Download Mendelian Genetics and Chromosomes PPT

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Who is the
GREATEST
BIOLOGIST
EVER?
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Why Gregor Mendel is the
GREATEST
BIOLOGIST
EVER…
Even though he wasn’t really a biologist
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Ch 14
Mendelian
Genetics
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Pre-Mendel


Predominate belief in “blending”, child is a mix of
parents
problem with this was traits skipping generations
Terms early genetic study
 Character = detectable, inherited feature, ex. color
 Trait = variant of an inheritable character, ex. green or
red color
 True-Breeding = always produce plants with same
traits as parents, self fertilization
 Cross-Breeding = cross parents with different traits to
create hybrids
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Generations are named

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P = parental
F1= results of PxP
F2= results of F1 x F1
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Mendel’s experiment

Mendel looked at 7 characteristics,
each had 1 alternate form that did
not “blend” when cross-bred

His experiment– if a cross of purple
& white P’s
gives all purple, then a cross
between F1’s, self-pollinating, would
produce white again in F2
generation
results – 3:1 ratio of purple to white
flowers,
conclusions – ?

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Mendel’s experiment

Mendel looked at 7 characteristics, each had 1
alternate form that did not “blend” when cross-bred


His experiment– if a cross of purple & white P’s
gives all purple, then a cross between F1’s, selfpollinating, would produce white again in F2
generation
results – 3:1 ratio of purple to white flowers,

conclusions

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Heritable trait for whiteness is masked
Purple trait is dominant
Extension

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If 2 purple P’s were mated, what ratio of traits would you
expect to observe?
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The ratio does
not match the
ideal. Create a
plan to test if
this difference is
acceptable.
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So…

there are alternate forms of the same gene = alleles,
p265

we inherit one allele from each parent
if alleles are different, one is dominant (noted by
capital letter), one is recessive (lowercase letter)
When do alleles segregate?

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So…

there are alternate forms of the same gene = alleles,
p265

we inherit one allele from each parent
if alleles are different, one is dominant (noted by
capital letter), one is recessive (lowercase letter)
When do alleles segregate? Anaphase I
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More Terms
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homozygous – 2 identical alleles for a trait, ex. DD, dd
heterozygous – 2 different alleles for a trait, carrier, ex. Dd
phenotype – organism’s expressed traits, ex. color, height
genotype – organism’s genetic makeup, letters, ex. PP, Pp
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
Testcross – a cross between
a recessive and an unknown

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tells if it is homo or
heterozygous
monohybrid cross – dealing
with 1 trait
dihybrid cross – 2 traits
Trihybrid – 3 traits
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Mendel’s first postulate:
Law of Segregation



= allele pairs separate
randomly during meiosis,
p. 266
There are 2 alleles for flower
color, if 1 purple and 1 white:
there is a 50% chance of
getting either allele
Punnett square used to
predict the results
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Mendel’s secondpostulate:
Law of Independent
Assortment



when dealing with 2
or more traits, each
allele of the different
genes segregates
independently of
each other
WHY?
If cross 2 dihybrid
heterozygotes, get
9:3:3:1 ratio
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Probability
= mathematical chance of an event happening

Rule of multiplication- probability of 2 events
occurring at the same time = product of their
individual probabilities
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Ex: 2 coins both coming up heads = ?
Ex: If DdRr x DdRr what is probability of getting DDRR is ?
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Probability
= mathematical chance of an event happening

Rule of multiplication- probability of 2 events
occurring at the same time = product of their
individual probabilities

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Ex. 2 coins both coming up heads = ½ x ½ = ¼
Ex: If DdRr x DdRr what is probability of getting DDRR is ?
chance of DD = ¼, chance of RR = ¼ so ¼ x ¼ = 1/16
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Rule of addition –p.270, probability that either of
two or more mutually exclusive events will occur
is calculated by adding the individual
probabilities.

What are the chances you will get heads or
tails when you flip a coin?

Ex. cross of 2 heterozygotes, what are chances of result being hetero?

Use → trihybrid AaBbCc x AaBbCc
? chance of AabbCC?
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Rule of addition –p.270, probability that either of
two or more mutually exclusive events will occur
is calculated by adding the individual
probabilities.

What are the chances you will get heads or
tails when you flip a coin?



½+½ =1
Ex. cross of 2 heterozygotes, what are chances of result being hetero?
 Chance of recessive egg + dominant sperm = ½ x ½ = ¼
 Chance of dominant egg + recessive sperm = ½ x ½ = ¼
 chance of hetero child is ¼ + ¼ = ½
Use → trihybrid AaBbCc x AaBbCc
? chance of AabbCC?
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Extensions:

Mendel’s laws were not
perfect, in fact, he was
lucky (or wise) that he
choose peas which have
simple inheritance (except pod
shape)

Incomplete dominance =
1 allele is not completely
dominant over the other
thus, there is a 3rd
phenotype, intermediate,
ex.Carnations/snapdragons
p. 271
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Codominance
= both alleles are expressed
 Level of expression varies at different levels

ex: Tay-sachs
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at the molecular level – looks codominant – both alleles
transcribed
at the biochemical level – looks like incomplete→ a partial
level of lipid-metabolizing activity
at the organismal level – heterozygotes are symptom free,
homoygote recessives express disorder
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Multiple Alleles
= genes that have more than 2
alleles

Ex. blood groups A, B, AB, O
(surface carbohydrates)

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blood type is the antigen present
on the RBC, p. 273
also contains Rh factor, + or –
with standard Mendelian rules
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Pleiotropy = a single gene has multiple effects
ex: sickle-cell
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Epistasis = one gene affects the expression of another
gene, Ex. pigments in mice
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Polygenic inheritance = many genes affect the same trait
Ex: skin color, very dark to very light, p. 274
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Environment plays an important part in gene
expression, how much is dependent on the
gene, nature vs. nurture argument
Norm of Reaction = The phenotypic range for
a genotype, p.275
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Humans
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Pedigree – family tree that shows inheritance over many
generations, shows patterns
 = male, O = female, ●= affected, ○= non-affected
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Recessive human disorders
- usually caused by a defective protein
 - heterozygotes are carriers
 Why more common than dominant disorders?
Examples
 Cystic Fibrosis – most common amongst Europeans (4%
carry), membrane protein that controls Cl⁻ traffic, causes
increase mucus in lungs infections persist
 Tay-Sachs – higher in Ashkenazic Jews, can’t break
down a type of lipid. How can it be high in a particular
pop?
 Sickle cell – substitution in one hemoglobin, causes RBC
to sickle and clog, carriers are immune to malaria, p. 278


In which pop. would sickle cell predominate?
Consanguinity – mating with relatives, increases
expression
of recessive disorders. Why?
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Dominant inherited disorders
– rarer than recessive. Why?
Examples
 Achondroplasia – type of dwarfism
 Huntington's – late acting degeneration
of
nervous system, due to single allele on tip of
chromosme #4
 Knowledge of this makes disease detectable.

Multifactorial disorders
many different factors affect onset, but genetic
predisposure present
 ex. Heart disease, diabetes, cancer
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Genetic testing and counseling

1) carrier recognition - help make decisions about
whether or not to reproduce
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2) fetal tests
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Can test for Tay-Sachs, sickle-cell, and cystic fibrosis, etc.
amniocentesis – take amniotic fluid from around fetus, do
karyotype
chorionic villus sampling (CVS) – take villi, do karyoptype, fast,
earlier, more risk, p. 280
ultrasound – imagery using sound waves, look for physical
problems
fetoscopy – fiber optics
Culturing escaped fetal blood cells in mother’s blood
3) Newborn screening – ex. PKU
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Big Picture of Inheritance…

must be looked in integrated light…i.e. it is a
product of genes working collectively and is
influenced by environmental cues

Must view emergent properties of organism as a
whole, not a reductionist view of single genes
acting in isolation
So, why is Gregor Mendel the
GREATEST
BIOLOGIST
EVER?
Even though he wasn’t really a biologist
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Ch 15
Chromosomes and Inheritance
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
Chromosome theory of inheritance: genes are located on
chromosomes, they segregate and independently assort
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T.H.Morgan
rediscovered Mendel’s work 1900’s
QUESTION: specific genes on specific chromosomes?


work on fruit fly, why?

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fast repro., easy to handle, 4 pairs of chromosomes (1 pair
are sex chromosomes)
gene symbol is based on the mutant or recessive
ex. curly is recessive = Cy, if normal then Cy+
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“wild type” is the type seen in nature = +
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Experiment- p 289

white eyed male (♂)→
crossed with a red eyed
female (♀)→ in F2 only males
had white eyes ?

how is no independent
assortment possible (sex and
white eyes)?
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Experiment- p 289
white eyed male (♂)→
crossed with a red eyed
female (♀)→ in F2 only
males had white eyes ?
→ eye color and sex are
linked
 Linked genes = when
genes are on the same
chromosome, so they are
inherited together

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
Sex linked traits = located
on a sex chromosome, p.
290, ex. Hemophilia

few genes on the Y, thus
most sex-linked diseases are
seen in males b/c on the X
(not masked), females often
carriers, p. 290

X-inactivation = females
inactivate one of their X’s
(see cat diagram)


inactive X becomes a Barr body
Typically both chromosomes’
genes are expressed
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Why do X
chromosomes
get
inactivated?
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Genetics in the lab
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Genetics
in the lab:
How could you
determine if a two
genes were
“linked”?
How could you tell
distance between
two genes?
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How could you
determine if a two
genes were
“linked”?
How could you tell
distance between
two genes?
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Recombination
= offspring with different combinations of traits than the
parents
Parental types – same phenotype as a parent
Recombinants – differ from parents, *p. 293-294
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What is % of recombination of the peas?
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Recombination
= offspring with different combinations of traits than the
parents, caused by crossing over or mutations
Parental types – same phenotype as a parent
Recombinants – differ from parents, *p. 293-294
What is % of recombination of the peas?
50% - one-half of the offspring
are expected
to inherit either of the
two phenotypes

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Recombination

What would a recombination of 25% tell
you about the chromosomal location of
two given genes?
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Recombination

What would a recombination of 25% tell
you about the chromosomal location of
two given genes?

The genes’ loci are on the same chromosome
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
Why is the recombination % not 0?
What would a recombination of 0.5% tell
you about their respective locations?
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Recombination

What would a recombination of 25% tell
you about the chromosomal location of
two given genes?

The genes’ loci are on the same chromosome

Why is the recombination % not 0?

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Crossing-over separates them
What would a recombination of 0.5% tell
you about their respective locations?
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That their respective loci are in close
proximity on the same chromosome
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Sturtevant and gene mapping
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use recombination frequency to determine distance of genes
The farther apart two genes are, the higher the probability that
crossover will occur between them and ∴ the higher the
recombination frequency
made chromosome maps
 find relative distance between farthest genes, find distance of
an end and a middle, fill in other genes

Made distance unit: 1 map unit = 1% recombination

double crossovers can occur too, throw # off a little
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Final product: a genetic (linkage) map
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HUMAN GENETIC DISORDERS
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
Nondisjunction –two chromosomes stuck together or not present

Why and when would this occur?
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
Nondisjunction –two chromosomes stuck together or not present

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Why and when would this occur?
Aneuploidy = having an abnormal # of chromosomes
 Trisomy – 3 copies of 1 chromosome
 Monosomy – 1 copy of the chromosome
Polyploidy = more than normal chromosome set
 Triploidy – 3 chromosome sets (3N)
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
Nondisjunction –two chromosomes stuck together or not present

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Why and when would this occur?
Aneuploidy = having an abnormal # of chromosomes
 Trisomy – 3 copies of 1 chromosome
 Monosomy – 1 copy of the chromosome
Polyploidy = more than normal chromosome set
 Triploidy – 3 chromosome sets (3N)
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Basic Mutations
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Deletion – chromosome loses a piece, p. 298
Duplication – double of gene
Inversion – chromosome is in reverse
Translocation – gene moves to another chromosome
→caused by UV light, chemicals or random
→effects can be silent, lethal or in between
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Human aneuploid conditions
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Down Syndrome – trisomy 21, female age makes more
frequent?
Klinefelters – XXY, XXXY male, sterile, some female
features
XYY – male, usually normal, XXX- female, usually normal
Turner syndrome – X, female, sterile, few sexual features
Some effects of chromosomal abnormalities depend on
what parent inherited by (genomic imprinting, p.300)
- Prader–Willi disorder– deletion of part of #15 from dad
- Angelman syndrome– deletion of same part of # 15 from mom,
motor issues
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Genomic Imprinting

is the activation or deactivation of a gene
depending upon whether it was inherited from
mom or dad

Mechanism is typically methylation (adding of
methyl group, –CH3 ). Methyl group acts as an
on/off switch.

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Key player in epigenetics
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Organelles and their genes

Do not follow Mendelian rules of inheritance.
Why?

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Organelles and their genes

Do not follow Mendelian rules of inheritance.

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Why? They do not undergo meiosis
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Study tip of the day: always be able
to explain text chapter concepts

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Concept 15.1: Mendelian inheritance has its physical
basis in the behavior of chromosomes
Concept 15.2: Sex-linked genes exhibit unique patterns
of inheritance
Concept 15.3: Linked genes tend to be inherited together
because they are located near each other on the same
chromosome
Concept 15.4: Alterations of chromosome number or
structure cause some genetic disorders
Concept 15.5: Some inheritance patterns are exceptions
to standard Mendelian inheritance
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Chapter 14 & 15
White-board challenge
pedigree charts
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Pedigree Practice Problems: Identify each pedigree
as autosomal
recessive, autosomal dominant, X-linked, or Y-linked (aka “patterns of inheritance”)
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Pedigree Practice Problems: Identify each pedigree
as autosomal
recessive, autosomal dominant, X-linked, or Y-linked (aka “patterns of inheritance”)
ANSWERS
a. autosomal
recessive
b. autosomal
dominant
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ANSWERS
c. autosomal
dominant
d. autosomal
recessive
e. x-linked
recessive
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ANSWERS
f. autosomal dominant
g. autosomal recessive
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Chapter 14
1.
2.
If a plant with purple flowers produces
only the same variety as the parent plant
over many generations, what is the plant
said to be?
What is Mendel’s law that states that the
two alleles for a heritable character
separate from each other during gamete
formation?
Chapter 14
1.
2.
True-breeding plants
Law of Segregation
3. If two heterozygous purple flowers produce
offspring, what are the odds that the offspring
will have white flowers?
4. The child’s mother has blonde hair, and their
father is heterozygous and has brown hair. If
blonde hair is a recessive trait, what are the
odds that the child would have blonde hair?
5. When red snapdragons are crossed with
white snapdragons their offspring is pink. What
type of dominance is this?
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3. 25%
4. 50%
5. Incomplete
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Chapter 15
1. The law that states that alleles of genes on
nonhomologous chromosomes assort
independently during gamete formation is?
2.
Who has a greater chance to receive Xlinked recessive disorders? Males or
females?
3.
Genes located near each other on the same
chromosome tend to be inherited together in
genetic crosses are called
?
Chapter 15
1.
Law of Independent Assortment
2.
Males
3.
Linked Genes
4. When the members of a pair of
homologous chromosomes do not move
apart properly during meiosis I they can
alter the chromosome structure. What is
this error called?
5. What are the four types of changes that
can occur to chromosome structure?
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4. Nondisjunction
5. Deletion,
duplication, inversion,
translocation
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Test your skills at…

practice pedigree problems

Or more on following pages 
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?
Dominant or recessive?
autosomal, x-, or y-linked?
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A
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