Mendel Genetics Chapter 14

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Mendel GeneticsChapter 14
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Genetics

• The study of heredity

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Heredity

• Transmission of traits from one generation to
  another
• Inherited features are the building blocks of
  evolution

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Historically

• Blending of parental contributions
• Example
• Tall parent short parent
• Medium child
• Example
• Brown hair parent a red hair parent
• Reddish-brown hair child

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Problem

• No outside genes
• All parents traits blended
• Over time all members of the species will soon
  look the same.

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Variation

• Differences in offspring

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Vocabulary

• Character
• Inheritable feature
• Ex color
• Trait
• Alternate forms of the character
• Purple or white

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Vocabulary

• True-breeding
• Produced same variety as the parent
• P generation
• Parental generation
• True breeding parents

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Vocabulary

• First filial generation (F1)
• Offspring from the first cross
• Second filial generation (F2)
• Offspring from the second cross

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Vocabulary

• Alleles
• Alternate versions of the gene
• Dominant
• Trait that is expressed
• Recessive
• Trait that is not expressed or hidden

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Vocabulary

• Homozygous
• Pair of the same alleles
• Heterozygous
• Pair of different alleles
• Genotype
• Genetic make-up
• Phenotype
• Appearance of organism

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Vocabulary

• Hybridization
• Crossing of parents that are not alike
• Hybrids
• Offspring with two alleles for trait
• Testcross
• Cross with a homozygous recessive individual
• Determines the genotype of an individual.

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Vocabulary

• Self-fertilization
• Fertilization can take place in plant if
  undisturbed.
• Cross-fertilization
• Remove the male parts
• Introduce pollen from another strain
• Different traits

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Vocabulary

• Punnett square
• Diagram displays the allele possibilities of
  fertilizations
• Monohybrid
• Individuals that are heterozygous for one trait
• Dihybrid
• Individuals that are heterozygous for two traits

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Gregor Mendel

• Austrian monk
• Studied math science at the University of
  Vienna
• Studied pea plants at the monastery

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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

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• 4. Male female sex organs located on the same
  plant

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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)

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Mendels experiments

• Allowed the peas to self-fertilize
• Used true-breeding or pure-breeding plants

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Mendels experiment

• Crossed plants with alternate forms of
  characteristics
• Example
• Cross-pollinate white plants with plants that
  produced purple flowers

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Mendels experiment

• Hybrid offspring were able to self-fertilize for
  several generations
• Allowed alternate forms to segregate among the
  offspring
• Allowed him to see alternate forms
• ie purple vs white flowers

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Mendels experiment

• Crossed white flowered plants with purple
  flowered plants
• F1 always revealed purple flowered plants
• Crossed the hybrid offspring
• F2 filial generation
• Some were purple
• Some were white

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Mendels experiment

• F1 trait was hidden
• F2 trait reappeared
• Ratio in the F2 generation
• 31 dominantrecessive
• 31 purplewhite
• All traits revealed this ratio

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Mendels 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
  31 ratio

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Mendels experiment

• Concluded that the F2 generation was really 121
  
• ¼ pure-breeding dominant individuals
• ½ non-pure breeding
• ¼ pure-breeding recessive individuals

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Mendels model

• 1. Plants did not produce intermediate offspring.
  
• Plants received both alternate forms of the
  characteristic.
• 2. Alternate trait was there only not expressed

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Mendels model

• 3. Alternate traits segregated in the offspring
• Some individuals showed one trait and some the
  other.
• 4. Mendelian ratio 31 in the F2 generation
• ¾ dominant
• ¼ recessive

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Mendels model

• Alleles remain discrete
• Do not influence the other
• Do not blend
• Are passed on in the gametes

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Mendels 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

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Mendels experiment

• Crossed dihybrids
• F1 generation demonstrated dominant phenotype for
  both traits
• F2 generation showed a 9331 phenotype (16
  gamete combinations)
• Each trait showed a 31 ratio similar to a
  monohybrid cross

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Mendels second law of heredity

• Law of Independent Assortment
• Genes located on different chromosomes
• Assort independently
• Assuming the genes are on separate chromosomes

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Mendel

• Phenotypes may be influenced by many factors
• Many different genes
• Environment

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Incomplete dominance

• Not all chromosomes are dominant or recessive
• Heterozygous genotype can cause an intermediate
  between the parents

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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

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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

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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

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Multiple alleles

• ABO blood type
• Gene codes an enzyme
• Adds a sugar to the lipids on the surface of the
  RBC
• Sugars act as recognition markers for the immune
  system

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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

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ABO

• Type A IAIA Homozygous
• Type A IAi Heterozygous
• Type B IBIB Homozygous
• Type B IBi Heterozygous
• Type AB IAIB Heterozygous
• Type O ii Homozygous

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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
• Will develop antibodies against Rh blood.

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ABO

• Problem is 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
  its Rh factor
• RhoGam

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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

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Pleiotropic

• Inherited diseases that one gene produces many
  symptoms
• Sickle cell anemia
• Anemia
• Joint pain/swelling
• Heart failure
• Splenomegaly
• Renal failure

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Sickle cell

• Single aa change in the 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

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Pleiotropic

• Cystic fibrosis
• Mutation in the gene that encodes the chloride
  ion trans membrane channel
• Increased mucous
• Salty sweat
• Liver/pancreatic failure
• SOB

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Epistasis

• One gene can interfere with the expression of
  another gene
• Interaction between two non-allelic genes
• Controls phenotypic expression of a single trait

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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

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Epistasis

• Both dominant genes present
• Corn will be purple (AABB, AaBb)
• One dominant one recessive
• Corn will be white. (aaBb, aaBB, Aabb, AAbb)
• 97(purplewhite)
• 9/16 vs 7/16

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Epistasis

• Labrador retrievers has two genes that affect
  fur, nose

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Epistasis

• E gene is the gene for color
• EE or Ee genotype
• Dark pigment will be deposited
• ee no pigment

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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

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Fig. 14-12
?
BbCc
BbCc
Sperm
1/4
1/4
1/4
1/4
BC
bC
Bc
bc
Eggs
1/4
BC
BBCc
BBCC
BbCC
BbCc
1/4
bC
BbCC
bbCC
BbCc
bbCc
1/4
Bc
BBcc
Bbcc
BBCc
BbCc
1/4
bc
BbCc
bbCc
Bbcc
bbcc
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3
4
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Polygenes

• Additive effect of two or more genes determines a
  single phenotypic character.

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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

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Continuous variation

• Three genes with the dark-skin allele (A, B, C)
  contribute to the phenotype
• A cross between two AaBbCc individuals
  (intermediate skin shade) produce offspring
  covering a wide range of shades.
• Range of phenotypes forms a normal distribution.

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Continuous variation
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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

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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

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Fig. 14-14
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• 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.

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Pedigree

• Graphical representation of mating over multiple
  generations for a particular trait

Male
Affected Male
Mating
Offspring, in birth order (first-born on left)
Female
Affected Female
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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

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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 Marthas Vineyard
• Single gene
• Parents are heterozygous for deafness
• 25 chance of having a deaf child

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Recessive disorders

• Cystic Fibrosis 1/1800 European Americans
• Albinism 1/22000
• PKU 1/10,000

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Fig. 14-16
Parents
Normal
Normal
Aa
Aa
?
Sperm
a
A
Eggs
Aa
AA
Normal (carrier)
A
Normal
Aa
aa
Normal (carrier)
a
Albino
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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

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Dominant disorders

• Achondroplasia
• Form of dwarfism
• Head and torso develop normally
• Arms and legs are short
• 1/25,000

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Genetic counseling

• Identifies parents at a risk of producing a child
  with a genetic disorder
• Helps parents plan

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Amniocentesis

• Needle removes fluid from the pregnant female
• Analyzes fluid for genetic anomalies
• Needle is guided by ultrasound.

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Amniocentesis
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Fig. 14-18a
Amniotic fluid withdrawn
Centrifugation
Fetus
Placenta
Cervix
Uterus
Fluid
Bio- chemical tests
Fetal cells
Several hours
Several weeks
Several weeks
Karyotyping
(a) Amniocentesis
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Chorionic villi sampling

• Can be done earlier
• Removes cells from the membrane of placenta
• Less invasive

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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