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Alleles
• Dominant - expressed when paired with a
different allele
– Represented by an uppercase letter (RR) or (Rr)
• Recessive - no effect when paired with a
dominant allele
– Represented by a lowercase letter (rr)
GENOTYPE AND PHENOTYPE
• The genotype is the genetic constitution of
the individual, in other words, the genes (and
alternate forms) that are carried.
• Alternate forms of the same gene are called
alleles.
• The phenotype is the observable trait
(characteristic) produced by the genotype
(gene).
GENERATION DESIGNATIONS
• The parental generation (P1) is the first
generation of the controlled cross.
• The first filial generation (F1) is the result of
crossing the parental generation.
• The second filial generation (F2) is produced
from the crossing of the F1 progeny.
PUNNETT SQUARE
• Graphical means of visualizing a monohybrid
cross and applying probability to the
outcome.
• E.G. cross 2 heterozygous individuals [Yy]
Genotypic
Ratio=1:2:1
¼ YY
½ Yy
¼ yy
Y
y
Y
YY
y
Yy
yellow
yellow
Yy
yy
yellow
green
Phenotypic
Ratio=3:1
¾ yellow seeds
¼ green seeds
Alleles in an Individual
• Homozygote - same two alleles (AA or aa)
• Heterozygote - two different alleles (Aa)
Phenotype versus Genotype
Phenotype
Purple
3
Purple
Genotype
PP
(homozygous)
1
Pp
(heterozygous)
2
Pp
(heterozygous)
Purple
1
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
1
Monohybrid cross
yellow seeds X green seeds [parental generation]
YY
yy [P1]
yellow seeds [first filial generation]
Yy [F1]
¾ yellow seeds [F2]
¼ green seeds
[second filial generation]
Dihybrid Cross – two characters
EXPERIMENT Two true-breeding pea plants—
one with yellow-round seeds and the other with greenwrinkled seeds—were crossed, producing dihybrid F1 plants.
Self-pollination of the F1 dihybrids, which are heterozygous for
both characters, produced the F2 generation. The two
hypotheses predict different phenotypic ratios. Note that
yellow color (Y) and round shape (R) are dominant.
P Generation
YYRR
yyrr
Gametes
YR
F1 Generation
yr
YyRr
Hypothesis of
independent
assortment
Hypothesis of
dependent
assortment
Sperm
RESULTS
1⁄
Sperm
1⁄
CONCLUSION The results support the hypothesis of
independent assortment. The alleles for seed color and
seed shape sort into gametes independently of each other.
F2 Generation
(predicted
offspring)
2
2
1⁄
2
YYRR
yr
yr
YyRr
yyrr
YyRr
3⁄
4
1⁄
YR
4
Yr
1⁄
4
yR
1⁄
4
yr
Eggs
1⁄
Eggs
1⁄
2 YR
1⁄
YR
4
1⁄
4
YR
4
Yr
4
yR
4
yr
9⁄
16
1⁄
1⁄
YYRR
YYRr
YyRR
YyRr
YYrr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
4
Phenotypic ratio 3:1
1⁄
3⁄
16
3⁄
16
1⁄
16
Phenotypic ratio 9:3:3:1
315
108
101
32
Phenotypic ratio approximately
9:3:3:1
Dihybrid cross
DIHYBRID CROSS WITH GENOTYPES
• A cross involving two traits.
R=round
r=wrinkled
Y=yellow
y=green
round,yellow seeds X wrinkled, green
RRYY
rryy
All round, yellow [F1]
RrYy
MENDEL’S EXPERIMENT
THE DIHYBRID CROSS
• The dihybrid cross, a cross involving two traits.
round,yellow seeds X wrinkled, green
All round, yellow [F1]
Phenotypic
ratio=9:3:3:1
9/16 round, yellow
3/16 wrinkled, yellow
3/16 round, green
1/16 wrinkled, green
• Incomplete Dominance: One does not completely
cover the other. Halfway between two extremes, so
blending of one another.
-Classic example: A red and white flower is cross, so
you end up with a pink flower.
• Codominance: There are equally strong, so nothing
over powers the other. Equal in strength, you see
both phenotypes.
-Classic example: Cow having brown and white spots
(Roam).
INCOMPLETE DOMINANCE
• Characterized by an absence of complete
dominance in one allele.
• This manifests as a “blending” of traits, or a
“hybrid” phenotype.
• Common in flower color genes.
4 O’ CLOCKS
red flowers X white flowers
RR
X
rr
[ R1R1]
[R2R2]
Rr [R1R2] F1
pink
1:2:1 genotypic
1:2:1 phenotypic
When 2 heterozygotes
are crossed
¼ RR [R1R1] red
½ Rr [R1R2] pink
¼ rr [R2R2] white
IDENTIFYING CHARACTERISTICS OF
INCOMPLETE DOMINANCE
1.
2.
Traits are blended.
Crossing two heterozygous individuals in a monohybrid
cross produces a 1:2:1 genotypic ratio and 1:2:1
phenotypic ratio.
Incomplete dominance is an apparent exception to Mendel’s
First Law because a different phenotypic ratio is obtained.
The alleles are in fact segregating according to Mendel’s first
law, the mechanism by which the phenotype is produced is
different than in pea plants.
CODOMINANCE
• A codominant gene in a heterozygous
individual will express the phenotype of both
alleles. The phenotype of both alleles are
expressed independently.
• ABO blood groups in humans are an example.
The I gene (isoagglutinogen) has three alleles
(A, B, and O).
• The A and B alleles are dominant to O and
codominant to each other.
ABO GENOTYPES
PHENOTYPE GENOTYPE
ANTIGEN
ANTIBODY
A
IAIA or IAIO
A
ANTI-B
B
IBIB or IBIO
B
ANTI-A
O
IOIO
NONE
BOTH
AB
IAIB
A and B
NONE
Codominance
• Both alleles are expressed
• Seen in blood types
–
–
–
–
IAIA or IAi = type A
IBIB or IBi = type B
ii = type O
IAIB = type AB
The ABO blood group in
humans Is determined by
multiple alleles.
ABO Blood Groups
Recessively Inherited Disorders
• Many genetic disorders
– Are inherited in a recessive manner
• Recessively inherited disorders
– Show up only in individuals homozygous for the
allele
• Carriers
– Are heterozygous individuals who carry the
recessive allele but are phenotypically normal
Inheritance of Sex-Linked Genes
• The sex chromosomes
– Have genes for many characters unrelated to
sex
• A gene located on either sex chromosome
– Is called a sex-linked gene
Other sex-linked conditions
• Some recessive alleles found on the X chromosome in
humans cause certain types of disorders
– Color blindness
– Duchenne muscular dystrophy
– Hemophilia
Sex-linked genes follow specific patterns of inheritance
XAXA
XaY
(a) A father with the disorder will transmit the
mutant allele to all daughters but to no
sons. When the mother is a dominant
Xa Y
homozygote, the daughters will have the
normal phenotype but will be carriers of
Ova XA XAXa XAY
the mutation.
Sperm
XA XAYa XAY
XAXa 
(b) If a carrier mates with a male of
normal phenotype, there is a 50%
chance that each daughter will be a
carrier like her mother, and a 50%
chance that each son will have the
disorder.
XA
XAY
Y
Sperm
Ova XA XAXA XAY
Xa XaYA XaY
(c) If a carrier mates with a male who
has the disorder, there is a 50%
chance that each child born to them
will have the disorder, regardless of
sex. Daughters who do not have the
disorder will be carriers, where as
males without the disorder will be
completely free of the recessive
allele.
gure 15.10a–c
XAXa 
XaY
Sperm
Xa
Y
Ova XA XAXa XAY
Xa XaYa XaY
Questions
If an allele for tall plants (T) is dominant to
short plants (t), what offspring would you
expect from a TT x Tt cross?
A. ½ tall; ½ short
B. ¾ tall; ¼ short
C. All tall
Questions
If an allele for tall plants (T) is dominant to
short plants (t), what offspring would you
expect from a TT x Tt cross?
A. ½ tall; ½ short
B. ¾ tall; ¼ short
C. All tall
Questions
Fur color in rabbits shows incomplete dominance.
FBFB individuals are brown, FBFW individuals are
cream, FWFW individuals are white. What is the
expected ratio of a FBFW x FWFW cross?
A. 3 white: 1 brown
B. 3 white: 1 cream
C. 2 white: 2 cream
Questions
Fur color in rabbits shows incomplete dominance.
FBFB individuals are brown, FBFW individuals are
cream, FWFW individuals are white. What is the
expected ratio of a FBFW x FWFW cross?
A. 3 white: 1 brown
B. 3 white: 1 cream
C. 2 white: 2 cream
Questions - Monohybrid Cross
• Height in pea plants is determined by the
genes T (dominant) and t (recessive).
• Cross a homozygous tall pea plant with a
dwarf pea plant and determine the probability
of producing a tall plant.
Questions - Monohybrid Cross
• Height in pea plants is determined by the
genes T and t.
• Cross two heterozygous tall plants and
determine the probability of producing a
dwarf plant.
Questions
• Note that blood type genotypes may be written using
an "I" before the A and B, such as IAIA and IBi, etc. In this
problem I’m not using "I".
• Hazel has type B blood (genotype BO) and Elijah has
type O blood (genotype OO). If they have children,
what is the probability that they will have a type B
child? What is the probability they will have a type A
child?
• In this problem you are given the genotypes so you
know both genes for each blood type.
Questions - Dihybrid Cross
• When a genetic cross involves the consideration of two
factors (such as shape and colour in pea seeds), the cross is
called a "dihybrid".
• Cross a completely heterozygous round/yellow seeded
plant with a completely homozygous round/green seeded
plant.
• Then determine the probability of obtaining a round/yellow
seeded plant in the offspring.
• R = round seeds, r = wrinkled seeds
Y = yellow seeds, y = green seeds
Questions - Page 7 - Lab Book
Questions - Page 7 - Lab Book
Questions - Page 7 - Lab Book
Questions - Page 7 - Lab Book