Download Genetics and Punnet Squares Lecture

Gregor Mendel
• Gregor Mendel:
– Austrian monk lived from
1822-1884
– Mendel developed
principles of heredity
without any knowledge of
genes or chromosomes
– His principles were
established through
experiments with pea plants
Why was Mendel successful with the pea?
• Used pure breeding, 7 contrasting traits
• Studied characteristics one at a time for many
generations
• Used mathematics in analyzing his results
• Obtained large numbers of offspring
• Chose pea plants which normally self-fertilize
• Inexpensive
• Used scientific method
• Easy to pollinate (transfer of male pollen to egg)
Mendel’s 7 contrasting traits
Genetics Terms
• Define genes:
factors that control organism’s traits.
the part of chromosome that contains the
genetic code.
• Every organism requires a set of coded
instructions for specifying its traits
• For offspring to resemble their parents, their must
be a reliable way to transfer hereditary information
from one generation to the next
Genetics Terms
• homozygous (pure): the alleles on homologous
chromosomes are the same
• heterozygous: (hybrid): the alleles on
homologous chromosomes are different
• parental generation (P): the two original
organisms being crossed - usually pure
• first filial generation (F1) : the first generation
of offspring from the parents
• second filial generation (F2) : generation of
offspring arising from the first filial generation
Genetics Terms
• Genotype:
the genetic makeup of an organism
– Homozygous Dominant: TT
– Homozygous Recessive: tt
– Heterozygous: Tt
• Phenotype: the appearance of an organism
– Describes what it looks like
TT - Tall
tt - short
Tt - Tall
Punnett Squares
• a model used to predict the results of a
genetic cross
• BB X bb
B
B
b
Bb
Bb
b
Bb
Bb
Three Laws by Mendel
1. Law of Dominance: a pattern of heredity
in which one allele of a gene may express
itself by masking the presence of the other
allele
Dominant Trait: the trait or allele that is
expressed (capital letter) R
Recessive Trait: the trait or allele that is
present but that is not expressed
(lowercase letter) r
Three Laws by Mendel
1. Law of Dominance:
Example:
red flower (RR) X white flower (rr)  red flower (Rr)
X

Example of Dominance
Problem:
Cross homozygous dominant
with homozygous recessive
• R = red
• r = white
• RR x rr
R
R
r
Rr
Rr
r
Rr
Rr
Results:
Phenotype:
100% red
Genotype:
100% heterozygous
2. Law of Segregation
Mendel’s second law
– When gametes are formed during meiosis:
• There is a random segregation of homologous
chromosomes
• Random segregation of sister chromatids & alleles
• The result: new gene combinations are likely to be
produced
• Segregation means separation and can
lead to genetic recombination.
Example of Segregation
Problem:
Cross two offspring from 1st cross (2 heterozygous parents)
• R = red
• r = white
• Rr x Rr
R
R
r
RR
Rr
Rr
rr
Phenotype:
75
25
____%
red, ____%
white
3 red : ___
1 white
___
Genotype:
___%
25 homozygous dominant,
25
___% homozygous recessive,
___% heterozygous
50
r
3. Law of Independent
Assortment
Mendel’s third Law
– Scenario: Two different traits located on two different
chromosomes
• They segregate randomly during meiosis
• May be inherited independently of each other
• The cross of two organisms heterozygous for a trait is known
as a dihybrid cross
Law of Independent Assortment
Dihybrid Cross
Problem:
Cross homozygous tall
and homozygous
wrinkled seeds with
homozygous short and
homozygous smooth
seeds
T = tall
t = short
Q = wrinkled
q = smooth
What are the genotypes
for these plants?
TTQQ x ttqq
TTQQ x ttqq
TQ TQ
TQ
TQ
tq
TtQq
TtQq
TtQq
TtQq
tq
TtQq
TtQq
TtQq
TtQq
tq
TtQq
TtQq
TtQq
TtQq
tq
TtQq
TtQq
TtQq
TtQq
Phenotype: 100% Tall & Wrinkled
Dihybrid Cross
• What is the phenotype from this cross?
– 100% Tall and Wrinkled
• What is the genotype from this cross?
– We don’t worry about genotype for dihybrid
crosses
Complete the following Dihybrid cross
Step 1 - set up gamettes(sex cells)
(1 3, 1 4, 2 3, 2 4)
TtQq x TtQq
Complete the following Dihybrid cross
TtQq x TtQq
TQ
T-Tall
t-short
Q-Wrinkled
q-smooth
tq
Tq
tQ
TQ TTQQ
TTQq
TtQQ
TtQq
Tq
TTQq
TTqq
TtQq
Ttqq
tQ
TtQQ
TtQq
ttQQ
ttQq
tq
TtQq
Ttqq
ttQq
ttqq
What are the phenotypes for the
above cross???
• 9 - Tall & Wrinkled
• 3 - Tall & smooth
• 3 - Short & wrinkled
• 1 - Short & smooth
Incomplete Inheritance
• Two examples of Incomplete Inheritance:
– Incomplete Dominance & Codominance
• Incomplete Dominance:
– A case where one allele is partially dominant over the other
– Examples of Incomplete Dominance:
red snapdragons X white snapdragons  pink snapdragons
cross between black and white Andulusian
fowl gives blue (gray) fowl
Example of Incomplete Dominance
Pink Snapdragons
X

Example of Incomplete Dominance
Problem:
Cross offspring from 1st cross
(2 heterozygous parents)
• R = red
• r = white
• Rr x Rr
R
R
RR
r
Rr
r
Rr
rr
Results:
25 red, ___%
50 pink, 25
Phenotype: ___%
___% white
Genotype:
25 homozygous dominant
___%
___%
50 heterozygous
25 homozygous recessive
___%
Ratio
Ratio
1 : ___
2 : ___
1
___
1 : ___
2 : ___
1
___
Intermediate Inheritance
• Codominance: a case in which neither allele is
dominant over the other
– Alleles have equal power
• Examples:
– Cross between red and white short horned cattle
gives roan cattle
– Checkered black & white chicken
– Sickle-cell Anemia - a blood disease where RBCs are
sickle shaped or half moon. Most common African.
– Heterozygous - half normal half sickle shape
Roan Cattle
X
Red Cattle

Roan Cattle
White Cattle
Example of Codominance
Problem:
Cross offspring from 1st cross
(2 heterozygous parents)
• R = red
• r = white
• Rr x Rr
R
R
RR
r
Rr
r
Rr
rr
Results:
25 red, ___%
50 red & white, ___%
25 white
Phenotype: ___%
Genotype:
25 homozygous dominant
___%
___%
50 heterozygous
25 homozygous recessive
___%
Ratio
Ratio
1 : ___
2 : ___
1
___
1 : ___
2 : ___
1
___
Problem 1
Phenotype of tt ------------------------- Short
Genotype of tt--------------------------- Homozygous recessive
Phenotype of TT ----------------------- Tall
Genotype of TT------------------------- Homozygous dominant
Phenotype of pure dominant-------- Tall
Genotype of pure dominant---------
TT
Phenotype of pure recessive-------- Short
Genotype of pure recessive--------- tt
Problem 2:
A married couple want to know
their chances of having girl
X
Y
X
XX
XY
X
XX
XY
X __
Y x __
X __
X
__
Results:
Phenotype: 50% male,
Phenotype ratio:
50% female
2 male
: 2 female
Problem 3: Cross two
R
r
heterozygous parents
• R = red
• r = white
R
RR
Rr
r
Rr
rr
R __
r x __
R __
r
• __
Phenotype: 75
___% red, white ___%
25
Phenotypic Ratio:
Genotype:
3__red
:
1 ___
white
___%
50 heterozygous
___%
25 homozygous dominant
25 %
____
homozygous recessive
25
R
Problem 4: Pure dominant
R
crossed with hybrid
• R = red
• r = white
R
R
R
r
• __ __ x __ __
R
RR
RR
r
Rr
Rr
Results:
Phenotype: 100
___% red, white ___%
0
Phenotype Ratio:
Genotype:
4__red :
0_ _white
__
___%
50 heterozygous
___%
50 homozygous dominant
0% homozygous recessive
___
Problem 5: The male’s genotype
is homozygous recessive. The
female is phenotypically dominant
but does carry the recessive allele.
• R = red
• r = white
r __
r x __
R __
r
• __
r
R
Rr
Rr
r
rr
rr
Phenotype: ___% red, white ___%
50
Phenotypic Ratio: 2 red
__
Genotype:
r
50
white
: 2 ___
50 heterozygous
___%
0
___%
homozygous dominant
___ % homozygous recessive
50
Problem 6: Law of Codominance
Cross two heterozygous parents
R
• R = red
• r = white
R
RR
Rr
r
Rr
rr
R __
r x __
R __
r
• __
r
Results:
Phenotype: 25
___% red, ___
50 % red and white, white ___%
25
Phenotypic Ratio:
__
1 red : __
2 red and white : ___
1 white
Genotype: 50
___% heterozygous
___%
25 homozygous dominant
___
25 % homozygous recessive
Problem 7:
Law of Incomplete Dom
R
r
Cross two heterozygous parents
• R = red
• r = white
R __
r x __
R __
r
• __
R
RR
Rr
r
Rr
rr
Results:
Phenotype: 25
___% red, ___
50 % pink, white ___%
25
Phenotype Ratio:
1 red : __2pink : ___
1 white
__
Genotype: ___%
50 heterozygous
___%
25 homozygous dominant
25 % homozygous recessive
___
Test Cross:
It will determine if a horse/dog in question is pure or
carrying a recessive gene.
Problem 8: A test cross uses
an individual that is homozygous
recessive. It will determine if the
dog in question is pure or
carrying a recessive gene.
• B = brown
• b = white
B
B
b
Bb
Bb
b
Bb
Bb
B
b
b
Bb
bb
b
Bb
bb
B __
B x __
b __
b
__
or
B __
b x __
b __
b
__
Multiple alleles
• Traits that are controlled by more than 2 alleles
• Results in multiple phenotypes
• Examples:
– Pigeons
BA dominant over B
BA and B are dominant over b
– Blood groups in humans
Four blood types A B AB & O
X-linked Inheritance
Hemophilia, Color Blindness,
Loss of Hearing & Muscular Dystrophy
Examples:
X-linked Recessive Inheritance
XDXd
XDY
XD
XDXD
XDY
XDXd
XdY
XD XDXD
Y
XDY
Xd
XDXd
XdY
75
___% NORMAL HEARING of TOTAL OFFSPRINGS,
___ % HEARING LOSS of TOTAL OFFSPRINGS
25
___% NORMAL HEARING OF FEMALES
100
___ % HEARING LOSS OF MALES
50