probability that two alleles will come
together at fertilization, is equal to the product
of their separate probabilities
Ex.: TtRr x TtRr
-the probability of getting a tall offspring is ¾
-the probability of getting a red offspring is ¾
-the probability of getting a tall red offspring is ¾
x ¾ =9/16
Comment
-Use the Product Rule to calculate the results of
complex crosses rather than the Punnett square
Inheritance patterns are often more complex
than predicted by simple Mendelian Genetics
which demonstrates complete dominance
Other than complete dominance
Incomplete Dominance
-When the F1 hybrids show a phenotype
somewhere between the phenotypes of the two
parents
-Unlike complete dominance problems, where
only the allel letter is used, for these problems,
the character is the letter we use and the allele is
written as a superscript
Ex.: Red (CR CR) x White (CW x CW)
F1= all pink
F2= 1 red: 2pink: 1 white
-No hidden recessive
Codominance
-Both alleles are expressed equally in the
phenotype
-You get both alleles expressed in the
heterozygous form, here a pink flower ( CP x CP)
and a white flower (CW x CW) form a pink and
white flower (CP x CW)
Result:
-No hidden recessive
-3 phenotypes and 3 genotypes (but not a does
effect)
Multiple Alleles
-A form of codominace where there are more
than two alleles for a trait
Ex.: ABO blood group
--IA= A type antigen
--IB=B type antigen
--i= no antigen
Result:
-Multiple genotypes and phenotypes
-Very common event in many traits
Alleles and Blood Types
Blood Type
Type
Genotypes
A
IAIA or IAi (AA or Ai)
B
IBIB or IBi (BB or Bi)
AB
IAIB
O
ii
-Rh blood factor is a separate factor from the
ABO blood group
-Rh+ = Dominant
-Rh- = Recessive
-A+ blood = Dihybrid Trait
Polygenic Inheritance
Environment affect
-Two or more genes have an addictive effect on a
single character in the phenotype
-When several genes are involved, the
phenotype described by a bell-shaped curve
Ex.: Skin color, height
Ex.: Skin color is likely controlled the trait by
atleast 4 genes, each dominant gives a darker
skin color
Results:
-Traits tend to “run” in families
-Offspring often between parental types
-Can affect the expression of genes
Ex.: In terms of skin color, although your genes
may code for a little melanin production,
prolonged exposure to UV radiation may alter
the expression rate of the gene and you will
produce more as a response to you environment
Morgan
-Choose to use fruit flies as a test organism in
genetics
-All owed the first tracing of traits to specific
chromosomes
Morgan discovered:
-Sex linked traits
Sex- Linked Genes
-Genetic traits whose expression are dependent
on the sex of the individual
-Are located on the sex chromosomes X and Y
-If a gene is located on the X chromosome,
fathers pass sex-linked genes on to their
daughters but not sons( they get Y)
-Females will express a sex-linked trait exactly
like any other trait but males will express the
allele on the X chromosome from mother
-The vast majority of genes on the X
chromosome have nothing to do with sex
Males
-Hemizygouse: one copy of X chromosome
-Show all X traits (Dominant or recessive)
-More likely to show X linked recessive gene
disorders than fermales
Several Sex-Linked Disorders have Medical
Significance:
-Duchenne Muscular Dystrophy: is a sex-linked
disorder resulting in progressive loss of muscular
coordination until the late 20s when usually
lethal
-Hemophilia: Also a sex-linked disorder resulting
in the inability to clot blood normally b/c of the
absence of proteins required to do so
-Colorblindness
X-Linked Patterns
-Trait is usually passed from a carrier mother to
½ of sons
-Affected father has no affected children, but
passes the trait on to all daughters who will be
carriers for the trait
Comment
-Watch how questions with sex linkage are
phrased:
Chance of children? All Possibilities
Chance of males? Male Possibilities
Extranuclear Inheritance
-Extranuclear genes are those found in the
mitochrondria and chloroplasts. They were
discovered in plants in 19009. Since then, they
have been linked to several rare and severe
inherited diseases in humans. Defects in the
mitochrondrial DNA reduce the amount of ATP a
cell can make, Therefore the organs most affect
by these mutations are the ones that require the
most energy: the nervous system and muscle
Extranuclear Inheritance Cont.
-Because the mitochrondria passed to the zygote
all come from the cytoplasm of the egg, these
dieases are always inherited from the mother
and because of this, do not follow Mendelian
rules of inheritance
Genetic Studies in Humans
-Often done be pedigree charts
-Pedigree: a chart that shows the relationship b/t
parent and offspring across two or more
generations
-Can help determine the genome of individuals
that comprise them and also genome of future
offspring
-Male = square
- Female = circle
-Person with trait = colored in
Pedigree Chart Tips
-Dominant Trait: never skip generations
-Recessive Trait: skips generations
-Sex linked: Predominant gender
-Autosomal: Both genders equally get disorder
Human Recessive Disorders
-Albinism
-Sickle Cell Anemia
-Tay-Sachs Disease
-Cystic Fibrosis
-PKU
-Galactosemia
Sickle-cell Disease
-Caused by an allele that codes for a mutant
hemoglobin molecule that forms long rods when
O 2 in the blood is low
-Reduced O 2 carrying capacity
-Codominant inheritance
Tay-Sachs
-Caused by an allele that codes for a
dysfunctional enzyme that breaks down certain
lipids in the brain
-Brain cells unable to metabolize type if lipid
accumulation and causes brain damage
-Death in infancy or early childhood
Cystic Fibrosis
-Most common lethal genetic disease in U.S
-Most frequent in Caucasian populations (1/20 a
carrier)
-Produces defective chloride channels in
membranes causing high levels of chloride
outside cells that make muclus thick and heavy
leading to organ damage
Recessive Pattern
-Usually rare
-Skips generations
-Often an enzyme defect
Human Lethal Dominant Disorders
-Less common than recessive. Require only one
copy of the dominant allele in order for the
disorder to be expressed
Ex.:
-Huntingtons Disease(progressive degeneration
of brain cells)
-Achondroplasia (Dwarfism)
-Famllial Hyperchlosteroemia (High cholesterol)
Inheritance Pattern
-Each affected individuals had one affected
parent
-Doesn’t skip generations
-Homozygous causes show worse phenotype
symptoms
-May have post-maturity onset of symptons
Genetic Screening
-Risk assessment for an individual inheriting a
trait
-Uses probability to calculate the risk
Carrier Recognition
-Fetal Testing
--Amiocentesis
--Chorionic Villi Sampling
-Newborn Screening
Fetal Testing
-Biochemical testing
-Chromosome analysis
Amniocentsis
-Administrated between 11-14 weeks
-Extract amnionic fluid = cells and fluid
-Biochemical tests and karyotype
-Requries culture time for cells
Chorionic Villi Sampling
-Administrated between 8-10 weeks
-Extract tissue from chorian (placenta)
-Greater risk but no culture time required
Newborn Screening
-Blood tests for recessive conditions that can
have the phenotypes treated at avoid damagegenotypes are not changed
Ex.:
-PKU
Multifactorial Diseases
-Where genetic and environmental factors
interact to cause the disease
-Becoming more widely recognized in medicine
Ex.”
-Genetic
-Diet
-Exercise
-Bacterial infection
Chi-Square
-Is a statistical test used to determine how well
observed ratios of data that you collect fits
expected ratios of data
Expected Ratios
-Come from a Punnett square
-Can count offspring to see how well our
observed results match the expected outcomes
-In genetic crosses, b/c Mendel’s Laws we
assume that there is a random assortment of
alleles from parents to offspring, so the
probability of receiving a particular gene
combination in an offspring is due to chance and
a punnett square should predict the outcome
Null Hypothesis
-N 0 : Our observed data should match our
expected data
-If our data matches the expected ratios of
offspring within a reasonable amount, we accept
our null hypothesis and our results are
statistically significant
-If our data does not match the expected ratios
within a reasonable amount, we reject the null
hypothesis and our data is not statistically
significant
The formula for Chi Square
-Where O is the observed frequency
-E is the expected frequency
-df is the degree of freedom (n-1)
-x2 is chi square
Critcial Value
-P =.o5 always
-If chi-square results are below the CV we accept
the null hypothesis
-If chi-square results are above the CV we reject
the null hypothesis
Use the row based on the df, which is one
subtracted from the number of phenotypes
possible