Download Genetics 2 - MaxSkyFan

More Mendelian genetics
Real Biologists of Genius
• We salute you Mr.
Gregor Mendel. An
Austrian monk with a
love for peas, you
published data that
showed blending
inheritance was
incorrect and
introduced hereditary
factors occurring in
discrete pairs.
Mendelian Genetics
• Mendel knew that his
'factors' were discrete
and non-blending.
• He also knew much
more about the
behavior of these
units of inheritance.
• So let’s revisit his
peas!
Law of Segregation
• Mendel's First Law (Law
of Segregation): Mendel
determined that each
individual has two copies
of each gene (e.g., Pp).
• These copies are called
alleles. If both alleles are
the same, then the
individual is homozygous
(e.g., PP or pp).
• If the two alleles are
different, then the
individual is heterozygous
(e.g., Pp).
• When an individual
creates gametes (sex
cells: egg or sperm in
humans, egg or pollen
grain in plants), only one
of each allele is packaged
in the gamete.
• Mendel determined that
which allele appears in
the gamete is random,
with each allele having a
50% chance. This rule is
the Law of Segregation.
Flower color
• Pea flowers are either
purple or white.
• Peas fertilize
themselves, so
• white  white and
purple  purple.
• called true-breeding
• But…
• …if you cross a truebreeding purple with
a true-breeding
white…
• …all of the offspring
have purple flowers.
• Hence Mendel said
that purple was
dominant to white.
• PP: purple
• pp: white
• Pp: purple!
Terms to understand
• gene: stretch of DNA that
codes for a particular
trait. (e.g., flower color)
• allele: a particular variant
of a gene (e.g., purple)
• genotype: what alleles
an individual has for a
particular trait or set of
traits (e.g., Pp)
• phenotype: the
expression of the genes;
what the individual looks
like (e.g., purple)
• dominant trait: an allele
that is expressed no
matter what the other
allele is (e.g., purple
flower color being
dominant to white flower
color in pea plants)
• recessive trait: an allele
that is only expressed if it
is the only allele present
(i.e., both alleles are the
same) (e.g., white flower
being recessive to purple
flower color)
Terms to understand
• homozygous: has 2
copies of the same allele
for a given trait (e.g., PP
or pp)
• heterozygous: has 1
copy of each of two
alleles for a given trait
(e.g., Pp)
• F1 generation: the kids
of the parents
• F2 generation: the
grandkids of the parents
(kids of F1)
• gamete: sex cell (egg or
sperm); only has ONE
allele for each gene since
it only has one
homologous chromosome
(either the one you
received from Mom or the
one you received from
Dad)
• True-breeding:
homozygous for the trait.
Forming gametes
• How many different
gametes can PP
make?
• 1
• P
• How many different
gametes can Pp
make?
• 2
• P or p
• When forming
gametes, you always
need one allele for
each gene.
• How many different
gametes can PPTt
make?
• 2
• PT or Pt
Determining the number of different
gametes possible
•
•
•
•
•
•
•
•
•
•
AaBBCc?
2x1x2=4
AaBbCC?
2x2x1=4
AaBbCcDd?
2 x 2 x 2 x 2 = 16
AAbbCCddEE?
1x1x1x1x1=1
What is it?
AbCdE
• Which of the following
gametes can this parent
(AABbCCDdeeFf) make?
a. AAbCEf
b. ABCDEF
c. abcdef
d. ABCdef
• d is the answer.
• What is the chance of
that parent producing that
gamete?
• 1/8 Why?
Determining the number of different
alleles
•
•
•
•
•
•
•
•
AaBBCc?
2 + 1 + 2 = 5 alleles
AaBbCC?
2 + 2 + 1 = 5 alleles
AaBbCcDd?
2+2+2+2=8
AAbbCCddEE?
1+1+1+1+1=5
How many different
genes are shown at
right?
• 3, 3, 4, and 5 (top to
bottom)
Other terms not on the handout
• Incomplete dominance:
in this case, the presence
of a single gene to code
for a particular protein
(enzyme) is insufficient to
produce the full trait.
• Why?
• Because you don’t have
enough of the enzyme to
fully express the trait!
Ex. In snapdragons,
• RR = red,
• rr = white,
• Rr = pink!
Incomplete Dominance
Co-dominant alleles: Human
ABO blood type
• There are 2 dominant
alleles (A and B) and one
recessive (O).
• A and B alleles determine
sugars present on cell
membrane of red blood
cells.
• If you have A, then you
produce type A sugars.
• If you have B, then you
produce type B sugars.
• If you have O, then you
produce no sugars.
Possible
Genotypes
Possible
Phenotypes
AA type A
AO
type A
BB
type B
BO
type B
AB type AB
OO
type O
Transfusions
• When you need a blood
transfusion, they try to
match blood types.
• If you give type A blood to
someone without type A
blood, they have no type
A blood sugars on their
own red blood cells so
their immune system will
attack the transfused
blood because it
recognizes that it is
foreign.
• While they try to give type
A blood to a person with
blood type A, type O
could also be used.
• Why? Because there are
no blood sugars in type O
blood that the type A
person’s body hasn’t
seen.
• Therefore, type O is
called the universal donor
and type AB is the
universal recipient.
What about positive and negative?
• That’s a different gene.
• The Rh factor is another
sugar on red blood cells.
• It’s called Rh for Rhesus, as
it was first found in a Rhesus
monkey.
• You are Rh positive if you
have the blood sugar, but Rh
negative if you do not.
• Thus the ultimate donor is?
• O negative
• Ultimate recipient?
• AB positive
What are the relative frequencies of
these blood types in humans?
•
•
•
•
•
•
•
•
O Positive
O Negative
A Positive
A Negative
B Positive
B Negative
AB Positive
AB Negative
37%
6%
34%
6%
10%
2%
4%
1%
Some More Terms
• Monohybrid cross:
cross between two
monohybrids (only a
single trait is tracked)
(e.g., Pp x Pp)
• Dihybrid cross:
cross between two
dihybrids (e.g., PpYy
x PpYy).
Dihybrid Cross
Some More Terms
• Pleiotropic: when a
single gene determines
more than one phenotype
for an organism (gene
that lengthens bones
lengthens legs and arms).
• Gene for sickle cell
affects vulnerability to
malaria and sickle cell
anemia.
Polygenic traits
• A trait that is affected
by multiple genes
• These traits are not
discrete (yes or no)
but show continuous
variation.
• E.g. skin color, height,
etc.
Test Cross
• Test cross: When a
single trait is being
studied, a test cross is a
cross between an
individual with the
dominant phenotype but
of unknown genotype
(homozygous or
heterozygous) with a
homozygous recessive
individual. If the unknown
is heterozygous, then
approximately 50% of the
offspring should display
the recessive phenotype.