Transcript Chapter 23 PATTERNS OF GENE INHERITANCE

PATTERNS OF GENE
INHERITANCE
Gregor Mendel
• Mathematician,
Monk, Professor
• 1860’s  Made
observations of pea
plants in monastery
garden
• 7 contrasting “traits”
observed
Mendel’s
Particulate Model of Heredity
Allele for purple flowers
Locus for flower-color gene
Allele for white flowers
Homologou
s
pair of
chromosom
es
• “Particle” is passed
from parent to
offspring  controls
visible “traits”
• Mendel’s “particles” 
genes
• Alternative “forms” of
genes = alleles
Learning Target
2. Understand Mendel’s laws of
inheritance
Mendel’s Laws of Inheritance:
1) Dominance/Recessivity
– 1 allele can “dominate”/hide another allele
– Dominant allele  Capital letter (A)
– Recessive allele  Lower case letter (a)
Mendel’s Laws of Inheritance:
2) Segregation
– There are at least 2 alleles for each trait (1 on
each homologous chromosome)
– Alleles separate (segregate) during Anaphase
I
– Gametes can only pass on 1 allele
Mendel’s Laws of Inheritance:
3) Independent Assortment
– Each pair of homologous chromosomes separate
independently of other pairs  anaphase I
– Each pair of sister chromatids separate independently
of other pairs  anaphase II
– All possible allele combinations can occur in gametes
Learning Target
11. Understand the Chromosomal
Theory of Inheritance
Chromosomal Theory of
Inheritance
• Mendelian Genes have specific loci
(positions) along chromosomes, and
it is the chromosomes that undergo
segregation and independent
assortment.
Learning Target
1. Explain how genotypes relate to
phenotypes
GENOTYPE
• * “Spelling” of alleles 
AA or Aa or aa
• Homozygous  allelic
pairs “spelled” the same
 AA or aa
• Heterozygous  allelic
pairs  “spelled”
different  Aa
Phenotype
• Physical
characteristics
• May refer to
appearance or
function
• Genotype causes
Phenotype
Learning Check
If black is dominant what is Fido’s
genotype?
Tracking Generations
• P1 Generation 
Parental or Pure strains
– Always homozygous
– RR or rr
• F1 Generation  First
Filial Generation
– Cross between pure strains
 RR x rr
– Always heterozygous Rr
• F2 Generation 
Second Filial Generation
– Cross between F1
generations  Rr x Rr
– All genotypes are possible
Learning Target
4. Predict the probability of the
occurrence of a specific trait by using
monohybrid crosses.
6. Predict the probability of the
occurrence of specific traits in an
offspring by using dihybrid crosses.
Types of Crosses
• Monohybrid  looks
at probability of
inheriting one set of
alleles/trait
• Dihybrid  looks at
probability of
inheriting two sets of
alleles at the same
time
Punnett Squares
• Developed by R. Punnett
to help students predict
offspring
• Conventions:
– Female  top of square
– Male  side of square
– Capitals always precede
lower case letters
• Testcross  cross
unknown genotype with
homozygous recessive
parent
Learning Check
• Monohybrid  (assume dad is
homozygous)
• Black is dominant
• White is recessive
Learning Check
• Albinism is a recessive trait. Two
alligators with normal phenotypes, but
they both have on parent that was
albino. What are their offspring’s
chances of being albino?
Learning Check
• Black fur is
dominant to
brown fur.
• Short tails are
dominant to long
tails
Part 3
• Probabilities & Ratios
• Incomplete Dominance
• Codominance
Learning Target
10. Apply mathematics to determine
patterns of inheritance
Inheritance and Probability
• Inheritance follows the rules of
probability
F1 genotypes
Bb male
Formation of sperm
Bb female
Formation of eggs
1
2
1
2
B
1
2
b
1
2
B
B
B
b
b
B
1
4
1
4
F2 genotypes
B
b
1
4
b
b
1
4
Inheritance and Probability
• FOIL to get possible gametes
– AaBb  AB, Ab, Ba, a,b
• Phenotypic ratio = # with dom for both:
# dom 1st and rec 2nd : # rec 1st and dom
2nd : # rec for both
• Dihybrid (AaBb X AaBb) ratio is 9:3:3:1
Hypothesis: Independent assortment
Hypothesis: Dependent assortment
RRYY
P generation
Gametes
rryy
RRYY
ry
RY
Gametes
rryy

RY
ry
RrYy
RrYy
F1 generation
Sperm
Sperm
1
2 RY
1
ry
4
1
4 RY
1
ry
4
RrYy
1
4 RY
1
2 RY
F2 generation
1
4 RY
1
2 ry
Eggs
RRYY
RrYY
RRYy
RrYY
rrYY
RrYy
1
4 ry
1
2 ry
Eggs
rrYy
9
16
1
4 Ry
Actual results
contradict hypothesis
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
1
4 ry
Actual results
support hypothesis
3
16
3
16
1
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
• Monohybrid
ratios 
• Dihybrid ratio 
Learning Target
3. Explain inheritance patterns that cannot be
accounted for by Mendelian genetics (e.g. sex-linked,
sex-influenced, multiple alleles, incomplete dominance).
5. Solve problems relating to inheritance patterns that
cannot be explained by Mendel’s model of inheritance
(incomplete dominance, codominance, sex-linked,
multiple alleles)
9. Explain deviations from Mendel’s Model of
Inheritance of traits
• Mendel’s principles apply to all
sexually reproducing organisms
• However, it’s not that simple….
Incomplete vs Codominance
• Incomplete
– Results in a blend
of phenotypes
– Ex:
• Red flower
Dominant
• White flower
recessive
• Red +White =Pink
• Codominance
– Results in BOTH
phenotypes being
expressed
– Ex:
• Brown coat dominant
in horses
• White coat recessive
• Brown + White =
Pinto (Brown &
White)
Codominance
Codominance
• Figuring out
Blood types…
• Blood type A is
dominant with
Blood type B
• O is recessive to
both A & B
• Possible
genotypes?
Learning Check
• A red flower
pollinates a white
flower. They get
a pink flower.
What is this
called?
Learning Check
• A brown horse and a white horse
produce a horse that has white and
brown patches all over. What type
of inheritance is this?
Learning Check
• I have O blood type. My husband
also has O blood type. What are
the chances that our kids will have
A-type blood?
Part 4
• Sex-linked
• Sex influenced
• Other inheritance patterns
(Multiple alleles, pleiotropy,
polygenic)
Learning Target
7. Explain how Genes located on the
same chromosome tend to be inherited
together
• (3 &5)
Linkage groups
• Genes on the same
chromosome tend to be
inherited together
• Do not follow the law of
Independent assortment
Experiment
Purple flower
 PpLI
PpLI
Observed
offspring
Phenotypes
Purple long
Purple round
Red long
Red round
Long pollen
Prediction
(9:3:3:1)
215
71
71
24
284
21
21
55
Explanation: linked genes
PL
Parental
diploid cell
PpLI
PI
Meiosis
Most
gametes
PL
PI
Fertilization
Sperm
Most
offspring
PL
PI
PL
PL
PL
PI
PI
PI
PL
PI
PL
Eggs
PI
3 purple long : 1 red round
Not accounted for: purple round and red long
Sex-Linked Traits
• Sex chromosomes determine gender
 but also contain genes for lots of
other stuff.
• Sex-linked traits are genes that are
located on the X or Y chromosome
• Examples:
Hemophilia
Red-green color blindness
Color Blindness
• Red-green color blindness is a recessive
trait.
• Sex-linked traits represented differently
 XB
• Red-green color blindness is X-linked
(the Y chromosome does not have an
allele for it)
Learning Check
• Is it possible for a girl to be color
blind?
• If a little boy is color blind, what are
the possible phenotypes of Mom and
Dad
Sex influenced
• Autosomal (chromosomes 1-22)
genes that are influenced by
hormones
• Example:
– Male pattern baldness  testosterone
influences the “bald” allele
• High levels of testosterone  “bald” allele
causes baldness
• Low levels of testosterone  “bald” allele
doesn’t really do anything
Other inheritance patterns
• Multiple alleles
– When there are more than 2 alleles for a
gene
• Pleiotropy
– When a single gene affects more than
one phenotype
• Polygenic
– When many genes affect one phenotype
Pleiotropy
Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickle cells
Clumping of cells
and clogging of
small blood vessels
Breakdown of
red blood cells
Physical
weakness
Impaired
mental
function
Heart
failure
Anemia
Paralysis
Pain and
fever
Pneumonia
and other
infections
Accumulation of
sickled cells in spleen
Brain
damage
Damage to
other organs
Rheumatism
Spleen
damage
Kidney
failure
Polygenic
Part 5
• Disorders
• Pedigrees
Learning Targets
1. How genotypes relate to phenotypes
2. Understanding Mendel’s Laws of
Inheritance
GENETIC DISORDERS:
Autosomal dominant disorders
• Caused by
presence of at least
1 dominant gene
• Symptoms may be
mild or severe
• Ex. Huntington’s
Disease ,
Neurofibromatosis
Huntington
• Brain Disorder that affects the
ability to move, talk and think.
• Symptoms typically appear in 50s
and gets worse
– Poor memory
– Lack of
coordination
– Difficulty
walking and/
or swallowing
Neurofibromatosis
• Growth of noncancerous tumors
under skin, on eyes or brain
• Symptoms vary widely, appear w/in
first 2 years of life
GENETIC DISORDERS:
Autosomal recessive disorders
• Caused by presence
of both recessive
alleles
• Heterozygotes may
be “carriers”  do
not have disorder,
can pass it on
• Ex. Tay-Sachs, Cystic
Fibrosis,
Phenylketonuria
Tay-Sachs
• Progressively destroys nerves cells in
brain and spine
• Symptoms appear 3-6 months
–
–
–
–
–
Development slows & muscles weaken
Seizures
Vision & hearing loss
Intellectual disabilities
paralysis
Cystic Fibrosis
• Buildup of thick, sticky mucus
• More common in Caucasians
• Symptoms vary, but can lead to
– Lung damage
– Digestive issues
– Reproductive problems
Phenylketonuria (PKU)
• If not caught soon after birth,
intellectual disabilities can result.
• No symptoms at first, but later can
include
–
–
–
–
–
Seizures
Musty body odor
Skin rash
Small head
Fair skin
Pedigrees
• Rules: