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Transcript
Genetics
What is Genetics?
• The branch of biology that seeks to
explain biological variation
• Heredity: Transmission of
characteristics (traits) from parent to
offspring
Vocab You Should Recall:
• Diploid– a cell with 2 sets of homologous
chromosomes (2n)
• Haploid– a cell with 1 set of
chromosomes (n) without a
homologous pair
• Gamete– a male or female sex cell (n)
• Zygote– a fertilized egg cell (2n)
More to Recall:
• Chromosome
– a long piece of DNA
containing many alleles
• Autosome
– All chromosomes not
involved in determining
sex
• Gene
– The length of DNA that
codes for a trait.
– Genes come in pairs that
separate in the formation
of gametes.
New Vocab
• Genotype
– An organisms
genetic make-up
(allele combination)
•Phenotype
- The physical
appearance of a
gene (visible trait)
What is the relationship between genes (genotype)
and observable characteristics (phenotype)?
Phenotype = Genotype + Environment.
Genetically identical hydrangeas growing in soils of different acidity
(different environments).
The phenotype = genotype + environment principle applies equally to
human traits.
Different Genotypes Can Produce
the Same Phenotype
• Allele
For example, the gene for seed
shape in pea plants exists in
two forms:
– Alternative versions
of a gene (one from
each parent 2 alleles
•one form or allele for round
= one gene)
seed shape (R)
• Homozygous
•the other for wrinkled seed
– Having a pair of
identical alleles for a shape (r).
characteristic (pure)
•A homozygous plant would
• Heterozygous
contain the following alleles for
– Having 2 different
seed shape:
alleles for a
•RR or rr.
characteristic
(hybrid)
•A heterozygous plant would
contain the alleles Rr
Classroom Genetics
Characteristic: capital=dominant / lower
case=recessive
Hairline

widow's peak (H)

continuous hairline (h)
Earlobes

unattached (E)

attached (e)
Hitchhiker's thumb

top segment of thumb straight (T)

top segment of thumb can be bent back (t)
U-shaped tongue

able to roll tongue in trough-like shape (R)

unable to roll tongue (r)
Personal Phenotype and
Genotype
Number w/trait
% of Class with Trait
• Dominant– In heterozygote, the allele that is
expressed in phenotype
• Recessive
– In heterozygote, the allele that is
completely masked in the
phenotype
Earlobes:
Free ear lobes (dominant trait)
Attached ear lobes (recessive trait)
Dimples:
Dimples (dominant trait)
No dimples (recessive trait)
Tongue-Rolling:
Rolling up edges (dominant trait)
not rolling (recessive)
• Monohybrid Cross
– a genetic cross
between individuals
differing in one trait
•Dihybrid Cross
- a cross between
individuals differing in
two traits
• F1 Generation
– The first generation of
hybrid offspring in a
genetic cross
• F2 Generation
– Offspring resulting from
interbreeding of the
hybrid F1 generation.
• Test Cross
– Breeding of recessive
homozygote with
dominant phenotype,
but unknown
genotype
• Punnett square– diagram used by
biologists to predict
the possible outcome
of a genetic cross
Mendelian Genetics
• Gregor Mendel (1822- 1884)
– Known as the “Father of
Modern Genetics”
– Austrian Monk (and HS
science teacher) who
wondered how plants obtain
atypical characteristics
– Wrote “Experiments with
Hybrid Plants”
Mendel’s Experiments…..
Gregor Mendel’s Rules of Inheritance
• Rule of Dominance and Recessiveness:
– The allele that expresses itself in the phenotype when a gene is
heterozygous (hybrid) is the dominant allele. The allele that is
masked is the recessive allele. Ex. Tongue rolling.
• Rule of Incomplete Dominance:
– When a gene is heterozygous, incomplete dominance (or codominance) results when the phenotype is a mixture of the two
genotypes. Ex. Red, pink and white snapdragons.
• Rule of Segregation:
– During Meiosis, two alleles of a gene separate during the
formation of gametes (egg and sperm).
• Rule of Independent Assortment:
– Alleles of one gene separate independently of the alleles of any
other gene. In other words, the way in which one pair of alleles
segregates has nothing to do with the way any other pair of
alleles segregate.
Nondisjunction
• Failure of homologous chromosomes
to separate in meiosis
• Web
Karyotyping
activity
Punnett Square:
• Predicts offspring genotype from parental
gamates in a visual form:
Tongue Rolling
• Dolly cannot roll her tongue, so we
know she is homozygous recessive
for that trait.
– Phenotype: non roller
– Genotype: tt
• Dolly’s dad cannot roll his tongue
either
– Phenotype: non roller
– Genotype: tt
• Dolly’s mom can roll her tongue.
– Phenotype: roller
– Genotype: TT or Tt
• In order to have a daughter who
cannot roll her tongue, what does
Dolly’s mom genotype have to be?
In order for Dolly to
be a non-tongue
roller, her mom has
to be heterozygous
for tongue rolling
T
t?
t
t
Tt
Tt
-t
tt
tt-t
Incomplete
Dominance
•gene expression in which
the phenotype of a
heterozygous individual is
intermediate between
those of the parents.
Some general rules for genetics problems:
• Two alleles are necessary for a trait
• The genes are symbolized by the first
letter of the dominant gene.
• The letter for the dominant gene is
always capitalized.
• The letter for the recessive trait is
always lower case (make sure you can
tell the difference between the two)
• Wild Type is the typical form of the
organism, strain, or gene
• Pure traits are those with identical
genes (homozygous).
• Hybrids have mixed genes for the
same trait (heterozygous).
• Gametes only carry one allele for each
trait (they are haploid)
Probability
• Mathematic tool used for predicting the
likelihood of events
• Equal to the expected frequency of a
particular event when an experiment is
repeated an infinite number of times
• Probabilities in genetics are often predicted
based on a hypothesis, which is tested with
real data
• Mendel predicted outcomes of pea plant
crosses and then tested them thousands of
times over 8 years!
Probability = # times event is expected to happen
# opportunities (trials)
• usually expressed as a fraction
(or %).
–Ex.
• The chance of a coin landing heads up is one
out of two or ½ (50%)
• The chance of drawing an ace out of a deck
of cards is 4 out of 52 or 1/13 (7.7%)
• Why can the principles of probability be
used to predict the outcomes of genetic
crosses?
Segregation of alleles in meiosis &
uniting of a certain egg and sperm is
random, like a coin flip.
• Cystic fibrosis is an autosomal recessive
disorder.
– To have the disease, an individual has to be
homozygous recessive (ff)
• A man and woman are both carriers for cystic
fibrosis (Ff). What are the chances of them
having a child with the disorder?
F
f
F
f
FF
Ff
Ff
ff
First Law of Probability
• the results of one chance event have no
effect on the results of subsequent chance
events.
If
my My
friend
Victoria
and herhas
husband
PeterHis
arewife
• ex.
cousin
already
3 boys.
carriers
of cystic What
fibrosisisand
already had
a
is pregnant.
thehave
probability
of his
son with the disorder, what are their chances of
fourth child being a boy?
having another child with the disorder?
¼ or 25%
½
Two Other Rules of Probability
you’ll need to understand
• Rule of Multiplication– the probability of a compound event is equal to the
PRODUCT of the separate probabilities of the
independent single events
– prob(a and b) = p(a)p(b)
Ex. The probability of flipping a coin and getting 2 tails in a row:
The
probability of
flipping tails
on the 1st
penny= ½
x
The
probability of
flipping tails
on the 2nd
penny= ½
=
The probability
of getting tails
on both
pennies= ¼
Rule of Addition• The probability of an event that can occur in
two or more alternative ways is the SUM of
the separate probabilities of the different
ways.
Ex. The probability of flipping 1 head and 1 tail in a toss with 2 coins:
The probability of
flipping tails on
the 1st penny
and heads on
the 2nd penny =
(½)(½) = ¼
+
The probability of
flipping heads on
the 1st penny and
tails on the 2nd
penny= (½)(½) =
¼
=
The probability of
getting one tail
and one head in a
toss with 2 coins
=½
Practice Problems
•What
What
is the
probability
any Victoria and
is the
probability
of myoffriend
couple
having
4 boys
in with
a row?
her
husband
having
two kids
cystic fibrosis?
¼ x ¼ = 1/16
½ x ½ x ½ x ½ = 1/16 or 6.25%
Is the following sentence true or false?
The past outcomes of coin flips
greatly affect the outcomes of
future coin flips.
False
How can you be sure of getting the expected
50 : 50 ratio from flipping a coin?
You must flip the coin
many times.
Therefore, the _____ the
number of offspring from a
genetic cross, the closer
the resulting numbers will
get to expected values.
GREATER
How to determine the expected
outcome of a coin toss
• a = heads. On a coin, there is a ½
probability of tossing heads
• b = tails. On a coin, there is a ½ probability
of tossing tails
• When tossing coins simultaneously, use
binomial expansion to determine all the
possible outcomes/frequencies.
Chi Square (X2)
• How well do the observed results fit with the
expected?
• X2 indicates the degree of deviation, tells you the
% of cases in which such a deviation might be
expected by chance.
• The higher the X2 Probability (P value from
table), the more likely that the results are “true.”
The lower the X2 P, the more likely there’s
something askew (coin weighted differently on
each side, sticky side, etc)
Degrees of Freedom
• # of possibilities – 1
For a coin, you can toss heads or tails
• 2 possibilities
• Degrees of freedom would be 2-1= 1
For a die, the degrees of freedom would be
6 – 1= 5
Dihybrid Crosses
• In this type of cross, each trait is considered separately
(just as in a monohybrid cross)
• How many traits are involved in a dihybrid cross?
2
• How many pairs of genes are involved?
2
• How many total genes are involved?
4
• How many genes for each trait are found in gametes?
1
• How many total genes are involved in each gamete?
2
Practice #1
•
A tall green plant is crossed with a short yellow
plant. All of the offspring are tall and green.
– Genotypes of the parents:
– Genotypes of the gametes:
Practice #2
•
A green wrinkled pea plant is crossed with a
yellow round pea plant. All of the offspring
have green round peas. Show how genes will
arrange to do this:
Genotype and
– Genotypes of the parents:
– Genotypes of the gametes:
phenotype of
offspring:
•
genotype
• Phenotype
Practice #3
•
If a plant that is hybrid for green and pure for wrinkled
peas is crossed with a yellow wrinkled pea plant,
predict the genotypes and phenotypes of the offspring.
•
Genotypes of the parents:
•
Genotypes of the
Genotype and phenotype
of offspring:
gametes: genotype
Phenotype
Rule of Independent assortment
• The random selection of one trait will not determine
the random selection of another
• In other words, the genes for your eyes are
transmitted independently of the genes for your
height
See the dihybrid example to the right:
• As you can see, there are 4
possible outcomes.
•One letter does not affect the
selection of the other.
What Works for Peas Also Works
for Humans
Consider a cross
between parents
heterozygous for
both deafness
and albinism.
This is the same
9:3:3:1 ratio seen for
Mendel’s cross
involving pea color and
shape.
Non-Mendelian Genetics
• i.e. exceptions to Mendel’s Rules
• Covers traits that have a range of
phenotypes, not just 2 possibilities
Incomplete
Dominance
•pattern of gene
expression in which the
phenotype of a
heterozygous individual
is intermediate between
those of the parents.
INCOMPLETE DOMINANCE
cont’d:
• Yellow coat color in guinea pigs
is produced by the homozygous
genotype YY
• Cream color by the heterozygous
genotype Yy
• White by the homozygous
genotype yy.
What genotypic and phenotypic
ratios are matings between
cream-colored individuals likely
to produce?
1:2:1
yellow: cream: white
Codominance
• the condition in which both alleles in a
heterozygous organism are expressed.
•
Ex. Roan horses or cows
CO-DOMINANCE cont’d:
• In cattle, reddish coat color is
not completely dominant to
white coat color.
Heterozygous individuals
have coats that are roan
colored (ie. reddish, but with
spots of white hairs).
• What would be the results of
the following crosses:
a. red x roan:
b. white x roan:
c. red x white:
d. roan x roan:
Genotype
1:1 RR:Rr
1:1 Rr:rr
All Rr
1:2:1 RR:Rr:rr
phenotype
1:1 Red: Roan
1:1 Roan:White
all Roan
1:2:1 Red:Roan:White
Multiple Alleles
• genes with three or
more alleles
• Ex. Blood types- there
is an A, B, and O allele
(IA, IB, i)
• (More on this when we
discuss blood type
genetics)
Polygenic Trait• when several genes influence one trait.
• Ex. Eye color, height, hair, and skin color
Height is a
polygenic trait
Pleiotropy
• one gene
that affects
more than
one
seemingly
unrelated
phenotype
Anemia,
infections,
weakness,
impaired
growth, liver
and spleen
failure,
death.
Traits (phenotypes)
associated with the
sickle cell allele.
Epistasis
• one gene masks the
expression of a different
gene for a different trait
• Ex. The gene for
albinism masks the
effects of genes for skin,
hair, and eye color.
Epistasis cont’d
• In Labs, black is
dominant to chocolate (B
or b). Yellow is
recessive epistatic
(when present, it blocks
the expression of the
black and chocolate
alleles) E or e.
• Determine the number of
chocolate labs produced
from a black female
and a yellow male (BbEe
x bbee)
Phenotype
Possible Genotypes
BBEE
BbEE
BBEe
BbEe
bbEE
bbEe
BBee
Bbee
bbee
Influence of Environment
• phenotype depends on conditions in the
environment.
• Ex. Arctic fox, Siamese cats, height, and
skin color
Temperature sensitive –
The cold extremities (ears,
nose, tail, and feet) express
pigmentation while the warm
body does not)
Height in humans is polygenic. Each upper case letter adds
3”. Males (aabbcc) are 5’ and females (aabbcc) are 4’ 7”.
How tall would a male with the genotype AaBbCc be?
5’9”
A female?
5’4”
Probability that mating with this male & female result in a
child that is homozygous recessive for all three traits?
(1/4)3 = 1/64
How tall would that child be if it were a girl?
4’7”
BbEe x bbee
BE
Be
bE
be
be
BbEe
Bbee
bbEe
bbee
Color
Black
Yellow
Brown
Yellow
Pea Plants
• In sweet peas, purple flower color (P) is dominant over white
(p), but there is also a control gene such that if the plant has a
“C”, the purple has “permission” to express itself.
– If the plant is “cc”, the purple does not “have permission” to
express itself and the flower will be white anyway.
– If a plant with homozygous purple, controlled flowers(CC)
is crossed with a plant with white, non-controlled(cc)
flowers, diagram the Punnett square for the F1 and F2
generations and calculate the phenotype ratios.
• First, what are the genotypes of the parents in the first generation?
PPCC and ppcc
• What are the genotypes of their offspring?
PpCc
• What is always the phenotypic ratio for a
dihybrid cross:
genotype
P-C9: Dominant for both traits
3: Dominant for first trait and recessive for second P-cc
3: Recessive for first trait and dominant for second ppC1: Recessive for both traits
ppcc
P-CP-cc
ppCPpcc
What are the phenotypes?
purple
white
white
white