Chapter 14 Mendel and the
Gene Idea
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).
Gregor Mendel Father of Modern Genetics.
Mendel was a pea picker.
He used peas as his study organism.
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
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 DomTt X Tt 1TT:2Tt:1tt 3 Dom: 1 ResTT X TT all TT all Domtt X tt all tt all ResTT X Tt 1TT:1Tt all DomTt X tt 1Tt:1tt 1 Dom: 1 Res
Test Cross Cross of a suspected
heterozygote with a homozygous recessive.
Ex: T_ X ttIf TT - all dominantIf 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 TtRrEach 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 floweredOr: 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 Dominance2. Codominance3. Multiple Alleles4. Epistasis5. 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
Result No hidden Recessive. 3 phenotypes and
3 genotypes Red = CR CR
Pink = CRCW
White = CWCW
Another example
Codominance Both alleles are expressed
equally in the phenotype. Ex. MN blood group
MM MN NN
Result No hidden Recessive. 3 phenotypes and
3 genotypes
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
Result Multiple genotypes and
phenotypes. Very common event in many
traits.
Alleles and Blood Types
Type Genotypes A IA IA or IAi B IB IB or IBi AB IAIB
O ii
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
Gerbils
In Gerbils
CcBb X CcBbBrown X Brown
F1 = 9 brown (C_B_) 3 black (C_bb) 4 albino (cc__)
Result Ratios often altered from the
expected. One trait may act as a
recessive because it is “hidden” by the second trait.
Epistasis in Mice
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.
Pedigree Chart Symbols
Male
Female
Person with trait
Sample Pedigree
Dominant Trait Recessive Trait
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 .25R = .25There 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.
Carrier Recognition Fetal Testing
Amniocentesis Chorionic villi sampling
Newborn Screening
Fetal Testing Biochemical Tests Chromosome Analysis
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.
Multifactorial Diseases Where Genetic and
Environment Factors interact to cause the Disease.
Ex. Heart Disease Genetic Diet Exercise Bacterial Infection