Transcript Unit 4: Genetics & Heredity

Unit 4: Genetics &
Heredity
Chapters 14 and 15
Biology, 9th Ed
By Campbell & Reece
Chapter 14 – Mendel & the Gene Idea
 Many suggested the
“blending” hypothesis:
genetic material from
parents mixes
 Correct model is the
“particulate” hypothesis:
genes are passed to
offspring in units called
genes.
Gregor Mendel
 Around 1857, Mendel began breeding garden
peas to study inheritance.
 Used experimental method
 Used quantitative analysis – b/c he collected
data & counted peas
 Excellent example of the scientific method
Mendel’s Experiment
 Why peas? Mendel
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noticed many
variations in peas.
Control the mating
of the pea plants
& record the results!
Traits were distinct!
Started w/ true-breeding
plants
Most traits are controlled by
a single gene & each gene
has 2 alleles (one is
completely dominant to the
other)
Mendel’s Work
 Bred pea plants
 Cross pollinated true-breeding parents (P)
 Raised seeds & then observed traits (F1)
 Allowed offspring (F1) to cross-pollinate &
observed the next generation (F2)
 P = parents
 F – filial generation
Mendel Collected Data for 7 Traits
Overview of Mendelian Genetics
 Character/Gene: heritable
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feature; i.e., fur color
Trait: variant of a
character; i.e. brown
fur or white fur
Allele: form of a gene;
represented by letters; i.e., B
or b
True-Bred: all offspring
are the same variety
Hybridization: crossing
of 2 different traits
P generation: parents
F1 generation: first filial
generation
Closer Look at Mendel’s Work
 P  purple flowers X white flowers
 F1  100% purple flowers
4 purple:0 white
Self-pollinate
 F2  75% purple & 25% white 3 purple:1white
Leading to the Law of Segregation
 Traits come in alternative
versions
Ex. Purple vs. white flower
color
 Alleles  different alleles
vary in the sequence of
nucleotides (nitrogen bases)
at the specific locus of a
gene
 Purple flower allele & white
flower allele are 2 DNA
variations at the flower color
locus
Law of Segregation
 Law of Segregation: The
alleles for each character
segregate (separate) during
the formation of gametes
(meiosis).
 When gametes are produced
during meiosis, homologous
chromosomes separate from
each other.
 Each allele for a trait
segregates (is packaged into
a separate gamete)
Law of Segregation
Law of Segregation
 What meiotic event creates the law of
segregation?
 Between anaphase I and telophase I when
the homologous chromosomes separate &
are packaged into different cells
 Remember, Mendel didn’t even know DNA or
genes existed!
Traits are inherited as discrete units
 For each gene/character, an organism
inherits two alleles, 1 from each parent
 Diploid: organism inherits one set of
chromosomes from each parent; 2 sets of
chromosomes
Law of Dominance
 If the two alleles differ, then the dominant
allele is fully expressed in the organism’s
appearance; the other, the recessive
allele, has no effect on the organism’s
appearance
 Purple X White = Light purple --- NO!!!!
 Purple masked white
Genetic Vocabulary
 Punnett Square: predicts
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the results of a cross b/w
individuals of a known genotype
Homozygous: same alleles
for a character – PP or pp
Heterozygous: different alleles
for a character – Pp or pP
Phenotype: physical
appearance (words) – purple or
white flowers
Genotype: genetic make-up
(letters)
Testcross: crossing a
homozygous
recessive to a dominant
phenotype
(unknown genotype)
Genotype vs. Phenotype
 2 organisms can have the same phenotype,
but different genotypes
 PP = homozygous dominant; purple flowers
 Pp = heterozygous; also purple flowers
Monohybrid Cross Practice
Problems – Complete Dominance
1) A homozygous cream colored mouse (dd) is crossed with a
heterozygous (Dd) dark mouse.
a. What are the odds that this couple will
have a cream colored baby?
b. What are the odds of a dark mouse?
2) In sheep, white is due to a dominant gene (W), black is due to its
recessive allele (w). A white ewe mated to a white ram produces a
black lamb. How does this happen? What are the genotypic and
phenotypic ratios of the offpspring?
3) In chickens, yellow legs (Y) are dominant over white legs (y). A
yellow legged rooster was crossed with a white legged hen. Both
kinds of offspring were produced. What are the genotypes of the
parents and the offspring?
Law of Independent Assortment
 Law of Segregation involves
1 character/gene
(monohybrid)
 What about two different
genes? (dihybrid)
 The two pairs of alleles
segregate independently
of each other
(in Metaphase I of meiosis)
 Law of Independent
Assortment
Law of Independent Assortment
 Each pair of alleles – for each trait –
segregates into gametes independently
 YyRr  YR, Yr, yR, yr (four gametes formed)
Law of Independent Assortment
Law of Independent Assortment
 What meiotic event creates the law of
independent assortment?
 When the homologous chromosomes line up
independently of each other during
metaphase I of meiosis
Interesting Historical Facts
 While Mendel was acknowledged by his
peers as an outstanding plant breeder, his
revolutionary work was overlooked for 34
years.
 Mendel published “Experiments on Plant
Hybrids” in 1865. In 1900, 16 years after his
death, a number of scientists independently
rediscovered his work.
Interesting Historical Facts
 Charles Darwin proposed that evolution by
natural selection was dependent on variation
in the population
 Darwin was unable to propose a mechanism
for how this variation was transmitted.
 The key was Mendel’s work, and nearly a
century after Mendel published his findings,
historians found a copy of Mendel’s paper in
Darwin’s study. He presumably never read it!
Probability and Genetics
 Mendel’s Laws:
A) Segregation
B) Independent Assortment
 Reflect same laws of probability that apply to
tossing coins or rolling dice
Probability & Genetics
 Calculating probability of making a specific
gamete is just like calculating the probability
in flipping a coin
 Probability of tossing heads?
 Probability of making a P gamete…….
P
P
Pp
= 50% or
PP
=100%
p
P
Probability & Genetics
 Outcome of one toss has no impact on the
outcome of the next toss
 Probability of tossing heads each time? 50%
 Probability of making a P gamete each time…
P
Pp
= 50%
p
Rule of Multiplication
 Chance that 2 or more independent events
will occur together
 Probability that 2 coins tossed at the same
time will land heads up
½X½=¼
 Probability of Pp X Pp  pp
½X½=¼
Rule of Addition
 Chance that an event can occur 2 or more different
ways
 Sum of the separate probabilities
Sperm
Egg
Offspring
P
½
p
½
Pp
¼
p
½
P
½
pP
¼
1/4
+1/4
-----1/2
Calculating Probability
 Pp X Pp = ?????
Sperm
Egg
Offspring
P
½
P
½
PP
1/4
P
½
p
½
p
½
P
½
Pp
¼
pP
+¼ = ½
p
½
p
½
pp
¼
Calculating Dihybrid Probability
 Rule of Multiplication also applies to Dihybrid
Crosses
 If you have heterozygous parents,YyRr, what is the
probability of producing yyrr offpspring?
A) Probability of producing y gamete = ½
B) Probability of producing r gamete = ½
C) Probability of producing yr gamete is …
½X½=¼
D) Probability of producing yyrr offspring is…
¼ X ¼ = 1/16
Dihybrid Cross Practice Problems
1) Cross a pea plant that is heterozygous for
purple (P) flowers and homozygous
dominant for yellow (Y) seeds with a plant
that is heterozygous for purple flowers and
homozygous recessive for green seeds.
Test Cross
 Cross-breed the dominant unknown
phenotype with a homozygous recessive to
determine the identity of the unknown allele.
 If parent is PP  offspring are all purple (Pp)
 If parent is Pp  offspring are ½ purple (Pp)
& ½ white (pp)
Test Cross
Test Cross Practice Problems
1)
In Border Collies, black coat (B) is dominant to red coat (b). A
breeder has a black male that has won numerous
awards. The breeder would like to use the dog for breeding if
he is purebred or BB. To learn this information, she
testcrosses him with a red female (bb). Answer the following
questions A, B, C, and D.
A. If the black male is BB, what kind of gamete (sperm) can he
produce?
B. If the red female is bb, what kind of gamete (eggs) can she
produce?
C. If the black male is Bb, what kind(s) of gametes (sperm) can
he produce?
D. If any of the puppies are red, what is the father's genotype?
Extending Mendelian Genetics
 Mendel worked with a simple system
A) Peas are genetically simple
B) Most traits are controlled by a single gene
C) Each gene has only 2 alleles; 1 of which
is completely dominant to the other
 The relationship b/w genotype and phenotype
is rarely this simple!!
Non-Single Gene Genetics –
Incomplete Dominance
 Incomplete Dominance:
appearance b/w
phenotypes of the 2
parents; an
intermediate/mixture;
Ex. Snapdragons
 RR = red flowers
RR’ = pink flowers
R’R’ = white flowers
 Red flower X White flower = ?
Non-Single Gene Genetics –
Co-dominance
 Codominance: Two alleles are both
dominant to each other, so they are both
expressed in a heterozygote
 Ex. Black & White Checkered Chickens and
M, N, and MN human blood groups
 B = Black chicken
W = White chicken
 Black Chicken X White Chicken = ?
 BB X WW = All BW (Black & White Chickens)
Incomplete Dominance & CoDominance Practice Problems
1)
2)
3)
4)
The color of fruit for plant "X" is determined by two
alleles. When two plants with orange fruits are crossed the
following phenotypic ratios are present in the offspring: 25%
red fruit, 50% orange fruit, 25% yellow fruit. What are the
genotypes of the parent orange-fruited plants?
Cross a red fruit with an orange fruit and give the phenotypic
ratio.
Cattle can be red (RR = all red hairs), white (WW = all white
hairs), or roan (RW = red & white hairs together). Predict the
phenotypic ratios of offspring when a homozygous white cow is
crossed with a roan bull.
What should the genotypes & phenotypes for parent cattle be if
a farmer wanted only cattle with red fur?
Dominant Alleles
 NOTE: Dominant alleles are NOT always
more common than recessive alleles!!
 Polydactyly – dominant alleles
 Only 1 in 400 people are polydactyl
 Most people are homozygous recessive for
polydactyly
Non-Single Gene Genetics –
Multiple Allele Problems
 Multiple Alleles: more
than 2 possible alleles
for a gene;
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Ex. Human blood
types (ABO)
 3 alleles = IA, IB, and I
 IA & IB are dominant to the i
allele
 IA & IB are co-dominant to
each other
 Phenotype
Genotype
A
IAIA or IAi
B
IBIB or IBi
AB
IAIB
O
ii
Human Blood Types
Genotype
Phenotype
Phenotype
Status
IAIA or IAi
Type A
Type A
Oligosaccharides
on the surface of
RBC
--------
IBIB or IBi
Type B
Type B
oligosaccharides
on surface of RBC
--------
I AI B
Type AB
Both Type A &
Type B
oligosaccharides
on surface of RBC
Universal
Recipient
ii
Type O
No
oligosaccharides
on surface of RBC
Universal
Donor
Blood Compatibility
 Matching compatible blood groups is critical
for blood transfusions
 A person produces antibodies against foreign
blood factors  oligosaccharides
 If a donor’s blood has an A or B
oligosaccharide that is FOREIGN to the
recipient, antibodies in the recipient’s blood
will bind to the foreign molecules
 Binding causes the donated blood cells to
clump together & can kill the recipient
Multiple Alleles Practice Problems
1) A woman with Type O blood and a man who is Type
AB have are expecting a child. What are the
possible blood types of the kid?
2) What are the chances of a woman with Type AB and
a man with Type A having a child with Type O?
3) A test was done to determine the biological father of
a child. The child's blood Type is A and the mother's
is B. Man #1 has a blood type of O & man #2 has
blood type AB. Which man is the biological father?
Non-Single Gene Genetics –
Polygenic Inheritance
 Polygenic Inheritance: an
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additive effect of 2 or more
genes on a single
phenotypic character
Ex. human skin color
and height
Phenotypes on a continuum
Skin Color – 3 genes
A, B, & C – dark skin
a, b, & c – light skin
Alleles have a cumulative
effect; therefore…….
AaBbCc 
intermediate/medium skin
color
Nature vs. Nurture
 Phenotype is controlled by both environment
and genes
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Color of hydrangea
flowers is influenced
by the acidity of
the soil
Chi-Square Test
 Test to see if your data supports your
hypothesis
 Compare “observed” vs. “expected” data
A) Is variance from “expected” due to
random chance?
B) Is there another factor influencing data?
Chromosomal Theory of Inheritance
 Genes have specific
locations on
chromosomes and
chromosomes undergo
segregation and
independent assortment
Chromosomal Linkage
 Thomas Hunt Morgan
 Drosophilia
melanogaster
 Sex Linkage – genes
located on sex
chromosomes (pair #23
in humans)
 Linked Genes – genes
located on the same
chromosome tend to be
inherited together!
Morgan’s Research – First Mutant
 1st to associate a specific gene with a specific
chromosome
 Fruit flies have 4 pairs of chromosomes
 Wild type (Normal Phenotype) Fly = red eyes
 Discovered mutant white-eyed male
Morgan’s Experiment
P = White eyed male X Red Eyed Female
F1 = All Red Eyed Males & Females
F2 = 3 red: 1 white; only males had white eyes
Q: How was the possible?
A: The trait was sex-linked!!!
Sex-Linked Traits
 Humans & other mammals have 2 sex
chromosomes  X & Y
 2 X chromosomes = female
 X & Y = male
Human Female Karyotype
Human Male Karyotype
Genes on Sex Chromosomes
 Y Chromosome:
 SRY: sex-determining region
 Master regulator for maleness
 Turns on genes for production of male
hormones
 X Chromosome:
 Other traits, rather than sex determination
 Hemophilia
 Colorblindness
 Duchenne Muscular Dystrophy
Sex-Linked Traits Summary
 X-Linked:
 Follow the X chromosome
 Males get their X from their mother
 Trait is never passed from father to son
 Y-Linked:
 Very few traits
 Only 26 genes
 Trait is only passed from father to son
 Females cannot inherit the trait
X-Inactivation
 Female mammals inherit two X chromosomes
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One X becomes inactivated during embryonic
development
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Condenses into a compact object called a Barr
Body
X Inactivation & Barr Bodies:
Tortoise Shell Cat
Sex-Influenced Traits
 Male Pattern Baldness
 autosomal trait influenced by sex hormones
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age effect as well = onset after 30 years old
dominant in males & recessive in females
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B_ = bald in males; bb = bald in females
Linked Genes
 Genes on the same chromosomes tend to be
inherited together
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Close Together  more likely to be inherited
together
Far Apart  More likely to inherited
separately (behave as if they are on separate
chromosomes); more likely to “cross over” in
meiosis
What is Recombination?
 Occurs when offspring have different
combinations of traits than the parents
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Parental Types: Same genotype as parents
Recombitants: different genotype from
parents
Chromosomal Basis of Recombination
 Unlinked Genes (genes on different chromosomes)
have a 50% frequency of recombination
 Linked genes do NOT assort independently b/c they
are on the same chromosome & tend to move
together through meiosis & fertilization
 Why are there recombitants w/ linked genes if they
do not assort independently?
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B/c crossing over exchanges genes b/w non-sister
chromatids
Crossing over b/w homologous chromosomes breaks
linkages in parent chromosomes to form new,
recombitant chromosomes.
Chromosomal Basis of Recombination
 Notice that crossing over b/w non-sister
chromatids makes recombitant chromosomes
 These recombitant chromosomes are
packaged into gametes
Production of Recombitant Offspring
Genetic Maps
 Crossing Over – Genes that
DO NOT assort
independently of each other
 Genetic Maps – the further
apart two genes are, the
higher the probability that a
crossover will occur b/w
them; therefore, the higher
the recombination frequency
 I Map Unit – 1%
recombination frequency
Genetic Maps Continued
 Linkage Maps – Genetic
maps based on
recombination frequencies
 Not a true picture of a
chromosome and the
relative distances b/w
genes
 Shows the sequence
of genes on a chromosome,
not an exact location
Genomic Imprinting
 Definition: parental
effect on gene
expression
 Identical alleles may
have different effects on
offspring; depending on
whether they arrive in
the zygote via the egg
or sperm
Genomic Imprinting
 Both disorders below are caused by a partial deletion
of chromosome #15……………………………………
 Prader-Willi Syndrome
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Mental retardation
Obesity
Short stature
Inherits abnormal chromosome from father
 Angelman Syndrome
 Jerky movements
 Spontaneous laughter
 Motor and/or mental symptoms
 Inherits abnormal chromosome from mother
Extranuclear Genes
 Small amounts of DNA are found in
mitochondria & chloroplasts
 This extranuclear DNA is randomly assorted
to gametes and does not follow simple
Mendelian rules of inheritance.
 Maternal inheritance is the rule for
mitochondrial DNA b/c it comes from the
cytoplasm of the egg/ovum
Genetic Diseases/Disorders
 Carried by Genes
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Autosomal – Chromosomes #1-#22
Sex-Linked – Chromosome #23
Dominantly Inherited
Recessively Inherited
Co-dominance
 Chromosomal Error
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Monosomy
Trisomy
Recessively Inherited Diseases –
Cystic Fibrosis
 Primarily whites of European descent
1 in 2500 births
 1 in 25 whites is a carrier
 Defective/absent Cl- channels cause high levels of Cl- in the
body
 Thick & sticky mucus coats
cells
 Build-up of mucus causes
infections & affects
pancreas, lungs &
digestive tract
 Live until 20’s with treatment
( 5 yrs. w/o treatment)

Cystic Fibrosis
Recessively Inherited Diseases –
Tay Sachs
 Primarily Jews of eastern European (Ashkenazi) descent &
Cajuns
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1 in 3600 births
Non-functional enzyme fails to breakdown lipids in brain cells
Symptoms begin a few months after birth
Seizures, blindness, degeneration of motor and mental skills
Death before 5 years of age
Sickle-Cell Anemia –
Co-dominance Inheritance
 Primarily Africans or of African descent
1 of 400 African Americans
 Caused by substitution of a single amino acid in
hemoglobin
 When oxygen levels are too low, sickle-cell
hemoglobin crystallizes into long rods
 2 alleles are co-dominant
 Both normal & abnormal hemoglobins are made in
the heterozygote (Ss)
 Carriers are usually healthy, although some suffer
some symptoms of sickle-cell disease under
oxygen stress

Sickle-Cell Anemia Inheritance
Heterozygote Advantage &
Sickle-Cell Anemia
 High frequency of heterozygotes is unusual for an allele with
severe detrimental effects
 May be a selective advantage for being heterozygote
 In Africa, where malaria is common….
 Homozygous normal: die of malaria
 Homozygous sickle-cell: die of sickle cell
 Heterozygote carriers: relatively free from both malaria &
sickle cell
Dominantly Inherited Diseases
 Only need one copy of the dominant allele to
have a dominantly inherited disease
 Huntington’s Disease:
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Degenerative disease of the nervous system
Occurs later in life (35-45 years of age)
Fatal
Children of a person with Huntington’s
Disease—what is their chance of getting it?
Carried on chromosome #4
Huntington’s Disease
Genetic Counseling & Testing
 Amniocentesis – uses needle
 Chorionic Villi Sampling – suction w/ a tube
 Ultrasound
 Fetoscopy
 Newborn Screening – blood tests

Phenylketonuria - PKU
 Pedigrees – traces family genes
Amniocentesis and
Chorion Villi Sampling
Pedigree Analysis
 Reveals patterns of
inheritance
 Square = male
 Circle = female
 Filled in square/circle =
person with trait
Royal Hemophilia Pedigree