Transcript Genetics - Mount Mansfield Union High School

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 containing 2 sets of
homologous chromosomes (2n)
• Haploid– a cell containing 1 set of
chromosomes without a homologous
pair (n)
• 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 makeup (DNA code passed on from parents
to offspring)
•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
Classroom Genetics
Characteristic: d=dominant / r=recessive
Hairline

widow's peak (d)

continuous hairline (r)
Earlobes

unattached (d)

attached (r)
Hitchhiker's thumb

top segment of thumb straight (d)

top segment of thumb can be bent back (r)
U-shaped tongue

able to roll tongue in trough-like shape (d)

unable to roll tongue (r)
Number w/trait
% of Class with Trait
Nondisjunction
• Failure of homologous chromosomes to
separate in meiosis
• Web Karyotyping
activity
• 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
•one form or allele for round seed
gene)
shape (R)
• Homozygous
– Having a pair of identical •the other for wrinkled seed
shape (r).
alleles for a
characteristic (pure)
•A homozygous plant would
• Heterozygous
contain the following alleles for
– Having 2 different alleles seed shape:
for a characteristic
•RR or rr.
(hybrid)
•A heterozygous plant would
contain the alleles Rr
• Dominant– In heterozygote, the allele that is
expressed in phenotype
• Recessive
– In heterozygote, the allele that is
completely masked in the phenotype
Earlobes:
MidDidget Finger Hair:
Free ear–lobes
Present
even(dominant
if just one trait)
hair (dominant trait)
Attached
Not
present
ear(recessive)
lobes (recessive trait)
Dimples:
Freckles:
Dimples
(dominant trait)
trait)
Freckles (dominant
No
dimples (recessive)
(recessive trait)
No freckles
Widow’s
Peak
Tongue-Rolling:
Presenttrait) trait)
Rolling up(dominant
edges (dominant
Not
present(recessive)
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 Cross– The first generation of
hybrid offspring in a
genetic cross
• F2 Cross
– 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 (1823- 1884)
– Known as the “Father of Modern
Genetics”
– Augustinian Monk who
wondered how plants obtain
atypical characteristics
– Wrote “Experiments with Hybrid
Plants”
Why Peas?
• Easy to grow
• Fast growing
– could grow many
generations during his
8-year experiment
• Self-fertilizing
– don’t need a mate
Traits he looked at:
•He DID NOT study traits with a continuous
range of variation (like seed size)
• He chose traits with an either-or form. For
example:
– Short or tall
– Smooth or wrinkled peas
– Green or yellow pods and seeds
Mendel’s Experiments
• 1st, he needed true-breeding varieties (homozygous
dominant) for each of the traits --this took years.
• He then found an “atypical” variety of plant
(homozygous recessive) and transplanted them next to
typical varieties and recorded which progeny arose.
• He found that all offspring retained the essential traits
from parents.
Mendel crossed plants with wrinkled peas and plants with
smooth peas
• Would the offspring plants produce
smoothish-rough pods?
• Would some of the pods be smooth
and some be rough?
•Mendel found that the next
generation of plants produced only
smooth peas.
•However, when those smooth pod
plants self-fertilized, some plants in
the next generation produced rough
pods.
A Color Cross:
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
tt
-t
tt-t
Some Alleles Are
Related Through
Incomplete
Dominance
•pattern of gene
expression in which the
phenotype of a
heterozygous individual is
intermediate between
those of the parents.
Some generalizations 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)
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.
Probability:
• It’s the mathematical tool used for predicting the
likelihood of events in everyday life
• The expected frequency of a particular event
when an experiment is repeated an infinite
number of times is the probability of the event.
• Probabilities in genetics are often predicted based
on certain hypotheses and then the predictions
are used to test the hypothesis using real data.
• Mendel actually did this with pea plants. He
predicted outcomes and then tested them
thousands of times over 8 years!
Probability = # times event is expected to happen
# opportunities (trials)
• It is usually expressed as a fraction.
– Ex.
• The chance of a coin landing heads up is one out of
two or ½
• The chance of drawing an ace out of a deck of cards is
4 out of 52 or 1/13
• Or explained as a percentage chance.
– There is a 50% chance of a coin landing heads up when
you toss it.
– There is a 7.7% chance of picking an ace up from a deck
of cards.
• Why can the principles of probability be used
to predict the outcomes of genetic crosses?
The way in which the alleles
segregate is completely random,
like a coin flip.
Segregation of Alleles
Principle of Segregation:
•Genes come in pairs that
separate in the formation of
sex cells (and these sex cells
unite randomly at
fertilization).
• Cystic fibrosis is an autosomal recessive
disorder.
– In order 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.
• ex. My cousin already has 3 boys. His wife is
If pregnant.
my friend Victoria
her probability
husband Peter
Whatand
is the
ofare
hiscarriers
fourth
ofchild
cysticbeing
fibrosisaand
have already had a son with the
boy?
disorder, what are their chances of having another child
with the disorder?
½
¼ or 25%
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
– Probability of event a and b happening = 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 =
1/2
Practice Problems
•What
What
is the
probability
any Victoria
couple and her
is the
probability
of myof
friend
havinghaving
4 boystwo
in kids
a row?
husband
with 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 _____
number of offspring from a
genetic cross, the closer the
resulting numbers will get to
expected values.
Larger
What Works for Peas Also Works for
Humans
Albinism is a autosomal recessive
condition.
• In order to show the disorder, an
individual has to be homozygous
recessive (aa)
• Refer to the following punnett
squares to see the possible offspring
for different scenarios:
Stop
• The Coin Lab
Are Different Characters Like Color and Shape
Inherited Together or Inherited Independently?
• Mendel performed dihybrid
crosses to find out.
•Mendel’s conclusion:
•Different characters are
inherited independently.
Incomplete
Dominance
•pattern of gene
expression in which the
phenotype of a
heterozygous individual is
intermediate between
those of the parents.
How to make a two gene cross:
Meiocytes - diploid cells that form haploid gametes–
a.k.a. germ cells
A
a
A
B
Egg meiocyte
b
A
b
Sperm meiocyte
b
Determine the types of gametes that each
parent forms, according to Mendel’s laws:
a) The Law of Equal Segregation
• the 2 members of a gene pair segregate equally
into gametes
b) The Law of Independent Assortment:
different gene pairs assort independently from one
another (unless otherwise noted)
How do I do this? Perform a cross.
1) Align the 2N genotype horizontally (one locus) and
vertically (the second locus) on a square. Do this for
each meiocyte.
egg meiocyte sperm meiocyte
A
a
B
b
A
A
b
b
2) Perform a cross, fill in each cell of the square. Each cell
is one of the types of gametes formed by the meiocyte.
B
b
A
AB
Ab
a
aB
ab
b
b
A
Ab
Ab
A
Ab
Ab
Or:
Figure out what the gametes are that
the parents can make.
Using the F.O.I.L. method from math
AaBb :
First-
AB
OuterAb
InneraB
Lastab
Next, combine the male and female gametes.
These combine at random upon fertilization.
AB
Ab
Ab
Ab
Ab
Ab
aB
ab
AB
Ab
aB
ab
Ab
AABb
AAbb
AaBb
Aabb
Ab
AABb
AAbb
AaBb
Aabb
Ab
AABb
AAbb
AaBb
Aabb
Ab
AABb
AAbb
AaBb
Aabb
4/16
25%
4/16
25%
4/16
25%
4/16
25%
The Dihybrid Cross: a cross between 2 individuals that are
heterozygote at the same two genes
Using Mendel’s strategy consider A=yellow, a=green. B=round,
b=wrinkled
P: AA BB x aa bb (cross two pure lines)
F1: Aa Bb x Aa Bb (the dihybrid cross, between 2 F1 progeny)
F2: genotypes
phenotypes
AB
AB
Ab
aB
ab
AABB AABb AaBB
AaBb
AABb
AAbb
AaBb
Aabb
AaBB
AaBb
aaBB
aaBb
AaBb
Aabb
aaBb
aabb
Ab
aB
ab
9 A- B- (yellow, round)
3 A- bb (yellow, wrinkled)
3 aa B- (green, round)
1 aa bb (green, wrinkled)