Download Allele

GENETICS
Genetics:
• The study of how traits encoded in
our DNA are passed on.
• Used to predict the possible
outcomes of a genetic cross
–Depends on probability
–We might expect 2/4 offspring to look
a certain way, and end up with 0/4
looking that way!
History of Genetics as a science:
Gregor Mendel is the “Father of Genetics”
- Austrian monk who studied pea plants to
figure out how parents pass traits to their
offspring
- Started with parents that were True breeding
- means that when they self fertilize, their
offspring all look like them.
Parents: (both true breeding)
white
x purple
Expect???
What he got:
So… he crossed two of them….
Expect???
What he got:
These crosses showed that
there were “factors” being
passed from parent to offspring
even if it wasn’t being “used”
Now we call these factors
GENES
Genes – control a heritable
feature;
Example: Hair color, seed shape,
height;
Allele – controls the variation of a
feature.
Example: brown, blonde,
black hair
Characteristic/Gene?
Trait/Allele?
CHARACTERS AND VARIANTS OF EACH
TRAITS
?
TRAITS
?
RARE DOMINANT PHENOTYPE
A chromosome = folded up string of many genes
What are alleles?
Variations of a gene that occupy
the same locus on homologous
chromosomes
Locus = position on a
chromosome.
GENE = STEM LENGTH
SHORT
t
T
LONG
GENE = FLOWER COLOR
P
p
Terms
• Diploid (2n)
• HAPLOID (n)
• Egg
• Sperm
• Parent
• Meiosis
• Testes
• Gamete
• Zygote
• Progeny
• Offspring
• Fertilization
• Ovary
Mendel’s laws of genetics
1. Law of segregation: only one allele for
each gene is passed from a parent to
the offspring.
Why? Has to do with separation of
homologous chromosomes during
meiosis.
Segregation
of Alleles
Tongue Rolling
2. Law of independent assortment:
Alleles for different genes are
passed to offspring independently of
each other.
The result is that new combinations
of genes present in neither parent is
possible.
3. Law of complete dominance – some
alleles over power others. So even if
both alleles are present, we only “see”
the dominant one.
- the “hidden” allele is called recessive
This only applies to SOME genes, not all.
Remember Mendel’s pea plants?
- Purple was crossed with white and we got
ALL purple. So which allele is dominant?
Genotype: the alleles that an organism has.
- alleles are abbreviated using the first letter
of the dominant trait. (with some exceptions
that we will get to)
- a capital letter represents the dominant
ex: P for purple flower allele
- a lower case represents the recessive.
ex: p for white flower allele
All diploid organisms have two alleles for
each trait:
- you can have two of the same alleles
Ex: PP or pp
- such an individual is described as
Pure or Homozygous.
OR
All diploid organisms have two alleles for
each trait:
- you can have two different alleles
Ex: Pp
- such an individual is described as
hybrid or heterozygous
Phenotype: physical appearance
Examples: brown hair, widows
peak
- the trait that “wins” in the case
of complete dominance;
- depends on the combination of
alleles
Terminology for Genetic Crosses
P generation: “parents;” First
generation in the cross
F generations: results of the
cross;
- F1 – 1st generation; offspring of
P generation
- F2 – 2nd generation; offspring of
F1 generation
Monohybrid cross: cross that
focuses on the alleles of a single
trait;
How do we show the possibilities?
- punnett square
PUNNETT SQUARE
Allele in sperm
1
Allele in sperm
2
Allele in Egg 1
Allele in Egg 2
Zygote formed
if sperm 1
fertilizes egg 1
Zygote formed
if sperm 1
fertilizes egg 2
Zygote formed
if sperm 2
fertilizes egg 1
Zygote formed
if sperm 2
fertilizes egg 2
In pea plants tallness is dominant
to short or dwarf. Cross a pure
tall male to a pure dwarf female
pea plant. Show both ratios
phenotype & genotype for the
offspring. Now cross two of the
F1.
• Take it step by step until you “get it”
• Step 1: what are the parent’s
genotypes?
–Mom?
–Dad?
tt
TT
• Step 2: Set up Punnett Square
t
t
T
Tt
Tt
T
Tt
Tt
• Step 3: ANSWER THE QUESTION
Offspring
t
t
genotypes:
T
T
Tt
Tt
Tt
Tt
Offspring
phenotypes:
• Step 4: Part II
T
t
T
t
TT
Tt
Tt
t t
Offspring
genotypes:
Offspring
phenotypes:
Inheritance Patterns:
Every gene demonstrates a distinct
phenotype when both alleles are
combined (the heterozygote)
Complete dominance is one
- when both alleles are present, only
the dominant trait is seen.
Inheritance Patterns:
Incomplete dominance
- when both alleles are present, the
two traits blend together and create
an intermediate trait
INCOMPLETE DOMINANCE
Inheritance Patterns:
Co-dominance
- when both alleles are present,
both traits are visible
Different notation: Use first letter of
the feature with a superscript for the
trait.
Example: CW or CB for white coat or
black coat;
Inheritance Patterns:
Co-dominance
- when both alleles
are present, both
traits are visible
Inheritance Patterns:
Each gene has a specific inheritance
pattern.
- you will either be told or be given a
hint; look at the heterozygote!
Women have two X’s
but men only have
one.
How do we deal with
the genes on the X
chromosome?
Sex-linked trait
Alleles for the trait are located on
the X chromosome in humans.
- works the same in women as all
the other traits.
BUT –
- men only inherit one such allele.
Sex-linked trait
For females: have to figure out
phenotype based on inheritance
pattern.
For Males: phenotype is that of
whatever allele they inherit.
Example: color blindness
Seeing color (XC) is dominant to
c
being color blind (X )
Identify the sex and trait of the
following:
XCY
XCXc
XcXc
XcY
XCXC
Example: Color Blindness
Set up a punnett square crossing a
heterozygous normal female with a
normal male:
- what is mom’s genotype?
- what is dad’s genotype?
- what gametes can each give?
- what are the offspring’s geno’s?
Cross Number 1:
XC
Xc
XC
C
C
X X
C
c
X X
Y
XC Y
c
X Y
What %
chance of
having color
blind daughter?
Son?
SEX-LINKED TRAITS
COLOR BLINDNESS
AFFLICTS 8% MALES AND 0.04% FEMALES.
If we are dominant, how can we
figure out our genotype?
What are the possibilities?
Test cross: a cross that
determines genotype of
dominant parent
- Cross unknown dominant
parent (possibilities BB or Bb)
with a recessive parent
then analyze the offspring.
If some of the
offspring have the
recessive trait,
then the unknown
parent has to be
b
heterozygous
B
?
Bb
?b
b
Bb
?b
If all offspring are dominant,
unknown parent HAS to be
homozygous
B
?
b
Bb
?b
b
Bb
?b
Multiple alleles: Some genes have more
than two variations that exist, although
we still only inherit 2
Example: Human blood types
Three alleles:
IA
IB
i
Genotype
IA IA
IA i
IB IB
IB i
IA IB
ii
Phenotype
A
A
B
B
AB
0
Polygenic –
Multiple genes each
with 2 alleles
Creates additive/
quantitative effect
SKIN PIGMENTATION
Dihybrid cross:
A cross that focuses on possibilities
of inheriting two traits
- two genes, 4 alleles
Black fur is dominant to brown fur
Short fur is dominant to long fur
What is the genotype of a guinea pig that is
heterozygous for both black and short fur?
Dihybrid cross:
Parent phenotypes: BbSs x BbSs
Figure out the possible gametes:
Then set up punnett square
Dihybrid cross:
BS
BS
Bs
bS
bs
Bs
bS
bs
Linked Genes: genes that are on
the same chromosome.
Does the law of independent
assortment apply?
Can they be separated? Will
they always separate?
What does this mean?
• It means that you can pass on an
allele that you got from your mom and
an allele you got from your dad ON
THE SAME CHROMOSOME
• However, it is more likely that two
alleles that start on the same
chromosome will get passed on
together.