Transcript Reebop Ratios - West Branch Schools

Reebop Ratios
Part 1: Introduction
Part 2: Mendelian Genetics
and Beyond
Part 1: Introduction
Remember your baby Reebops?
• Let’s think about the tail trait.
• What were the possible phenotypes?
curly tail
straight tail
• What were the possible genotypes?
TT = curly tail
Tt = curly tail
tt = straight tail
Important Words
• Homozygous: Having two alleles (forms
of a gene) that are identical (TT or tt).
• Heterozygous: Having two alleles (forms
of the gene) that are different (Tt).
• Dominant: The dominant allele is
expressed when homozygous (TT) and
when heterozygous (Tt).
• Recessive: The recessive allele is only
expressed when homozygous (tt).
• Which of the genotypes below are
heterozygous? Homozygous?
Tt TT tt
• Based on these genotypes and
phenotypes, which allele is dominant, T
or t? How do you know?
TT = curly tail
Tt = curly tail
tt = straight tail
Quick-Think Time
• If your original Reebop parents had 100
offspring, how many of them would you
expect to have a curly tail?
• How many of them would you expect to
have a straight tail?
• Why do you think so?
• Why would someone want to predict
the numbers of offspring to have a
particular trait?
Punnett’s Square
• We can find out the expected number of
curly-tailed and straight-tailed babies
with a tool called Punnett’s Square.
• We will call it Punnett’s “Fertilization
Probability” Square.
Who was Punnett
and what is his square?
• REGINALD CRUNDALL
PUNNETT (1875-1967)
was among the first
English geneticists. He
created the “Punnett
Square” to work out what
the possible offspring of
two parents will be.
Curly-tailed Dad
Tt
possible gametes
Tt
Curly-tailed Mom
T
t
T
TT
Tt
t
Tt
tt
Quick-Think Time
TT
T
t
T
TT
Tt
t
Tt
tt
Tt
t t
How many out of 100?
Expected Genotype
TT
Tt
50%
t t
25%
25%
T
t
T
TT
Tt
t
Tt
tt
Expected Phenotype
TT
Tt
50% + 25% = 75%
curly-tailed
t t
25% straight-tailed
T
t
T
TT
Tt
t
Tt
tt
From Percentages to Ratios
25% TT to 50% Tt to 25% tt =
Genotypic ratio of 1 to 2 to 1, or 1:2:1
75% curly-tailed to 25% straight-tailed =
Phenotypic ratio of 3 to 1 or 3:1
How can we test this prediction?
Expected genotypic ratio:
1:2:1
Expected phenotypic ratio:
3:1
STOP
to complete
Reebop Ratios
activity
Part 2: Mendilian Genetics
and Beyond
You and Gregor Mendel…
• In the first Reebop activity, you looked
at the variety of offspring produced by
two parents.
• You looked at 7 traits, the gene for each
residing on a different chromosome.
• You learned about how meiosis and
fertilization contribute to variation in
organisms.
…You and Gregor Mendel
• In the second Reebop activity, you
looked at the expected and
experimental ratios of offspring with
curly and straight tails.
• You, and a nineteenth-century monk
named Gregor Mendel have a lot in
common.
• Let’s find out why.
Mendel & the Garden Pea
• Gregor Mendel, Austrian Monk
• In 1866, Mendel performed breeding
experiments with garden pea plants
and observed consistent, predictable
patterns of inheritance.
• From his observations, Mendel
developed a number of principles,
today referred to as Mendel's Laws of
Inheritance.
Before Mendel…
• In the 19th century, when both Gregor
Mendel and Charles Darwin were alive,
“blending inheritance” was a popular idea.
• According to blending inheritance, traits
from parents “blended” in offspring
producing intermediate traits.
• Example: The offspring of one short parent
and one tall parent would be of medium
height. This offspring would pass the new
“medium sized” trait to its own offspring.
Quick-Think Time
• Imagine that blending inheritance
occurs in Reebops.
• What type of tail would the offspring of
curly-tailed and straight-tailed parents
have?
• What would happen to variation in this
population over time?
Darwin and Mendel…
1809–1882
1822–1884
Darwin and Mendel…
• Darwin observed that variation, in the
form of individual differences, exists in
every species, and is not reduced from
one generation to the next.
• He proposed that natural selection acts
on this variation.
• Darwin knew that in order for his idea
to be valid, a mechanism for preserving
and inheriting variation had to exist.
Darwin and Mendel…
THE BIG IDEA:
•
Mendel’s work answered Darwin’s
difficult question.
–
Mendel showed that traits from parents do
not blend. The traits remain distinct and
can be passed on to later generations.
Genetic variation is preserved.
–
Mendel also proposed a mechanism for
how this variation is inherited and
preserved.
Mendel summarized his findings
in three theories…
 Mendel’s first law, or the principle of
segregation:
 During the formation of eggs and sperm,
“paired factors” (now known as alleles,
which reside on chromosomes) segregate,
or separate.
Quick-Think Time
 Explain how Mendel’s Principle of
Segregation was demonstrated in the
Reebop activities.
Mendel summarized his findings
in three theories…
 Mendel’s second law, or the principle of
independent assortment:
 Each pair of “factors” (now known as
alleles) separate and recombine into
gametes independently of each other.
Quick-Think Time
 Explain how Mendel’s Principle of
Independent Assortment was
demonstrated in the Reebop activities.
Mendel summarized his findings
in three theories.
 Mendel’s third law, or the law of
dominance:
 Each characteristic is determined by the
interaction of two hereditary factors (now
called alleles), one from each parent.
Mendel found that one allele was always
dominant over the other.
Quick-Think Time
 Explain how Mendel’s Law of
Dominance was demonstrated in the
Reebop activities.
Mendel & Probability
• coin (heads/tails)
- probability of heads or tails = ½
- probability of getting heads 3 times in a row
- multiplication rule:
(½ x ½ x ½ = 1/8)
Extensions of Mendelian Genetics
• Many researchers have encountered
exceptions to Mendelian Principles –
was he wrong?
• Majority of heritable characters not as
simple as peas
– Phenotypes affected by: many genes,
sequential, jointly, environment, etc.
Extensions of Mendel…
• Incomplete Dominance – appearance between
dominant and recessive
CR
F2 generation
1 : 2 : 1
CRCR:CRCW:CWCW
F1 Generation
All CRCW
Eggs
CRCR
CR
CW
CRCR
CRCW
CRCW
CWCW
Sperm
CR
Cw
CWCW
CW
1:2:1 phenotypic ratio
1:2:1 genotypic ratio
Another example of
Incomplete Dominance
Chestnut,
CC
Palomino,
CCcr
Cremello,
CcrCcr
Extensions of Mendel…
• Codominance – both alleles affect
phenotype in separate ways
– Only observed at the cellular or molecular
level
– e.g., M N bloodgroups (RBC surface marker)
• MM
• NN
• MN (both equally expressed)
Extensions of Mendel…
• Multiple alleles of a single gene– ABO
blood groups
– A, B, AB, O
– Combinations of 3 different alleles
– IA, IB, I
– A and B dominant to O, but not to each other
– A and B are codominant
Extensions of Mendel…
•Multiple alleles of a single gene– ABO blood groups
• I – enzyme that adds sugar molecules to lipids
on surface of RBC recognized by our immune
system
• IB –adds galactose (IBIB or IBi) “B”
• IA – adds galactosamine (IAIA or IAi) “A”
• i– does not add a sugar (ii) “O”
• IA IB adds both sugars “AB”
Multiple Alleles—ABO Blood Groups
Possible alleles from male
Possible alleles from female
IA
IB
I
IA
IAIA
IAIA
IAi
IB
IAIB
IBIB
IBi
i
IAi
IBi
ii
Blood Types
A
AB
B
O
Extensions of Mendel…
• Epistasis – one gene influences a second
gene (gene interaction)
For example in dogs:
– B gene: determines whether pigment
(melanin) is brown or black.
– E gene:enables or prevents melanin from
being deposited in the coat.
Epistatic Ineractions on Coat Color
ee
No dark pigment in fur
E_
Dark pigment in fur
Yellow Lab
eebb
Yellow fur,
brown nose,
lips, eye rims
eeB_
Yellow fur,
black nose,
lips, eye rims
E_bb
Chocolate Lab
E_B_
Black Lab
Brown fur,
nose, lips,
eye rims
Black fur,
nose, lips,
eye rims
Pedigree Analysis
• Display of family relationships to help
keep track of relationships and traits
– phenotypes
– genotypes
– e.g., X-linked clotting disorder
XX (normal)
XXh (carrier)
XhXh (hemophilia)
XY (normal)
XhY (hemophilia)
Hemophilia in European Royalty
XXh
XXh
XhY
Click and choose the
button
Genetic Testing
• fetal testing
• amniocentesis – needle inserted in
uterus to remove ~10 mL of fluid from
amniotic sac (14-16th week)
– chromosomal analysis
• chorionic villus sampling (CVS)
– tube inserted through cervix into uterus
removing sample of fetal tissue from placenta