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Inheritance
The passing of traits from parents to offspring.
Humans have known about inheritance for thousands of years.
Genetics
The scientific study of the inheritance.
Genetics is a relatively “new” science (about 150 years).
Why Use Peas?
Short life span. Bisexual. Many traits known. Cross- and self-pollinating. (You can eat the failures).
Cross-pollination
Two parents. Results in hybrid offspring
where the offspring may be different than the parents.
Self-pollination
One flower as both parents. Natural event in peas. Results in pure-bred
offspring where the offspring are identical to the parents.
Mendel's Work
Used seven characters, each with two expressions or traits.
Example: Character - height
Traits - tall or short.
Monohybrid or Mendelian Crosses
Crosses that work with a single character at a time.
Example - Tall X short
P Generation
The Parental generation or the first two individuals used in a cross.
Example - Tall X short Mendel used reciprocal crosses,
where the parents alternated for the trait.
Offspring
F1 - first filial generation. F2 - second filial generation,
bred by crossing two F1 plants together or allowing a F1 to self-pollinate.
Another Sample Cross
P1 Tall X short (TT x tt)
F1 all Tall (Tt)
F2 3 tall to 1 short
(1 TT: 2 Tt: 1 tt)
Results - Summary In all crosses, the F1
generation showed only one of the traits regardless of which was male or female.
The other trait reappeared in the F2 at ~25% (3:1 ratio).
Mendel's Hypothesis
1. Genes can have alternate versions called alleles.
2. Each offspring inherits two alleles, one from each parent.
Mendel's Hypothesis3. If the two alleles differ, the
dominant allele is expressed. The recessive allele remains hidden unless the dominant allele is absent.
Comment - do not use the terms “strongest” to describe the dominant allele.
Mendel's Hypothesis
4. The two alleles for each trait separate during gamete formation. This now called: Mendel's Law of Segregation
Vocabulary
Phenotype - the physical appearance of the organism.
Genotype - the genetic makeup of the organism, usually shown in a code. T = tall t = short
Helpful Vocabulary
Homozygous - When the two alleles are the same (TT/tt).
Heterozygous- When the two alleles are different (Tt).
6 Mendelian Crosses are Possible
Cross Genotype PhenotypeTT X tt all Tt all Dom
Tt X Tt 1TT:2Tt:1tt 3 Dom: 1 Res
TT X TT all TT all Dom
tt X tt all tt all Res
TT X Tt 1TT:1Tt all Dom
Tt X tt 1Tt:1tt 1 Dom: 1 Res
Test Cross
Cross of a suspected heterozygote with a homozygous recessive.
Ex: T_ X tt
If TT - all dominant
If Tt - 1 Dominant: 1 Recessive
Dihybrid Cross
Cross with two genetic traits. Need 4 letters to code for the
cross. Ex: TtRr
Each Gamete - Must get 1 letter for each trait. Ex. TR, Tr, etc.
Dihybrid Cross
TtRr X TtRr
Each parent can produce 4 types of gametes.
TR, Tr, tR, tr
Cross is a 4 X 4 with 16 possible offspring.
Results
9 Tall, Red flowered 3 Tall, white flowered 3 short, Red flowered 1 short, white flowered
Or: 9:3:3:1
Law of Independent Assortment
The inheritance of 1st genetic trait is NOT dependent on the inheritance of the 2nd trait.
Inheritance of height is independent of the inheritance of flower color.
Comment
Ratio of Tall to short is 3:1 Ratio of Red to white is 3:1 The cross is really a product
of the ratio of each trait multiplied together. (3:1) X (3:1)
Probability
Genetics is a specific application of the rules of probability.
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: 3:175% chance of the dominant25% chance of the recessive
Rule of Multiplication
The probability that two alleles will come together at fertilization, is equal to the product of their separate probabilities.
Example: 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 work out the Punnett Squares.
Ex: TtrrGG X TtRrgg
Variations on Mendel
1. Incomplete Dominance
2. Codominance
3. Multiple Alleles
4. Epistasis
5. Polygenic Inheritance
Incomplete Dominance
When the F1 hybrids show a phenotype somewhere between the phenotypes of the two parents.(blending)
Ex. Red X White snapdragons F1 = all pink F2 = 1 red: 2 pink: 1 white
Multiple Alleles
When there are more than 2 alleles for a trait.
Ex. ABO blood group IA - A type antigen IB - B type antigen i - no antigen
Comment
Rh blood factor is a separate factor from the ABO blood group.
Rh+ = dominant Rh- = recessive A+ blood = dihybrid trait
Epistasis
When 1 gene locus alters the expression of a second locus.
Ex: 1st gene: C = color, c = albino 2nd gene: B = Brown, b = black
Result
Ratios often altered from the expected.
One trait may act as a recessive because it is “hidden” by the second trait.
Polygenic Inheritance
Factors that are expressed as continuous variation.
Lack clear boundaries between the phenotype classes.
Ex: skin color, height
Genetic Basis
Several genes govern the inheritance of the trait.
Ex: Skin color is likely controlled by at least 4 genes. Each dominant gives a darker skin.
Result Mendelian ratios fail. Traits tend to "run" in
families. Offspring often intermediate
between the parental types. Trait shows a “bell-curve” or
continuous variation.
Genetic Studies in Humans
Often done by Pedigree charts. Why?
Can’t do controlled breeding studies in humans.
Small number of offspring. Long life span.
Human Recessive Disorders
Several thousand known: Albinism Sickle Cell Anemia Tay-Sachs Disease Cystic Fibrosis PKU Galactosemia
Sickle-cell Disease Most common inherited disease
among African-Americans. Single amino acid substitution
results in malformed hemoglobin. Reduced O2 carrying capacity. Codominant inheritance.
Tay-Sachs Eastern European Jews. Brain cells unable to metabolize
type of lipid, accumulation of causes brain damage.
Death in infancy or early childhood.
Cystic Fibrosis
Most common lethal genetic disease in the U.S.
Most frequent in Caucasian populations (1/20 a carrier).
Produces defective chloride
channels in membranes.
Recessive Pattern
Usually rare. Skips generations. Occurrence increases with
consaguineous matings. Often an enzyme defect.
Human Dominant Disorders
Less common then recessives.
Ex: Huntington’s disease Achondroplasia Familial Hypercholsterolemia
Inheritance Pattern
Each affected individual had one affected parent.
Doesn’t skip generations. Homozygous cases show
worse phenotype symptoms. May have post-maturity onset
of symptoms.
Genetic Screening
Risk assessment for an individual inheriting a trait.
Uses probability to calculate the risk.
General FormalR = F X M X D
R = riskF = probability that the female
carries the gene.M = probability that the male
carries the gene.D = Disease risk under best
conditions.
Example
Wife has an albino parent. Husband has no albinism in
his pedigree. Risk for an albino child?
Risk Calculation Wife = probability is 1.0 that
she has the allele. Husband = with no family
record, probability is near 0. Disease = this is a recessive
trait, so risk is Aa X Aa = .25 R = 1 X 0 X .25 R = 0
Risk Calculation
Assume husband is a carrier, then the risk is:
R = 1 X 1 X .25
R = .25
There is a .25 chance that every child will be albino.
Common Mistake
If risk is .25, then as long as we don’t have 4 kids, we won’t get any with the trait.
Risk is .25 for each child. It is not dependent on what happens to other children.
Amniocentesis Administered between 11 - 14
weeks. Extract amnionic fluid = cells
and fluid. Biochemical tests and
karyotype. Requires culture time for cells.
Chorionic Villi Sampling
Administered between 8 - 10 weeks.
Extract tissue from chorion (placenta).
Slightly greater risk but no culture time required.
Newborn Screening
Blood tests for recessive conditions that can have the phenotypes treated to avoid damage. Genotypes are NOT changed.
Ex. PKU
Newborn Screening
Required by law in all states. Tests 1- 6 conditions. Required of “home” births
too.
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