Genetics - Lancaster High School
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Transcript Genetics - Lancaster High School
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