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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 (studied what pea plants looked
like( and probability (statistics) to determine the
principles of genetics (how parents passed traits to
their offspring)
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 (allele)
CHARACTERISTICS STUDIED (SEE 2ND PAGE
OF NOTES)
• Height
• Flower position
• Pod color
• Pod appearance
constricted
• Seed texture
• Seed color
• Flower color
tall or short
axial or terminal
green or yellow
inflated or
smooth or wrinkled
yellow or green
purple or white
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.
HOW DID MENDEL CONTROL
FERTILIZATION?


THE NEXT FEW SLIDES SHOW YOU HOW
MENDEL “hand fertilize” the peas
You will need to know the definitions of
Pollination
 Cross-pollination
 Self-pollination


Other than that, the next six slides are for your
interest ONLY (they are NOT in the notes)
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!
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.
Parents: (both true breeding)
white
x purple
Expect ed : Light purple
What he got: All purple
So… he crossed two of them….
Expect??? All purple
What he got: ¾ purple, ¼ white!
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
VOCAB WORDS – BE APPLY THESE TO
GENETICS AND PUNNETT SQUARES
Diploid
(2n)
Haploid(n)
Egg
Sperm
Parent
Meiosis
Testes
Gamete
Zygote
Progeny
Offspring
Fertilization
Ovary
LINKING VOCABULARY – PRACTICE
PARAGRAPH

Review Mendel’s
process and substitute
in all words on the
previous slide, (used
in mitosis and
meiosis) to describe
how Mendel arrived at
the F1 generation.

Draw a Punnett
square and link terms
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
T
T
3: ANSWER THE QUESTION
Offspring
t
t
genotypes:
Tt
Tt
Tt
Tt
Offspring
phenotypes:
Step
T
t
4: Part II
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. The problems you
were given exhibit this pattern. Go do the
problems
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.
Based on phenotype, can you determine whether
you are homozygous or heterozygous?
WHAT ARE THE POSSIBILITIES?
Can we use the inheritance pattern?
Complete dominance
Incomplete dominance
Co-dominance
If the gene has a complete dominance inheritance
pattern, what phenotype should we breed to in
order to determine whether the genotype is
homozygous dominant or heterozygous????
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 GENES

While you only inherit two alleles, some genes
have 3, 4, 5 and even 6 possible alleles. BUT you
can only inherit two (and they must come from
your parents!
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 (6)
IA IA
IA i
IB IB
IB i
IA IB
ii
Phenotype (4)
A
A
B
B
AB
0
INHERITANCE PATTERN DEPENDS ON ALLELE
IA and IB are codominant
 i (the O allele) is recessive to IA and IB

POLYGENIC

Several genes control a single character trait

Phenotype shows a continuum

(quantitative) rather than discrete categories of color
Polygenic –
Multiple genes each
with 2 alleles
Creates additive/
quantitative effect
SKIN PIGMENTATION
EXAMPLE
Skin Color: Humans
 Height - humans

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 phenotypes: 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?
WHAT DOES THIS MEAN?
It
means that you can pass on an allele
that you got from your mom and an
allele you got from your dad ON THE
SAME CHROMOSOME
However,
it is more likely that two
alleles that start on the same
chromosome will get passed on
together.
MULTIPLE GENE INTERACTIONS

One gene affects another
On-off
 Darker  lighter
 The combination of alleles  phenotype

EPISTASIS
Expression of one gene is modified or
influenced by one or several other genes.
expression of another (on-off switch)



Labradors: recessive expression in one gene 
turns off a second gene
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
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.
There are three inheritance patterns
X-linked
 Autosomal recessive
 Autosomal dominant

PRACTICING WITH PEDIGREES



Use the handout called “Hemophilia in the
Descendants of Queen Victoria”
This contains the pedigree of several royal
families in Europe.
What can you tell me about the pattern?
HEMOPHILIA IS “X-LINKED”
Found ONLY on X chromosomes
 Most diseases are recessive
 One “disease allele” causes disease in males
(hemizygous)
 In females, two recessive required

How does this affect the pedigree?
Look at the carriers and the “affected”
individuals
What are their genders?
Can the “allele” be hidden? In what
gender?
X-LINKED RECESSIVE DISEASE
 Pedigrees
often show of those that have
the disease, virtually all are males,
and females as carriers
 If
a female has hemophilia, her father
must have the disease and her mother
must be a carrier
A
male ALWAYS inherits the disease
allele from his mother.
QUESTIONS

What inheritance pattern is demonstrated ?

How many generations are shown?



According to the pedigree, what is the earliest
known origin of the hemophilia gene?
Why do boys show the disease, and girls are
carriers?
How is it that some boys have the disease and
others do not?
REMEMBER HOW TO NOTATE X
CHARACTERISTICS

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
DISEASES – AUTOSOMAL RECESSIVE
Cystic Fibrosis
 Sickle Cell Disease (recessive or co-dominant)
 Tay Sachs


Who must the alleles be inherited from?

Can you have an allele, but not the disease?
RECESSIVE
Affected individuals may have 2 carrier parents
(unaffected)
 Affected x affected  100% affected
 Unaffected parents of affected child may be
related.

CYSTIC FIBROSIS
1
in 2500 Caucasians affected
 Chloride

transport is affected
Thick mucous forms
 Why does the gene “survive”?
 Heterozygotic Advantage?
(http://bric.postech.ac.kr/science/97now/98_5now/9805
06b.html)
 Protection from typhoid fever? Malaria? Cholera? Still
uncertain.
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
TAY SACHS


In Ashkenazic Jews, 1 out of every 30 people
carry the allele
Infantile form
Symptoms appear at 3-6 months
 Child dies by age 4 or 5
 Lacks hexosaminidase A, a protein that helps break
down a chemical in nerves (gangliosides)
 Without this protein, gangliosides, particularly
ganglioside GM2, build up in cells, especially nerve
cells in the brain deterioration of the brain and
nervous system  death.

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


How many generations are shown in pedigree
1?
Does HD show up in each generation?


nerve system damage
Could it skip a generation (have a recessive allele)
and then show up again? Explain
What gender could be a carrier?
EXAMPLE: DWARFISM

Dominant autosomal
Two dominant alleles  lethal
 Heterozygous is a “dwarf”
 Only homozygous recessive is “normal”

IDENTIFYING PEDIGREES WITH DOMINANT
TRAIT DISORDERS
Every Affected Individual has at least one Affected
Parent
 Generations not skipped (in other words, no “hidden”
genes

If one parent is affected, 50% of children will be affected
 If two parents are affected, 75% of children are affected
