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Mendel Genetics
Chapter 14
Genetics
The study of heredity
Heredity
Transmission of traits
One generation to another
Inherited features are the building
blocks of evolution
Historically
Blending of parental contributions
Example:
Tall parent + short parent
Medium child
Problem
No outside genes
All parents traits blended
Over time all members of the species
will look the same.
Variation
Differences in offspring
Vocabulary
Character:
Inheritable feature
Ex: color
Trait:
Alternate forms of the character
Purple or white
Vocabulary
True-breeding:
Produced same variety as the parent
P generation
Parental generation
Vocabulary
First filial generation (F1)
Offspring from the first cross
Second filial generation (F2)
Offspring from the second cross
Vocabulary
Alleles:
Alternate versions of the gene
Dominant:
Trait that is expressed
Recessive:
Trait that is not expressed or hidden
Vocabulary
 Homozygous:
 Pair of the same alleles
 Heterozygous:
 Pair of different alleles
 Genotype:
 Genetic make-up
 Phenotype:
 Appearance of organism
Vocabulary
Hybridization:
Crossing of parents that are not alike
Hybrids:
Offspring with two alleles for trait
Testcross:
Cross with a homozygous recessive
individual
Determines genotype of an individual.
Vocabulary
Self-fertilization:
Fertilization can take place in plant if
undisturbed.
Cross-fertilization:
Remove the male parts
Introduce pollen from another strain
Different traits
Vocabulary
Punnett square:
Diagram
Displays allele possibilities of
fertilizations
Vocabulary
 Monohybrid:
 Individuals are heterozygous for one trait
 Aa
 Tt
 Dihybrid:
 Individuals are heterozygous for two traits
 AaTt
Gregor Mendel
Austrian monk
Studied math & science
University of Vienna
Studied pea plants at the monastery
Why the pea??
1. Has been studied
Able to produce hybrid peas
2. Variety with 7 simple & easy to see
traits
Purple vs white flower
3. Small, easy to grow
Short generation time
4. Male & female sex organs located on
same plant
Mendel
 Chose comparable traits
 1. Flower color (white vs purple)
 2. Seed color (yellow vs green)
 3. Shape of seed (smooth vs wrinkled)
 4. Pod color (green vs yellow)
 5. Pod shape (inflated vs constricted)
 6. Flower location (axial vs terminal)
 7. Plant size (tall vs. short)
Mendel’s experiments
Allowed the peas to self-fertilize
Used true-breeding or pure-breeding
plants
Mendel’s experiment
Crossed plants with alternate forms of
characteristics
Example:
Tall plants with short plants
Mendel’s experiment
Parental generation
Pure white flowered plants X pure
purple flowered plants
F1 always revealed purple flowered
plants
Crossed the hybrid offspring
F2 filial generation
Some were purple
Some were white
Mendel’s experiment
F1 trait was hidden
F2 trait reappeared
Ratio in the F2 generation
3:1 dominant:recessive
3:1 purple:white
All traits revealed this ratio
Mendel’s experiments
F2 generation self-fertilized
White flowers always produce white
flowers
Purple flowers
1/3 produced only purple flowers
2/3 produced dominant & recessive
flowers in a 3:1 ratio
Mendel’s experiment
Concluded that the F2 generation was
really 1:2:1
¼ pure-breeding dominant individuals
½ non-pure breeding
¼ pure-breeding recessive individuals
Mendel’s model
1. Plants did not produce intermediate
offspring.
2. Alternate trait was there only not
expressed
Mendel’s model
3. Alternate traits segregated in the
offspring
4. Mendelian ratio:
3:1 in the F2 generation
¾ dominant
¼ recessive
Mendel’s model
Alleles remain discrete
Do not influence the other
Do not blend
Are passed on in the gametes
Mendel’s first law of heredity
Law of Segregation:
Alternate alleles of a character
Segregate (separate) from each other &
remain distinct.
Seen in meiosis when the homologous
chromosomes separate
Form gametes
Mendel’s experiment
Crossed dihybrids
F1 generation demonstrated dominant
phenotype for both traits
F2 generation showed a 9:3:3:1
phenotype (16 gamete combinations)
Each trait showed a 3:1 ratio similar to
a monohybrid cross
Mendel’s second law of heredity
Law of Independent Assortment:
Genes located on different
chromosomes
Assort independently
Assuming the genes are on separate
chromosomes
Mendel
Phenotypes may be influenced by many
factors
Many different genes
Environment
Incomplete dominance
Not all chromosomes are dominant or
recessive
Heterozygous genotype can cause an
intermediate between the parents
Codominance
Effect of both alleles can be seen
MN blood groups
Molecules on surface of RBC
MM, NN or MN
MN see affects of both
Codominance
Tay-Sachs disease (homozygous
recessive)
Brain cells unable to break down lipids
Lacking enzyme build up lipids
Retardation & early death
Heterozygous
50% the normal enzyme levels
Survive
Tay Sachs
1 in 300,000 births in the US
1 in 3500 births in Ashkenazi Jews
1 in 28 are carriers in this population
Multiple alleles
ABO blood type
Gene codes an enzyme
Adds a sugar to lipids
Located on the surface of the RBC
Sugars act as recognition markers for
the immune system
ABO
3 gene alleles
4 different blood types
I is the enzyme
IA (allele) adds galactose
IB (allele) adds galactosamine
i (allele) has no sugar
ABO
Type A
Type A
Type B
Type B
Type AB
Type O
IAIA Homozygous
IAi Heterozygous
IBIB Homozygous
IBi Heterozygous
IAIB Heterozygous
ii
Homozygous
Rh blood group
Cell surface marker on the RBC
85% have the marker
Rh +
Rh does not have the marker
If a Rh- person gets blood that is Rh +
Develops antibodies against Rh+ blood.
ABO
 Problem
 Rh- mother gives birth to a child that is Rh +
(Rh+ dad)
 She has built up antibodies
 They could cross into the babies blood.
 Erythroblastosis fetalis:
 Babies blood clumps due to antibodies against
it’s Rh factor
 RhoGam
Pleiotropic
 Allele has more than one effect on the
phenotype
 One gene has many effects
 Peas: gene for flower color
 Codes for seed cover color
 Yellow mice
 Gene for yellow fur
 Same for lethal developmental defect
 So homozygous dominant would die
Pleiotropic
Inherited diseases that one gene
produces many symptoms
Sickle cell anemia
Anemia
Joint pain/swelling
Heart failure
Splenomegaly
Renal failure
Sickle cell
Single aa change in beta-globin of
hemoglobin
Causes hemoglobin to be sticky
Sickle cell shape
Higher incidence to people of African
decent 1/500
Heterozygous for the disease
Have greater resistance to malaria
Pleiotropic
Cystic fibrosis
Mutation in the gene that encodes the
chloride ion trans membrane channel
Increased mucous
Salty sweat
Liver/pancreatic failure
SOB
Epistasis
One gene can interfere with the
expression of another gene
Interaction between two non-allelic
genes
Controls phenotypic expression of a
single trait
Epistasis
Corn (Zea Mays)
Purple pigment called anthocyanin
pigment
Requires two working enzyme genes to
produce the color
Dominant alleles have functional genes
Recessive alleles have non-functional
genes
Epistasis
Both dominant genes present
Corn will be purple (AABB, AaBb)
One dominant & one recessive
Corn will be white. (aaBb, aaBB, Aabb,
AAbb)
9:7(purple:white)
9/16 vs 7/16
Epistasis
Labrador retrievers has two genes that
affect fur, nose
Epistasis
E gene is the gene for color
EE or Ee genotype
Dark pigment will be deposited
ee no pigment
Epistasis
B gene determines darkness of pigment
Distributes melanosomes (hair)
EEBB, EeBb will be a black lab
EEbb, Eebb will be a chocolate lab
eeBB, eeBb will have yellow fur/black
nose
eebb will have yellow fur/brown nose
Fig. 14-12
BbCc

BbCc
Sperm
1/
4 BC
1/
4 bC
1/
4 Bc
1/
4 bc
Eggs
1/
1/
1/
1/
4 BC
BBCC
BbCC
BBCc
BbCc
BbCC
bbCC
BbCc
bbCc
BBCc
BbCc
BBcc
Bbcc
BbCc
bbCc
Bbcc
bbcc
4 bC
4 Bc
4 bc
9
: 3
: 4
Polygenes
Additive effect of two or more genes
determines a single phenotypic
character.
Continuous variation
When multiple genes jointly influence a
character
A range in the degree of expression
Such as height or weight
Quantitative traits:
Traits that cause a range in phenotype
Continuous variation
•
•
•
•
Three genes with the dark-skin allele (A, B, C)
Contribute to the phenotype
A cross between two AaBbCc individuals
Produce offspring covering a wide range of
shades.
• Range of phenotypes forms a normal
distribution.
Continuous variation
Environmental effects
Some alleles are heat sensitive.
Artic fox makes fur pigment only when
it is warm
During the winter it is white/summer
brown
Environmental effects
Siamese cats
Heat sensitive enzyme that codes for
Melanin
Above 330C it is inactive
Ear tips, nose are colder so they are
darker

Fig. 14-14
Mendelian Inheritance in humans is
difficult to study because:
1. Generation time is 20 years.
2. Humans produce relatively few
offspring.
3. Breeding experiments are impossible.
Pedigree
Graphical representation of mating over
multiple generations for a particular
trait
Male
Female
Affected
Male
Affected
Female
Mating
Offspring, in
birth order
(first-born on left)
Pedigree
Hemophilia:
Bleeding disorder
Affects one protein in series of
proteins to clot blood
Sex linked genetic abnormality
X-linked recessive allele
Heterozygous females are carriers but
do not have the disease
Human genetics does follows
Mendelian principles
Most genetic disorders are recessive
Majority of recessive disorders are
born to heterozygous parents that are
symptom free
Deafness in Martha’s Vineyard
Single gene
Parents are heterozygous for deafness
25% chance of having a deaf child
Recessive disorders
Cystic Fibrosis 1/1800 European
Americans
Albinism 1/22000
PKU
1/10,000
Fig. 14-16
Parents
Normal
Aa

Normal
Aa
Sperm
A
a
A
AA
Normal
Aa
Normal
(carrier)
a
Aa
Normal
(carrier)
aa
Albino
Eggs
Dominant disorders
Not too common
Huntington disease
Altered protein in nerve cells of the
brain
Leads to neural degeneration
Mental deterioration and uncontrollable
movements
Age of onset around 40-50
Dominant disorders
Achondroplasia
Form of dwarfism
Head and torso develop normally
Arms and legs are short
1/25,000
Genetic counseling
Identifies parents at a risk
Produce a child with a genetic disorder
Helps parents plan
Amniocentesis
Needle removes fluid from the pregnant
female
Analyzes fluid for genetic anomalies
Needle is guided by ultrasound.
Amniocentesis
Fig. 14-18a
Amniotic fluid
withdrawn
Centrifugation
Fetus
Placenta
Uterus
Cervix
Fluid
Fetal
cells
BioSeveral chemical
hours
tests
Several
weeks
Several
weeks Karyotyping
(a) Amniocentesis
Chorionic villi sampling
Can be done earlier
Removes cells from the membrane of
placenta
Less invasive
Genetic counseling
Identifies aneuploidy
Helps identify enzyme problems such as
PKU (phenylketouria)
Missing enzyme to break down
phenylalanine
Tay-Sachs disorder missing the enzyme
to break down gagliosides