Transcript h b 5555

Mendelelian
Genetics
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Gregor Mendel
(1822-1884)
Responsible
for the Laws
governing
Inheritance of
Traits
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Gregor Johann Mendel
Austrian monk
Studied the
inheritance of
traits in pea plants
Developed the laws
of inheritance
Mendel's work was
not recognized until
the turn of the
20th century
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Gregor Johann Mendel
Between 1856 and
1863, Mendel
cultivated and
tested some 28,000
pea plants
He found that the
plants' offspring
retained traits of
the parents
Called the “Father
of Genetics"
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Site of
Gregor
Mendel’s
experimental
garden in the
Czech
Republic
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Particulate Inheritance
Mendel stated that
physical traits are
inherited as “particles”
Mendel did not know
that the “particles”
were actually
Chromosomes & DNA
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Genetic Terminology
 Trait - any characteristic that
can be passed from parent to
offspring
 Heredity - passing of traits
from parent to offspring
 Genetics - study of heredity
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Types of Genetic Crosses
 Monohybrid cross - cross
involving a single trait
e.g. flower color
 Dihybrid cross - cross involving
two traits
e.g. flower color & plant height
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Punnett Square
Used to help
solve genetics
problems
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Designer “Genes”
 Alleles - two forms of a gene
(dominant & recessive)
 Dominant - stronger of two genes
expressed in the hybrid;
represented by a capital letter (R)
 Recessive - gene that shows up less
often in a cross; represented by a
lowercase letter (r)
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More Terminology
 Genotype - gene combination
for a trait (e.g. RR, Rr, rr)
 Phenotype - the physical
feature resulting from a
genotype (e.g. red, white)
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Genotype & Phenotype in Flowers
Genotype of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2
alleles affect a characteristic
Possible combinations are:
Genotypes
RR
Rr
rr
Phenotypes
RED
RED
YELLOW
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Genotypes
 Homozygous genotype - gene
combination involving 2 dominant
or 2 recessive genes (e.g. RR or
rr); also called pure
 Heterozygous genotype - gene
combination of one dominant &
one recessive allele
(e.g. Rr);
also called hybrid
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Genes and Environment
Determine Characteristics
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Mendel’s Pea Plant
Experiments
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Why peas, Pisum sativum?
Can be grown in a
small area
Produce lots of
offspring
Produce pure plants
when allowed to
self-pollinate
several generations
Can be artificially
cross-pollinated
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Reproduction in Flowering Plants
Pollen contains sperm
Produced by the
stamen
Ovary contains eggs
Found inside the
flower
Pollen carries sperm to the
eggs for fertilization
Self-fertilization can
occur in the same flower
Cross-fertilization can
occur between flowers
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Mendel’s Experimental
Methods
Mendel hand-pollinated
flowers using a paintbrush
He could snip the
stamens to prevent
self-pollination
Covered each flower
with a cloth bag
He traced traits through
the several generations
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How Mendel Began
Mendel
produced
pure
strains by
allowing the
plants to
selfpollinate
for several
generations
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Eight Pea Plant Traits
Seed shape --- Round (R) or Wrinkled (r)
Seed Color ---- Yellow (Y) or Green (y)
Pod Shape --- Smooth (S) or wrinkled (s)
Pod Color --- Green (G) or Yellow (g)
Seed Coat Color ---Gray (G) or White (g)
Flower position---Axial (A) or Terminal (a)
Plant Height --- Tall (T) or Short (t)
Flower color --- Purple (P) or white (p)
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Mendel’s Experimental Results
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Did the observed ratio match
the theoretical ratio?
The theoretical or expected ratio of
plants producing round or wrinkled seeds
is 3 round :1 wrinkled
Mendel’s observed ratio was 2.96:1
The discrepancy is due to statistical
error
The larger the sample the more nearly
the results approximate to the
theoretical ratio
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Generation “Gap”
Parental P1 Generation = the parental
generation in a breeding experiment.
F1 generation = the first-generation
offspring in a breeding experiment. (1st
filial generation)
From breeding individuals from the P1
generation
F2 generation = the second-generation
offspring in a breeding experiment.
(2nd filial generation)
From breeding individuals from the F1
generation
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Following the Generations
Cross 2
Pure
Plants
TT x tt
Results
in all
Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT, Tt, tt
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Monohybrid
Crosses
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P1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Wrinkled seeds
RR
x
rr
r
r
R
Rr
Rr
R
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Ratio: All alike
Phenotypic
Ratio: All alike
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P1 Monohybrid Cross Review
 Homozygous dominant x Homozygous
recessive
 Offspring all Heterozygous
(hybrids)
 Offspring called F1 generation
 Genotypic & Phenotypic ratio is ALL
ALIKE
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F1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Round seeds
Rr
x
Rr
R
r
R
RR
Rr
r
Rr
rr
Genotype: RR, Rr, rr
Phenotype: Round &
wrinkled
G.Ratio: 1:2:1
P.Ratio: 3:1
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F1 Monohybrid Cross Review
 Heterozygous x heterozygous
 Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
 Offspring called F2 generation
 Genotypic ratio is 1:2:1
 Phenotypic Ratio is 3:1
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What Do the Peas Look Like?
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…And Now the Test Cross
Mendel then crossed a pure & a
hybrid from his F2 generation
This is known as an F2 or test
cross
There are two possible
testcrosses:
Homozygous dominant x Hybrid
Homozygous recessive x Hybrid
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F2 Monohybrid Cross
st
(1 )
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Round seeds
x Round seeds
RR
x
Rr
R
r
R
RR
Rr
R
RR
Rr
Genotype: RR, Rr
Phenotype: Round
Genotypic
Ratio: 1:1
Phenotypic
Ratio: All alike
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F2 Monohybrid Cross (2nd)
Trait: Seed Shape
Alleles: R – Round
r – Wrinkled
Cross: Wrinkled seeds x Round seeds
rr
x
Rr
R
r
r
Rr
Rr
r
rr
rr
Genotype: Rr, rr
Phenotype: Round &
Wrinkled
G. Ratio: 1:1
P.Ratio: 1:1
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F2 Monohybrid Cross Review
 Homozygous x heterozygous(hybrid)
 Offspring:
50% Homozygous RR or rr
50% Heterozygous Rr
 Phenotypic Ratio is 1:1
 Called Test Cross because the
offspring have SAME genotype as
parents
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Practice Your Crosses
Work the P1, F1, and both
F2 Crosses for each of the
other Seven Pea Plant
Traits
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Mendel’s Laws
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Results of Monohybrid Crosses
Inheritable factors or genes are
responsible for all heritable
characteristics
Phenotype is based on Genotype
Each trait is based on two genes,
one from the mother and the
other from the father
True-breeding individuals are
homozygous ( both alleles) are the
same
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Law of Dominance
In a cross of parents that are
pure for contrasting traits, only
one form of the trait will appear in
the next generation.
All the offspring will be
heterozygous and express only the
dominant trait.
RR x rr yields all Rr (round seeds)
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Law of Dominance
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Law of Segregation
During the formation of gametes
(eggs or sperm), the two alleles
responsible for a trait separate
from each other.
Alleles for a trait are then
"recombined" at fertilization,
producing the genotype for the
traits of the offspring.
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Applying the Law of Segregation
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Law of Independent
Assortment
Alleles for different traits are
distributed to sex cells (&
offspring) independently of one
another.
This law can be illustrated using
dihybrid crosses.
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Dihybrid Cross
A breeding experiment that tracks
the inheritance of two traits.
Mendel’s “Law of Independent
Assortment”
a. Each pair of alleles segregates
independently during gamete formation
b. Formula: 2n (n = # of heterozygotes)
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Question:
How many gametes will be produced
for the following allele arrangements?
Remember: 2n (n = # of heterozygotes)
(count the number of traits that have
both the dominant and recessive.)
1. RrYy
2. AaBbCCDd
3. MmNnOoPPQQRrssTtQq
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Answer:
1. RrYy: 2n = 22 = 4 gametes
RY
Ry
rY ry
2. AaBbCCDd: 2n = 23 = 8 gametes
ABCD ABCd AbCD AbCd
aBCD aBCd abCD abCD
3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64
gametes
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Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round
r wrinkled
Y yellow
y green
RrYy
x
RrYy
RY Ry rY ry
RY Ry rY ry
All possible gamete combinations
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Dihybrid Cross
RY
Ry
rY
ry
RY
Ry
rY
ry
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Dihybrid Cross
RY
RY RRYY
Ry RRYy
rY RrYY
ry
RrYy
Ry
rY
ry
RRYy
RrYY
RrYy
RRyy
RrYy
Rryy
RrYy
rrYY
rrYy
Rryy
rrYy
rryy
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Round/Yellow:
Round/green:
9
3
wrinkled/Yellow: 3
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
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Dihybrid Cross
Round/Yellow: 9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
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Test Cross
A mating between an individual of unknown
genotype and a homozygous recessive
individual.
Example: bbC__ x bbcc
BB = brown eyes
Bb = brown eyes
bb = blue eyes
CC = curly hair
Cc = curly hair
cc = straight hair
bC
b___
bc
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Test Cross
Possible results:
bc
bC
b___
C
bbCc
bbCc
or
bc
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bC
b___
c
bbCc
bbcc
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Summary of Mendel’s laws
LAW
DOMINANCE
SEGREGATION
INDEPENDENT
ASSORTMENT
PARENT
CROSS
OFFSPRING
TT x tt
tall x short
100% Tt
tall
Tt x Tt
tall x tall
75% tall
25% short
RrGg x RrGg
round & green
x
round & green
9/16 round seeds & green
pods
3/16 round seeds & yellow
pods
3/16 wrinkled seeds & green
pods
1/16 wrinkled seeds & yellow
pods
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Incomplete Dominance
and
Codominance
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Incomplete Dominance
F1 hybrids have an appearance somewhat
in between the phenotypes of the two
parental varieties.
Example: snapdragons (flower)
red (RR) x white (rr)
r
r
RR = red flower
rr = white flower
R
R
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Incomplete Dominance
r
r
R Rr
Rr
R Rr
Rr
produces the
F1 generation
All Rr = pink
(heterozygous pink)
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Incomplete Dominance
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Incomplete dominance in carnations
Codominance
Two alleles are expressed (multiple
alleles) in heterozygous individuals.
Example: blood type
1.
2.
3.
4.
type
type
type
type
A
B
AB
O
=
=
=
=
IAIA or IAi
IBIB or IBi
IAIB
ii
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Codominance Problem
Example: homozygous male Type B (IBIB)
x
heterozygous female Type A (IAi)
IA
i
IB
IAIB
IBi
IB
IAIB
IBi
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1/2 = IAIB
1/2 = IBi
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Another Codominance Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IAi
IBi
i
IAi
IBi
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1/2 = IAi
1/2 = IBi
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ABO blood types
Multiple alleles for the ABO blood groups
Codominance
Question:
If a boy has a blood type O and
his sister has blood type
AB,
what are the genotypes
and
phenotypes of their
parents?
boy - type O (ii)
AB (IAIB)
X
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girl - type
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Codominance
Answer:
IA
IB
i
i
IAIB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
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More exceptions to the dominant/recessive
rule
Pleiotropy:
One genes having many effects. Only one gene
affects an organism in many ways.
Ex: sickle cell anemia and cystic fibrosis
Pleiotropy
Alleles at a single locus may have effects on two or
more traits
Classic example is the effects of the mutant allele
at the beta-globin locus that gives rise to sicklecell anemia
A single gene may affect many phenotypic
characteristics
In pleiotropy
A single gene may affect phenotype in many
ways
Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickle cells
Clumping of cells
and clogging of
small blood vessels
Breakdown of
red blood cells
Physical
weakness
Anemia
Impaired
mental
function
Figure 9.14
Heart
failure
Paralysis
Pain and
fever
Pneumonia
and other
infections
Accumulation of
sickled cells in spleen
Brain
damage
Damage to
other organs
Rheumatism
Spleen
damage
Kidney
failure
Genetics of Sickle-Cell Anemia
Two alleles
1) HbA
Encodes normal beta hemoglobin chain
2) HbS
Mutant allele encodes defective chain
HbS homozygotes produce only the defective hemoglobin;
suffer from sickle-cell anemia
Epistasis:
Interaction between the products of gene
pairs
Interaction between two genes in which one
of the genes modifies the expression of
the other.
Ex: fur /hair color in mammals and albinism
Albinism
Phenotype results when pathway for melanin
production is completely blocked
Genotype - Homozygous recessive at the
gene locus that codes for tyrosinase, an
enzyme in the melanin-synthesizing
pathway
Genetics of Coat Color in Labrador
Retrievers
Two genes involved
- One gene influences melanin production
Two alleles - B (black) is dominant over
b (brown)
- Other gene influences melanin deposition
Two alleles - E promotes pigment
deposition and is dominant over e
Allele Combinations
and Coat Color
Black coat - Must have at least one dominant allele
at both loci
BBEE, BbEe, BBEe, or BbEE
Brown coat - bbEE, bbEe
Yellow coat - Bbee, BbEE, BBee, bbee
An example of epistasis
More modifications to Mendel’s rule
Polygenic Inheritance:
In this case many genes have an additive effect.
The characteristic or trait is the result of the
combined effect of several genes. Ex: human
skin color, height. Controlled by more than one
pair of genes
Continuous Variation
Polygenic inheritance results in a continuous
range of small differences in a given trait
among individuals
The greater the number of genes that affect
a trait, the more continuous the variation
in versions of that trait
A simplified model for polygenic inheritance of skin color
Environmental effects:
The degree to which an allele is expressed
depends on the environment
Ex: Siamese cat fur color ( enzyme for
melanin production inhibited by heat),
hydrangea flowers ( depends on acidity of
soil), height (nutrition)
Temperature Effects
on Phenotype
Himalayan rabbits are
Homozygous for an allele that
specifies a heat-sensitive
version of an enzyme in
melanin-producing pathway
Melanin is produced in cooler
areas of body
Environmental Effects on Plant
Phenotype
Hydrangea macrophylla
Action of gene responsible for floral color is
influenced by soil acidity
Flower color ranges from pink to blue
The effect of environment of phenotype
Web sites to check
http://gslc.genetics.utah.edu/units/basics/tour/inhe
ritance.swf
http://science.nhmccd.edu/biol/genetics.html
http://library.thinkquest.org/20465/games.html
Thomas Hunt Morgan (1910) and Sex Linked
Inheritance
Morgan’s Experimental Evidence: Scientific
Inquiry
The first solid evidence associating a specific
gene with a a specific chromosome came from
Thomas Hunt Morgan
Morgan’s experiments with fruit flies (Columbia
University, 1910) provided convincing evidence
that chromosomes are the location of Mendel’s
heritable factors. He provided confirmation of
the correctness of the chromosomal theory of
inheritance.
Morgan’s experiments
Demonstrated the role
of crossing over in
inheritance
Experiment
Black body,
vestigial
wings
Gray body,
long wings
(wild type)

GgLI
ggll
Male
Female
Offspring
Gray long
Black vestigial
Gray vestigial
Black long
965
944
206
185
Parental
phenotypes
Recombinant
phenotypes
391 recombinants
Recombination frequency =
Explanation
GgLI
(female)
G L
2,300 total offspring
G L
g l
g l
g l
g l
Gl
gL
Eggs
G L
g l
ggll
(male)
g l
Sperm
g l
g l
Offspring
Figure 9.20 C
= 0.17 or 17%
G l
g l
g L
g l
Thomas Hunt Morgan
Performed some of the early studies of
crossing over using the fruit fly
Drosophila melanogaster
Figure 9.20 B
Sex-linked Traits
Traits (genes) located on the sex
chromosomes
Sex chromosomes are X and Y
XX genotype for females
XY genotype for males
Many sex-linked traits carried on
X chromosome
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Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
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In Drosophila
White eye color is a sex-linked trait
Figure 9.23 A
Sex-linked Trait Problem
Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female)
XRY
x
XrXr
Remember: the Y chromosome in males
does not carry traits.
Xr
Xr
RR = red eyed
Rr = red eyed
R
X
rr = white eyed
XY = male
Y
XX = female
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Sex-linked Trait Solution:
Xr
XR
XR
Xr
Y
Xr Y
Xr
XR
Xr
Xr Y
50% red eyed
female
50% white eyed
male
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SEX LINKED TRAITS ARE THOSE
CARRIED BY THE X CHROMOSOME
Red-Green color blindness
Inability to see those colors. Red and green
look all the same ,like gray
Hemophilia Blood clotting disorder.
The clotting factor VIII is not made,
individual can bleed to death.
Muscular dystrophy
X linked recessive, gradual and progressive
destruction of skeletal muscles .
Faulty teeth enamel
Extremely rare, X linked Dominant
Female Carriers
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• Types of genetic screening
• Test for carriers (parents)
• Newborn Screening
– Some genetic disorders can be detected at birth
• By simple tests that are now routinely performed in most
hospitals in the United States
• Antenatal options: fetal blood cell test from maternal
blood
• Chorionic Villus Sampling (CVS)
• Amniocentesis
• Fetal Testing
– Amniocentesis and chorionic villus sampling (CVS)
• Allow doctors to remove fetal cells that can be tested for genetic
abnormalities
Chorionic villus sampling (CVS)
Amniocentesis
Needle inserted
through abdomen to
extract amniotic fluid
Ultrasound
monitor
Ultrasound
monitor
Fetus
Suction tube inserted
through cervix to extract
tissue from chorionic villi
Fetus
Placenta
Placenta
Uterus
Chorionic
villi
Cervix
Cervix
Uterus
Amniotic
fluid
Centrifugation
Fetal
cells
Fetal
cells
Several
weeks
Figure 9.10 A
Table 9.9
Biochemical
tests
Several
hours
Karyotyping
DISORDERS RESULTING FROM AUTOSOMAL
RECESSIVE INHERITANCE
These are conditions in which the gene that is
defective is recessive.
It is only expressed when the child receives both
recessive genes for the disorder (one from
each parent)
If a person is heterozygous he/she will be a
CARRIER but not have the disorder. The
dominant allele will mask the expression of the
abnormal condition.
EXAMPLES:
ALBINISM: SICKLE CELL ANEMIA: CYSTIC
FIBROSIS: TAY- SACHS DISEASE;
PHENYLKETONURIA; GALACTOSEMIA:
DISORDERS RESULTING FROM RECESSIVE
INHERITANCE
ALBINISM: No pigmentation in skin This is also
an example of “EPISTASIS”(one pair of genes
modifies the expression of another)
SICKLE CELL ANEMIA: This is also an example
of “PLEIOTROPY”
Red blood cells curved shape. Decreased oxygen
to brain and muscles (offers resistance to
Malaria)
DISORDERS RESULTING FROM RECESSIVE
INHERITANCE
CYSTIC FIBROSIS: Excessive mucus secretions.Impaired
lung function, lung infections. Protein channel that
transport chloride across cell membrane does not
function. Protects against cholera.
This is also an example of “PLEIOTROPY”
TAY –SACHS DISEASE: Nervous system degeneration in
infants. Enzyme fails to breakdown lipids which
accumulate in nerve cells and kills the cells. Progressive
degeneration starting with the brain cells.
DISORDERS RESULTING FROM RECESSIVE
INHERITANCE
GALACTOSEMIA: Produces brain, liver, eye
damage. Enzyme that breaks down lactose
is lacking. It accumulates to toxic levels.
Death in infancy
PHENYLKETONURIA: Results in mental
retardation
Disorders resulting from Autosomal
Dominant Inheritance
Dominant traits appear in each generation
since the allele shows in the heterozygous
individual.
Dominant Disorders
Some human genetic disorders are
dominant
Figure 9.9 B
Disorders resulting from Dominant
Inheritance
Acondroplasia or dwarfism:
A condition where the bone does not grow
properly and can’t make proper cartilage. Person
is less than 4 feet with short arms and legs but
a regular size trunk.
Cholesterolemia:
High cholesterol levels in the blood causing
arteries to clog and high incidence of early
heart attacks.
Marfan Syndrome:
Abnormal connective tissue
Disorders resulting from Autosomal
Dominant Inheritance
Huntington’s Disorder:
Progressive degeneration of nervous system and
muscle control. Affects motor and mental abilities
and it is irreversible. Late onset, usually late 30’s.
Usually the person already had children.
Progeria:
Premature accelerated aging. Usually dead by 18.
Genes that bring about growth and development
are abnormal.
Polydactily:
Extra toes and fingers
Karyotype
A karyotype is a visual display of an individual’s
chromosomes. A man made picture of a person’s 23
pairs of chromosomes. ( the photo is taken during
metaphase when the sister chromatids are lined up
together)
It is useful in sex determination and diagnosis of
certain conditions.
CHROMOSOMES CHANGES
Chromosome changes can cause a lot of genetic
disorders as well as a lot of variety
WHEN AND HOW CAN A CHROMOSOME
CHANGE?
Mistakes in replication. During the S phase of the
cell cycle segments of a chromosome could be
deleted, duplicated, inverted or moved to a new
location. Also during Metaphase I (meiosis) there
can be improper separation after duplication.
This can change the total number of
chromosomes in each gamete of the new
individual.
If during meiosis the paired chromatids fail
to separate correctly this is called NONDISJUNCTION
ANEUPLOIDY means an abnormal number of
chromosomes.
When an individual ends up with the wrong
number of chromosomes most of the time
it is miscarried ( spontaneous abortion).
The wrong number of somatic chromosomes
are almost always lethal. Ex: trisomy
21(three chrom. 21): Down Syndrome
You can live with the wrong number of sex
pair chromosomes.
CHROMOSOMES
X
Turner syndrome One X instead of a pair.
This happens because of non disjuction of
sperm. Most are aborted spontaneously. If they
live, she is very short, infertily and with reduced
sex characteristics.
XXY Klinefelter syndrome One in 500 live male
births. Taller than average, infertile, some low
intelligence, some normal. Testosterone
injections help.
XYY “super male” about 1 in 1000. taller, mildly
retarded but normal phenotype.
Genetic Practice
Problems
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Breed the P1 generation
tall (TT) x dwarf (tt) pea plants
t
t
T
T
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Solution:
tall (TT) vs. dwarf (tt) pea plants
t
t
T
Tt
Tt
produces the
F1 generation
T
Tt
Tt
All Tt = tall
(heterozygous tall)
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112
Breed the F1 generation
tall (Tt) vs. tall (Tt) pea plants
T
t
T
t
copyright cmassengale
113
Solution:
tall (Tt) x tall (Tt) pea plants
T
t
T
TT
Tt
t
Tt
tt
produces the
F2 generation
1/4 (25%) = TT
1/2 (50%) = Tt
1/4 (25%) = tt
1:2:1 genotype
3:1 phenotype
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copyright cmassengale
115