Transcript Genetics Review - Net Start Class

Chapter 13: Mendel and the
Basics of Genetics
Terminology
• Chromosome: structure containing a single,
linear DNA molecule that physically transmits
hereditary info from one generation to the next.
• Chromatin: the DNA and protein of the
chromosome
• Nucleosome: a bead-like structure made of a
single DNA molecule spooled around a histone
protein core (necessary for fit in the nucleus)
Terminology
• Gene: sequence of DNA nucleotides
(sugar, phosphate, nitrogenous base) on a
chromosome that encodes a specific
protein
• Alleles: alternative forms of a gene (Ex.
brown vs. black for hair color)
• Codon: 3 DNA nucleotides in sequence
that code for a particular amino acid
(combine to form proteins)
Gregor Mendel
• Austrian Monk
– Background in mathematics and botany
– 1831 to 1853
– Studied inheritance patterns in peas.
• Discovered that:
– Heritable factors called traits were passed on from
one generation to the next.
– Traits are carried on chromosomes in structures
called genes.
• Different forms of the gene are called alleles.
• Autosomes: 22 matched chromosomes
• Sex Chromosomes: pair #23 (XX, XY)
The Experiment
• Mendel cross pollinated pea plants and followed
their traits.
• Stamen – male organ of the pea plant
– Produces pollen - the male gamete
– Pollen produced at the terminus of the stamen in a structure
called the anther.
• Carpel – female organ of the pea plant (pistil plus ovary)
– Produces the ova – female gamete.
• Union of pollen and ova creates a fertilized egg called the
zygote which develops into a seed and eventually grows into
an embryo.
– Mendel used True Breeders
• Self pollinating plants that produced clones of themselves.
What did Mendel Find?
• Mendel found that in the F1 generation (first filial)
all of the flowers were the same color.
• There was no blending of parental characteristics.
• Only one trait was expressed in the hybrid offspring.
• When he crossed two flowers from the F1
generation he found a 3:1 ratio of purple to white
flowers.
• This indicated that one trait was dominant over
the other trait.
Mendel’s Two Laws of Heredity
• #1. Law of Segregation: members of each pair
of alleles separate when gametes are formed
(get ONE allele from mom and ONE allele from
dad for each trait)
• #2. Law of Independent Assortment: when
two or more pairs of alleles are located on
different chromosomes or far apart on the same
chromosome, they separate independently of
one another during gamete formation (just cause
you got mom’s eye color does not mean you will
also get her hair color, even if found on her
same chromosome)
Mendel Made Four Assumptions
•
•
•
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1. There are alternative forms of heritable factors called alleles
2. The offspring inherit an allele from each parent.
3. The alleles separate during meiosis (Law of Segregation)
4.One allele can be masked by the presence of another allele.
• Phenotype
– Physical characteristics
• Purple flowers
• Genotype
– Genetic composition
• The genes on that particular locus on the chromosome.
– Locus is location of the gene on the chromosome.
– Heterozygous
» Different alleles Aa
– Homozygous
» Same alleles AA
Crosses and Following Traits
•
Test Cross
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Used to find the genotype of an unknown individual
Cross with a homozygous recessive to reveal the unknown genotype.
If all white, unknown must be homo recessive
If get some white, unknown must be hetero
Monohybrid Cross
– One trait is followed (generations)
•
Dihybrid Cross
– Two traits are followed (generations)
– According to the laws of probability and if all alleles segregate independently
then when crossing two hetero parents (for both traits) there should be a 9:3:3:1
phenotypic ratio in the offspring.
• 9 dominant/ dominant
• 3 dominant /recessive
• 3 recessive/dominant
• 1 recessive/ recessive
EXPECTED, BUT NOT NECESSARILY OBSERVED!
Not All Traits Exhibit Dominance
and Recessive Characteristics
• Incomplete Dominance
– The F1 hybrids express a phenotype that is
somewhere in between the phenotypes of the
parents.
• Snap dragons only express half the red pigment
and appear pink.
• Human hypercholesterolemia: the heterozygote
have ½ the LDL receptors and thus have high
cholesterol.
Incomplete Dominance
• Codominance:
– Both alleles are equally expressed
Ex. Black feathered chickens and white
feathered chickens when mated can produce
chicks with BOTH white feathers and black
feathers.
Multiple Alleles
• More than two alleles may exist for one trait
(not SOLELY a dominant and a recessive)
• Both alleles may be codominant
– Both alleles are equally expressed
Ex. Blood types are determined by proteins present on
the surface of the blood cell.
– Types:
» IA i or IA IA A blood type
» IB i or IB IB
B blood type
» IA IB
AB blood type (universal receiver)
» ii
O blood type (universal donor) BOTH PARENTS
MUST BE TYPE O or at least heterozygotes for type A
or B TO HAVE A TYPE O CHILD
Rh factor
• Rh factor is an additional antigen found on the red blood
cells and it is a separate gene from the ABO gene.
• If a person has two (+) genes for Rh or one (+) and one
(-) they will test (+).
• A person will be (-) ONLY if they have two (-) because
Rh(+) is dominant.
• Positive means you have the Rh antigen, negative
means you don’t.
• Other minor antigens include Kell, Lewis A, Lewis B, rho,
P etc. and doctors also attempt to match as many of
those minor antigens as possible when selecting blood
for transfusions.
Blood Types
• Types:
Red blood cells: Plasma:
– IA i or IA IA A: A antigen
B antibodies
– IB i or IB IB
B: B antigen
A antibodies
– IA IB AB: A and B antigens
NO antibodies
– ii
O: NO antigens A and B antibodies
Universal donor: Type O-negative (no antigens) is
compatible with all blood types because it has no
antigens for other blood types to recognize with their
antibodies
Universal Recipient: Type AB-positive has no
antibodies in its plasma so it can accept any type
without destroying those foreign cells
Pleiotrophy
• The inheritance of a single gene can have
multiple effects on the individual.
Ex. Sickle Cell Anemia
Causes defective hemoglobin molecules but that single
gene can ALSO cause:
1. heart failure
2. anemia
3. susceptibility to pneumonia
4. kidney failure
5. enlarged spleen
Sickle Cell Anemia
Epistasis
• An allele on one chromosome can affect the
expression of the allele on another
chromosome.
– Example
• Epistasis occurs in mice.
• One allele determines the coat color.
– Dominant B for black
– Recessive b for brown
• The other allele determines whether or not pigment is
deposited at all.
– Dominant C for pigment deposition.
– Recessive c for no pigment deposition
Epistasis
Polygenic Inheritance
• More than one copy of the allele determines the
degree in which the allele is expressed.
• Pigment deposition in human skin cells.
• Dominant alleles cause the pigment melanin to
be deposited.
– Multiple copies of the dominant allele cause more
melanin to be deposited.
– AABBCC – very dark and aabbcc very light.
– AaBbCc is intermediate shade
Environmental Factors
• The environment can have an impact on
phenotypic characteristics.
– Exercise changes the build of a person.
• The product of a genotype is not rigidly defined
by a phenotype but rather a range of
possibilities.
– This range is known as the norm of the reaction.
• Blood: a particular locus may determine the blood type,
however the number of blood cells from one individual to the
next varies and this affects physical fitness at certain
altitudes.
Ex. In Hydrangea an acidic soil
produces a pink flower
Pedigree Charts
• Pedigrees are used to track the
inheritance patterns of previous
generations to predict the future to
determine what characteristics are likely to
be inherited in future generations.
• Probabilities and Mendelian genetics are
used to determine the genotypes of
various family members
• Square=Male Circle=Female
Recessive Disorders
• The allele is recessive and in most cases codes
for a protein that is malfunctioning.
• The result can be a syndrome such as
– Tay Sachs Disease
• Malfunctioning protein that breaks down lipids in the brain
• Higher occurrence in Ashkenazic Jews due to ancestral
heritage.
• Infant experiences seizures, blindness and degeneration of
motor and mental performance.
• The heterozygote is the carrier.
– May produce enough normal protein to compensate
and the individual is normal.
Penetrance
The proportion of individuals who show the
phenotype expected from their genotype.
• 100% means all individuals with genotype
show phenotype.
• Tay-Sachs disease shows complete or
100% penetrance as all homozygote for
the allele develop disease and die.
Expressivity
• The degree to which a particular gene is
expressed in individuals showing the trait.
– Example Retinoblastoma ( type of eye tumor)
– Not all individuals who inherit the allele
develop the tumor ( incomplete penetrance)
and in those who do develop the tumor the
severity varies.
Dominant Allele Disorders
• The heterozygote has the disorder.
• Dominant Lethal disorder
– Huntington’s disease
• Eventual deterioration of the nervous system.
• Remains in the gene pool because the onset is in
individuals that are 35 years or older.
• Any child born to the parent carrying this allele has
a 50% chance of having the disease.
Achondroplasia
A type of dwarfism
Nondisjuntion
• Non-disjunction:
Failure of the chromosomes to separate
properly
• Ex. Extra chromosome #21 = Downs
Syndrome
•
XXY= Klinefelter
•
XYY= Jacob’s Syndrome
•
Single X (XO)= Turner Syndrome
Multifactorial Disorders
• Heart disease, cancer, diabetes manic
depression, schizophrenia and alcoholism
– Increased propensity due to heredity can be
circumvented by diet, exercise and behavior.
• Carriers for certain diseases can be
identified through the genetic testing.
– Genetic counseling may help couples decide
if they want to have children based on the
probability the disorder will be inherited.
Fetal Testing
• Amniocentesis
– Amniotic fluid is extracted from the womb during the 14th to 16th week of
pregnancy.
– The presence of certain chemicals can determine whether or not a
particular genetic disorder is present immediately.
• Cystic Fibrosis
– Cells can be grown in vitro for 2 weeks and chromosomal abnormalities
can be determined through a karyotype.
• Down syndrome
• Tay Sachs
– Chorionic Villis Sampling (CVS)
• A needle is inserted into the cervix and placental (fetal) fluid is extracted.
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Karyotype can be done in 24 hours
Can be performed in the 8th week of pregnancy
More risky than amniocentesis
Less available than amniocentesis
Cannot be done to look for abnormalities in amniotic fluid.
What genetic disorder will this
patient have?