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Chapter 14
Mendel and the Gene Idea
Genetics
• The scientific study of the
inheritance.
Gregor Mendel 1822 - 1884
• Father of Modern Genetics.
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x
Studied physics and math,
Became a monk to continue
learning for free.
Worked as a teacher as he
experimented in monastery
a
“Blending” hypothesis accepted
at the time.
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• Used peas as his
model organism.
Why Use Peas?
• Short life span.
• Many traits known.
• Cross- and self-pollinating.
Monohybrid or Mendelian
Crosses
• Crosses that work with a
single gene at a time.
Example:
Gene – Height of plant
Phenotypes Tall X short
P Generation
• The Parental generation or the
first two individuals used in a
cross.
Example - Tall X short
Offspring
• F1 - first filial (son or
daughter) generation.
• F2 - second filial generation,
bred by crossing two F1 plants
together or allowing a F1 to
self-pollinate.
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 Hypothesis
3. If the two alleles differ, the
dominant allele is expressed.
The recessive allele remains
hidden unless the dominant
allele is absent.
Allele for purple
Alleleflowers
Locus for flower-color Purple
gene
Allele for White
white flowers
Homologous
pair of
chromosomes
Mendel's Hypothesis
4. The two alleles for each trait
separate during gamete
formation.
This now called:
Mendel's Law of Segregation
Test Cross
• Cross an unknown genotype
with 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 x TtRr
• Each Gamete - Must get 1
letter for each trait.
• Ex. TR, Tr, etc.
Results
• 9 Tall, Red flowered
• 3 Tall, white flowered
• 3 short, Red flowered
• 1 short, white flowered
Or: 9:3:3:1
Dihybrid cross between 2
double heterozygotes always
gives 9:3:3:1 ratio
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.
Variations on Mendel
1.
2.
3.
4.
5.
Incomplete Dominance
Codominance
Multiple Alleles
Epistasis
Polygenic Inheritance
Incomplete Dominance
• When the F1 hybrids show a
phenotype somewhere
between the phenotypes of
the two parents.
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: Colors in cows
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
A
B
AB
O
Genotypes
IA IA or IAi
IB IB or IBi
IAIB
ii
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
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.
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:
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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.
Dominance vs Phenotype
• For any character,
dominance/recessiveness
relationships of alleles depend
on the level at which we
examine the phenotype.
Example -Tay-Sachs
• Disease is fatal; a dysfunctional
enzyme causes an
accumulation of lipids in the
brain.
• At the organismal level, the
allele is recessive.
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Tay-Sachs
• At the biochemical level, the
phenotype (i.e., the enzyme
activity level) is incompletely
dominant.
• At the molecular level, the
alleles are codominant.
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.