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GENETICS
Genetics
• Purpose: to understand how traits in our DNA are passed
on (parent to child)
• Used to predict possible outcomes of a genetic cross.
– This means that what we predict and what we see could
be different!
History of Genetics
(as a science)
• Gregor Mendel
• “Father of genetics”
• Conducted experiments
– Used phenotype and probability to
determine the principles of genetics
• Studied many plants including
– Pisum sativum (peas)
Why peas
(why choose this model)?
•
The Garden pea - Model system to
study heritability
–
–
–
–
–
–
small
easily cultured
short life span
exhibits great variability
true-breeding strains
dominant/recessive alleles
Mendel’s Experiments
• Looked at seven characteristics
– Characteristics are an inheritable factor, such
as color, size, seed texture, etc.
• Each characteristic occurred in only two
contrasting traits
– A trait is a genetically determined variant of a
characteristic
Question Asked:
• What will happen if I breed two different
plants with different versions of a
characteristic?
• Started with parents that were True
breeding
•
- means that when they self fertilize, all
offspring look like parents (for that trait)
How do plants make offspring?
• What is “natural pollination”?
– Pollination
• Pollen grains produced on anther are transferred
to the stigma (top of the female reproductive
system)
• Self pollination:
– Pollen from a plant pollinates a stigma on the same plant
(same flower or different flower)
• Cross pollination
– Pollen from a plant pollinates a stigma on a totally
different plant.
Results of pollination
• Flowers bloom- produce a pistil a stamen
• Female pistil:
– Stigma (sticky top)
– Style
– Ovule (seeds form)
• Male stamen
– Anther
– Filament
Fertilization
Embryo formation
What are plant embryos?
• Seeds! These are 2N!
Mendel’s method
• Manual pollination (Selective breeding)
– Occurs when anthers are removed from
the flowers of a plant (contain the pollen
grains at the top). Then you choose which
flowers to pollinate.
His Scientific Method
• Utilized monohybrid crosses
– ONE characteristic, two alleles, selective breeding
• Carefully recorded his data (PHENOTYPES).
– Parental characteristics and offspring characteristics
– 3 or more generations (P, F1, F2)
• Formed testable hypotheses.
• Tested hypotheses “statistically”
• Utilized seven traits in the garden pea.
Characteristics studied
•
•
•
•
•
•
•
Height
Flower position
Pod color
Pod appearance
Seed texture
Seed color
Flower color
tall or short
axial or terminal
green or yellow
inflated or constricted
smooth or wrinkled
yellow or green
purple or white
Parents: (both true breeding)
white
x purple
Expect???
What he got:
So… he crossed two of them….
Expect???
What he got:
A carpal is
another name for
_________?
These crosses showed that
there were “factors” being
passed from parent to offspring
even if it wasn’t being “used”
Now we call these factors
GENES
Genes – control a heritable
feature; characteristic
Example of characteristic: Hair
color, seed shape, height;
Allele – controls the variation of a
feature (characteristic) – AKA trait.
Example of trait: brown,
blonde, black hair
Characteristic/Gene?
Trait/Allele?
CHARACTERS (characteristics) AND VARIANTS (traits)
TRAITS
?
TRAITS
?
RARE DOMINANT PHENOTYPE - Polydactyly
A chromosome = folded up string of many genes
What are alleles?
Variations of a gene that occupy
the same locus on homologous
chromosomes
Locus = position on a
chromosome.
GENE = STEM LENGTH
SHORT
t
T
LONG
GENE = FLOWER COLOR
P
p
Terms – apply these to
genetics and Punnett squares
• Diploid (2n) • Gamete
• Haploid(n)
• Zygote
• Egg
• Progeny
• Sperm
• Offspring
• Parent
• Fertilization
• Meiosis
• Ovary
• Testes
Linking vocabulary
• Review Mendel’s
• Punnett squares
process and
should have been
substitute in all words
covered in grades 6on the previous slide,
8.
(used in mitosis and
meiosis) to describe
• Draw a Punnett
how Mendel arrived at
square and link terms
the F1 generation.
on the previous page
to the Punnett square
Mendel’s laws of genetics
1. Law of segregation: only one allele for
each gene is passed from a parent to
the offspring.
Why? Has to do with separation of
homologous chromosomes during
meiosis.
Segregation
of Alleles
Tongue Rolling
2. Law of independent assortment:
Alleles for one gene are passed to
offspring independently of alleles from
other genes.
The result is that new combinations of
genes present in neither parent is
possible.
This only applies to SOME genes, not all.
3. Law of complete dominance – some
alleles overpower others. So even if
both alleles are present, we only “see”
the dominant one.
- the “hidden” allele is called recessive
This only applies to SOME genes, not all.
Remember Mendel’s pea plants?
- Purple was crossed with white and we got
ALL purple. So which allele is dominant?
Genotype: the alleles that an organism has.
- alleles are abbreviated using the first letter
of the dominant trait. (with some exceptions
that we will get to)
- a capital letter represents the dominant
ex: P for purple flower allele
- a lower case represents the recessive.
ex: p for white flower allele
All diploid organisms have two alleles for
each trait:
- you can have two of the same alleles
Ex: PP or pp
- such an individual is described as
Pure or Homozygous.
OR
All diploid organisms have two alleles for
each trait:
- you can have two different alleles
Ex: Pp
- such an individual is described as
hybrid or heterozygous
Phenotype: physical appearance
Examples: brown hair, widows
peak
- the trait that “shows” in the
case of complete dominance;
- depends on the combination of
alleles
Terminology for Genetic Crosses
P generation: “parents;” First
generation in the cross
F generations: results of the
cross;
- F1 – 1st generation; offspring of
P generation
- F2 – 2nd generation; offspring of
F1 generation
Monohybrid cross: cross that
focuses on the alleles of a single
characteristic;
How do we show the possibilities?
- punnett square
PUNNETT SQUARE
Allele in sperm
1
Allele in sperm
2
Allele in Egg 1
Allele in Egg 2
Zygote formed
if sperm 1
fertilizes egg 1
Zygote formed
if sperm 1
fertilizes egg 2
Zygote formed
if sperm 2
fertilizes egg 1
Zygote formed
if sperm 2
fertilizes egg 2
In pea plants, tallness is dominant
to short or dwarf. Cross a pure
tall male to a pure dwarf female
pea plant. Show both ratios
phenotype & genotype for the
offspring. Now cross two of the
F1.
• Take it step by step until you “get it”
• Step 1: what are the parent’s
genotypes?
–Mom?
–Dad?
tt
TT
• Step 2: Set up Punnett Square
t
t
T
Tt
Tt
T
Tt
Tt
• Step 3: ANSWER THE QUESTION
Offspring
t
t
genotypes:
T
T
Tt
Tt
Tt
Tt
Offspring
phenotypes:
• Step 4: Part II
T
t
T
t
TT
Tt
Tt
t t
Offspring
genotypes:
Offspring
phenotypes:
Inheritance Patterns:
Every gene demonstrates a distinct phenotype
when both alleles are combined (the
heterozygote)
Complete dominance is one
- when both alleles are present, only the
dominant trait is seen.
This is the dominance pattern seen in the
characteristics Mendel used.
Other dominance patterns
• Incomplete Dominance
– Still use Capital and Small letters
– Heterozygous offspring ARE blended
Other Inheritance Patterns:
Incomplete dominance
- when both alleles are present, the two
traits blend together and create an
intermediate trait (Red + White = Pink)
Codominance
- When both alleles are present you
see both traits of the characteristic
are visible. Red + White = Red and
White
INCOMPLETE DOMINANCE
Inheritance Patterns:
Co-dominance
- when both alleles are present,
both traits are visible
Different notation: Use first letter of
the feature with a superscript for the
trait.
Example: CW or CB for white coat or
black coat;
Inheritance Patterns:
Co-dominance
- when both alleles
are present, both
traits are visible
Inheritance Patterns:
Each gene has a specific inheritance
pattern.
- you will either be told or be given a
hint; look at the heterozygote!
Still more inheritance patterns
• Sometimes depend on the gender
(male/female)
• Reason: males have “non-homologous”
sex chromosomes
Women have two X’s
but men only have
one.
How do we deal with
the genes on the X
chromosome?
Sex-linked trait
Alleles for the trait are located on
the X chromosome in humans.
- works the same in women as all
the other traits.
BUT –
- men only inherit one such allele.
Sex-linked trait
For females: have to figure out
phenotype based on inheritance
pattern.
For Males: phenotype is that of
whatever allele they inherit.
Example: color blindness
Seeing color (XC) is dominant to
c
being color blind (X )
Identify the sex and trait of the
following:
XCY
XCXc
XcXc
XcY
XCXC
Example: Color Blindness
Set up a punnett square crossing a
heterozygous normal female with a
normal male:
- what is mom’s genotype?
- what is dad’s genotype?
- what gametes can each give?
- what are the offspring’s geno’s?
Cross Number 1:
XC
Xc
XC
C
C
X X
C
c
X X
Y
XC Y
c
X Y
What %
chance of
having color
blind daughter?
Son?
SEX-LINKED TRAITS
COLOR BLINDNESS
AFFLICTS 8% MALES AND 0.04% FEMALES.
If we are dominant, how can we
figure out our genotype?
What are the possibilities?
Test cross: a cross that
determines genotype of
dominant parent
- Cross unknown dominant
parent (possibilities BB or Bb)
with a recessive parent
then analyze the offspring.
If some of the
offspring have the
recessive trait,
then the unknown
parent has to be
b
heterozygous
B
?
Bb
?b
b
Bb
?b
If all offspring are dominant,
unknown parent HAS to be
homozygous
B
?
b
Bb
?b
b
Bb
?b
Multiple alleles: Some genes have more
than two variations that exist, although
we still only inherit 2
Example: Human blood types
Three alleles:
IA
IB
i
Genotype
IA IA
IA i
IB IB
IB i
IA IB
ii
Phenotype
A
A
B
B
AB
0
Polygenic –
Multiple genes each
with 2 alleles
Creates additive/
quantitative effect
SKIN PIGMENTATION
Dihybrid cross:
A cross that focuses on possibilities
of inheriting two traits
- two genes, 4 alleles
Black fur is dominant to brown fur
Short fur is dominant to long fur
What is the genotype of a guinea pig that is
heterozygous for both black and short fur?
Dihybrid cross:
Parent genotypes: BbSs x BbSs
Figure out the possible gametes:
Then set up punnett square
Dihybrid cross:
BS
BS
Bs
bS
bs
Bs
bS
bs
Linked Genes: genes that are on
the same chromosome.
Does the law of independent
assortment apply?
Can they be separated? Will
they always separate?
Remember??? X-linkage
• X-linkage
– Transmission of genes located on the xchromosome.
– Males are hemizygous (half) for all alleles on
the x-chromosome.
• One recessive = recessive phenotype
Designating X-linked alleles
• Xh
XH
Y has no alleles
–You must specify
–The chromosome (X or Y)
–The gender (shown by XX or XY)
–Whether the transferred allele is
dominant or recessive
X-linked Genes
• Found ONLY on X chromosomes
• Most diseases are recessive
• One “disease allele” causes disease in
males
• In females, two recessive required
Gene Linkage
• Genes located on the SAME chromosome
and that tend to be inherited together
• Linked genes do not follow expected
inheritance patterns. No independent
assortment
Gene Linkage
• Crossing over
– DOES “unlink” genes
• Genes which are very close (in position)
termed “highly linked”.
• Linkage seen in one phenotype/genotype
being seen at a different frequency.
Linkage and Mapping in Prokaryotes and
Bacterial Viruses
• Bacteria often used to study Linkage
– Short generation time
– Less genetic material than eukaryotes
– Single “naked” chromosome
– Haploid
– All mutations are expressed.
Pleiotropy
• A single gene  protein
– Protein  affects more than one phenotypic
characteristic
– Albinism and
Crossed eyes
Length of right
and left leg!
Pleiotropy
• Legs are “symmetric” due to pleitropy!
Multiple Gene Interactions
• One gene affects another
– On-off
– Darker  lighter
– The combination of alleles  phenotype
Epistasis
•
One character affects expression of
another (on-off switch)
– Recessive expression in one gene 
second is turned off
•
Labrador retrievers
•
Black, chocolate and yellow labs
Labrador Retrievers
• Color of fur:
– Black (BB or Bb) with “black gums”
– Chocolate (bb) with “red gums”
• How do we get a Yellow lab?
– Second gene (E gene)
– If the E gene is recessive for both alleles (ee)
– pigment is not “expressed” in coat (but can
be seen in “gums”
Black nose
Red nose
What are
the
possible
genotypes
for each of
the dogs?
Example 2
• Could two black horses produce a white horse?
• Could a white horse be homozygous dominant
for the color gene? Heterozygous?
• Could you produce a black horse from the
mating of a white horse and a tan horse
X-inactivation
• See Calico Cat handout
Pedigrees
• Used to study past matings and
transference of specific disease alleles
Pedigree Symbols
Pedigrees and “disease”
• Pedigrees are used to look for “patterns in
certain diseases.
Diseases – autosomal recessive
• Cystic Fibrosis
• Who must the alleles be inherited from?
• Can you have an allele, but not the
disease?
Cystic Fibrosis
• 1 in 20 to 30 people
• Chloride transport is affected
– Thick mucous forms
• Why does the gene “survive”?
• Heterozygotic Advantage?
(http://bric.postech.ac.kr/science/97now/98_5now/980506b.ht
ml)
• Protection from typhoid fever??
PKU
• 1 in 50 people affected
• Symptom: mental retardation
– High phenylalanine  low tyrosine
• Affects neuron development (brain cells)
• Prevention: proper nutrition, infant testing
• Must have low/no phenylalanine in diet
• Phenotype is affected by environment!!!!
Sickle Cell
• 1 in 10 African Americans almost 1:2 in
some African countries
• REASON: protects against malaria
(heterozygous)
• This is a good case of “natural selection”
maintaining the allele
X-linked recessive disease
• Pedigrees often show a predominance of males
with the disease, and females as carriers
• For a female to show the disease, her father
must have the disease
her mother a carrier
Sickle Celland
Disease
• A male ALWAYS inherits the disease allele from
her mother.
Pedigrees –
Autosomal Dominant diseases
• Autosomal Dominant
– Requires ONE gene to show illness
– Frequency the same in Male/Female
• 50% chance offspring will inherit from either parent
Example – Huntington’s
Disease
• Late onset – 30s or 40s
• Progressive nerve system damage
– How many generations are shown?
– Does HD show up in each generation?
• Could it skip a generation and then show up again? Explain
– What gender could be a carrier?
Practicing with Pedigrees
• Use the handout called “Hemophilia in the
Descendants of Queen Victoria”.
• What type of trait is hemophilia?
• Who are carriers?
Practicing with Pedigrees
• Go through the Pedigree handout, answering all
questions.
• When you are finished…answer the following:
• What patterns do you see in the pedigrees that
can tell you whether the pattern is due to a
recessive, dominant, or X-linked allele?
Dominant trait pedigrees
•
•
•
•
Every Aff. Ind has at least one Aff. Parent
Affected x unaffected  50% affected
Affected x Affected -> 75% affected
Generations usually not skipped
– No “hidden” genes
X-linked pedigrees
• Affected males can have no affected
parents
• Only a female can be a carrier of an Xlinked allele
• Males are more likely to have the disease
than females
Recessive
• Affected individuals may have 2 carrier
parents (unaffected)
• Affected x affected  100% affected
• Can be “masked” for several generation
through carriers.
History of Blood Transfusions
•
•
•
•
•
•
•
•
•
•
1901 discover human blood groups
1907 cross-matching of blood suggested
1914 preservative to allow longer preservation of blood
1939 Rh blood groups
1940 National blood banks established
1947 ABO blood typing done on all donated blood
1961 platelet concentration (chemotherapy patients
1971 Apheresis (collect cells, not plasma)
1983 Figured out AIDS could be transmitted
1992 Testing of Donor blood for AIDS implemented
X-inactivation (Barr Bodies
• X-chromosome inactivation occurs early in embryonic
development. In a given cell, which of a female's X
chromosomes becomes inactivated and converted into a
Barr body is a matter of chance (except in marsupials
like the kangaroo, where it is always the father's X
chromosome that is inactivated). After inactivation has
occurred, all the descendants of that cell will have the
same chromosome inactivated. Thus X-chromosome
inactivation creates clones with differing effective gene
content. An organism whose cells vary in effective gene
content and hence in the expression of a trait, is called a
genetic mosaic.
• http://www.nature.com/scitable/topicpage/xchromosome-x-inactivation-323
Extranuclear Inheritance
• Maternal Inheritance-DNA from the mitochondria
(chloroplast) determines offspring phenotype.
•
• Maternal Effect- Phenotypic effects on the
offspring produced by factors transmitted
through the egg cytoplasm.
• Patterns (traits) established during early
development.
Creating a Chromosome Map
• Determined by # of crossovers that occur
– # of crossovers proportional to distance
between the genes
Genetics and Evolutionary Change
• Evolutionary change is caused by
– Changes in the genetic composition of a
population
• Alleles DO NOT occur with the same frequency
• Population genetics
– Link between genetics and evolution
What do we know
• Individual’s genetics do not change
• BUT…
– A population’s allele frequency can change
over time?
• WHAT CREATES CHANGE?