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CHAPTER 14
MENDEL AND THE GENE IDEA
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I. OVERVIEW
 What genetic principles account for the passing of
traits from parents to offspring?
 The “blending” hypothesis is the idea that genetic
material from the two parents blends together (like blue
and yellow paint blend to make green) offspring
 The “particulate” hypothesis is the idea that parents
pass on discrete heritable units (genes)
 Mendel documented a particulate mechanism through
his experiments with garden peas which began in
1860’s
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I. INTRODUCTION
A. Blending Theory of Heredity
• Pre-Mendel
• Proposed that hereditary material from each
parent mixes or blends in the offspring
B. Particulate Theory of Heredity
• Gregor Mendel’s theory
• Parents transmit to their offspring inheritable
factors (genes)
• Began in 1860’s
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II. GREGOR MENDEL
A.
B.
C.
D.
Used quantitative approach
Austrian monk
Father of Genetics
Two reasons he used peas in
his experiments:
1. Many easily recognized
characteristics (traits)
2. Easy to cross-pollinate
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III. VOCABULARY
1. gene—segment of DNA
--responsible for production of 1 protein
--Mendel’s factor
2. locus—specific position of a gene on a chromosome
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3.
4.
5.
6.
7.
8.
9.
characteristic—inherited feature (Ex: flower color)
trait—variant of a characteristic (Ex: white or red flowers)
diploid—refers to cells with 2 sets of chromosomes (2n)
haploid—refers to cells with 1 set of chromosomes (n)
allele—alternative forms of a gene
homozygous—two alleles for a particular characteristic are
identical
--TT or tt
heterozygous—two alleles for a particular characteristics
are different
--Tt
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10. genotype—actual genetic makeup
--symbols
--Tt, TT, or tt
11. phenotype—effects of the genes
--description of appearance
--tall or short
12. dominant—refers to trait that always appear in offspring
of parents with contrasting traits
tall x short
|
tall
13. recessive—refers to trait that does not appear in offspring
of parents with contrasting traits
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14. monohybrid cross—cross in which only one character is
considered
TT x tt
|
Tt (monohybrid)
15. dihybrid cross—cross in which two characters are
considered
TTyy x ttYY
|
TtYy (dihybrid)
16. P—parental generation
--true breeding
17. F1—first filial generation
--offspring of P
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18. F2—second filial generation
--offspring of F1
19. intercross—cross in which both parents are
heterozygous for a characteristic
--1 Factor Intercross (1FIC)
•Tt x Tt
•Phenotypic ratio 3:1
•Genotypic ratio 1:2:1
--2 Factor Intercross (2FIC)
•TtYy x TtYy
•Phenotypic ratio 9:3:3:1
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20. backcross—cross with a parent or parent-type
--Tt x TT or tt
21. testcross—cross unknown genotype with a homozygous
recessive
--used to attempt to determine if a dominant trait is
homozygous or heterozygous
--T_ x tt
22. true breeding—offspring have same traits as parents when
parents self-fertilize
--pure breeding
--homozygous
23. hybrid—offspring from a cross of parents differing in one or
more characteristics
--heterozygous
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IV. MENDEL’S EXPERIMENTS
P
purple x white
F1 purple (self-pollinate)
F2
705 purple
224 white
Ratio 3.15 :1 ~ 3:1
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Mendel’s Characters
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A. Mendel’s Conclusions
1. For each character, an organism inherits two genes
(factors), one from each parent.
2. Alternative forms of genes are responsible for
variations in inherited characters.
 If the two alleles (factors) differ, one is fully expressed
(dominant allele); the other is completely masked
(recessive allele).
•Symbol for dominant allele—capital letter
•Symbol for recessive allele—lowercase letter
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3. Law of Segregation
• The two genes
(factors) for each
character segregate
during gamete
formation
• Occurs during
meiosis
Law of Segregation
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4. Mendel’s Law of Independent Assortment
• States: Each allele pair segregates independently of
other gene pairs during gamete formation.
• Developed from dihybrid crosses
• Refers to behavior of genes during gamete formation
• Refers to genes on different chromosomes.
• Ex: RrTt x RrTt
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Law of Independent Assortment
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B. Punnett Square
• Named for R. C.
Punnett
• Special chart used to
predict outcome of
genetic crosses
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C. Inheritance as a Game of Chance
1. Laws of Probability
• Probability of 1 means event certain to occur
• Probability of 0 means event certain not to occur
• Probability of all possible outcomes for an event add up
to 1
• Two basic rules of probability:
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a. Rule of Multiplication
• the probability that independent events will occur
simultaneously is the product of their individual
probabilities
• What is the probability that offspring will be pp if the
parents are Pp x Pp?
-First determine the probability that an egg will
receive a “p” allele (½)
-Then determine the probability that a sperm
will receive a “p” allele (½)
-Solution:
½x½=¼
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b. Rule of Addition
• The probability of an event that can occur in two or more
independent ways is the sum of the separate probabilities of the
different ways.
• In the cross Pp x Pp, what is the probability of the offspring being
heterozygous (Pp)?
-2 ways:
egg
P (½)
p (½)
-Solution
¼+¼=½
sperm
probability
p (½) = Pp ¼
P (½) = Pp ¼
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IV. Concept 14.3: Extending
Mendelian Genetics
 The relationship between genotype and phenotype is
rarely as simple as in the pea plant characters Mendel
studied
 Many heritable characters are not determined by only
one gene with two alleles
 However, the basic principles of segregation and
independent assortment apply even to more complex
patterns of inheritance
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A. Extending Mendelian Genetics for a Single Gene
1. Inheritance of characters by a single gene may
deviate
from simple Mendelian patterns in the
following situations:
•When alleles are not completely dominant or
recessive
•When a gene has more than two alleles
•When a gene produces multiple phenotypes
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V. EXTENDING MENDELIAN
GENETICS
Complete Dominance
•
The phenotypes of the heterozygote and the
dominant homozygote are indistinguishable.
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V. EXTENDING MENDELIAN
GENETICS
A. Incomplete Dominance
• Dominant phenotype is not fully expressed in
heterozygote, resulting in a third phenotype that is
intermediate between homozygous dominant and
homozygous recessive.
• Example:
P
red flowers x white flowers
F1
F2
pink flowers (self-pollinate)
¼ red
½ pink ¼ white
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Degrees of Dominance
• Complete Dominance- the
phenotypes of the heterozygote and
the dominant homozygote are
indistinguishable.
• Incomplete Dominance- neither allele
is completely dominant and the F1
hybrids have a phenotype
somewhere between those of the two
parental varieties.
Fig. 14-10-3
P Generation
Red
CRCR
White
CWCW
CR
Gametes
CW
Pink
CRCW
F1 Generation
Gametes 1/2 CR
1/
CW
2
Sperm
1/
2
CR
1/
2
CW
F2 Generation
1/
2
CR
Eggs
1/
2
CRCR
CRCW
CRCW
CWCW
CW
• Symbols:
R—red
RR x WW
W—white
RW x RW
¼ RR ½ RW ¼ WW
R
W
R
RR
RW
W
RW
WW
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B. What is a dominant allele?
Complete
Dominance
Incomplete
Dominance
(A is dominant)
(A is incompletely
dominant)
AA and Aa
Same
phenotype
Aa
Intermediate
phenotype
between
AA and aa
Codominance
(no dominance)
Aa
Both alleles
expressed
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The Relation Between Dominance and Phenotype
1. A dominant allele does not subdue a recessive allele;
alleles don’t interact
2. Alleles are simply variations in a gene’s nucleotide
sequence
3. For any character, dominance/recessiveness
relationships of alleles depend on the level at which we
examine the phenotype
• Tay-Sachs disease is fatal; a dysfunctional enzyme
causes an accumulation of lipids in the brain
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B. Codominance
• Defined as the full expression of both alleles in a heterozygote
• Ex: MN blood type
Blood Type
M
N
MN
Genotype
MM
NN
MN
• Tay Sachs
• At the organismal level, the allele is recessive
• At the biochemical level, the phenotype (i.e., the
enzyme activity level) is incompletely dominant
• At the molecular level, the alleles are codominant
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Frequency of Dominant Alleles
1. Dominant alleles are not necessarily more common
in populations than recessive alleles
2. For example, one baby out of 400 in the United
States is born with extra fingers or toes
3. The allele for this unusual trait is dominant to the
allele for the more common trait of five digits per
appendage
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D. Multiple Alleles
• More than two alternative forms (alleles) of a gene
• Ex: ABO blood group
• The four phenotypes of the ABO blood group (A,
B, AB, O) in humans are determined by three
alleles for the enzyme (I) that attaches A or B
carbohydrates to red blood cells: IA, IB, and i.
• 3 alleles produce 4 possible phenotypes—A, B, O, AB
• IA and IB are dominant to i (recessive).
• IA and IB are codominant to each other.
• Each person carries only 2 alleles.
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ABO Blood Group
Blood Type Genotypes Antigen on
rbc
Antibodies
In serum
A
IA IA
IA i
A
Anti-B
B
IB IB
IB i
B
Anti-A
AB
I A I BB
A and B
None
O
ii
None
Anti-A and
Anti-B
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Blood Problem
• Identification bracelets were accidentally removed
from three newborn babies. Blood typings were
taken to help in the identification procedures. The
blood types for the babies and their parents were:
Baby 1- type A, Baby 2- type O, Baby 3- type AB
• Mr. Black = type A Mr. Black = type B
• Mr. Green = type AB
Mrs. Green = type O
• Mr. White = type O Mrs. White = type O
• Which baby could belong to Mr. and Mrs. Black?
• Which baby could belong to Mr. and Mrs. Green?
• Which baby could belong to Mr. and Mrs. White?
E. Pleiotrophy
• Ability of a single gene to have multiple phenotypic
effects
• For example, pleiotropic alleles are responsible for
the multiple symptoms of certain hereditary
diseases
• Ex: sickle cell anemia, cystic fibrosis
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F. Epistasis
• Interaction between two nonallelic genes in which one
alters the phenotypic expression of the other (a gene at
one locus alters the phenotypic expression of a
gene at a second locus)
• Dihybrid cross results will deviate from expected ratio of
9:3:3:1
• One gene determines the pigment color (with alleles B
for black and b for brown)
• The other gene (with alleles C for color and c for no
color) determines whether the pigment will be
deposited in the hair
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F. Epistasis
• Ex: albinism (no pigmentation)
BB, Bb-black coat color
bb-brown coat color
CC, Cc-can make pigment (melanin)
cc-albino (can not make pigment)
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BbCc x BbCc
BC
Bc
bC
bc
BC
BBCC
BBCc
BbCC
BbCc
Bc
bC
BBCc
BbCC
BBcc
BbCc
BbCc
bbCC
Bbcc
bbCc
bc
BbCc
Bbcc
bbCc
bbcc
9 Black
B_ C_
3 Brown bb C_
4 Albino
__ cc
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•Quantitative characters are those that vary in the
population along a continuum
•Quantitative variation usually indicates polygenic
inheritance
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G. Polygenic Inheritance
• Additive effect of two or more genes determines a
single phenotypic character
• Ex: Skin pigmentation in humans is controlled by at
least 3 separately inherited genes.
--incomplete dominance
--AABBCC- very dark
--aabbcc-very light
--AaBbCc-intermediate shade
--phenotype can be affected by
environmental factors (sun exposure)
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H. Environmental Effects on Phenotype
• Coat color can be influenced by temperature.
-Ex: Siamese cats
• Curly wing mutant of Drosophila loses the ability to
fly at less than 16° C
• Color of flowers of Hydrangeas because of soil pH
I. Phenocopy
• Environmentally produced phenotype that simulates
the effects of a particular gene
-Ex: The drug thalidomide mimics the birth
defect phocomelia.
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Environmental Genetic
Interaction
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Environmental Effects on Phenotype (Nature vs. Nuture)
•Another departure from Mendelian genetics arises
when the phenotype for a character depends on
environment as well as genotype
•Coat color can be influenced by temperature.
-Ex: Siamese cats
•Curly wing mutant of Drosophila loses the ability to fly at
less than 16° C
•Color of flowers of Hydrangeas because of soil pH
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Siamese Cat--Coco
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J. Norm of Reaction
• Refers to range of phenotypic possibilities due
to environmental influences
• The norm of reaction is the phenotypic
range of a genotype influenced by the
environment
• Nature vs. Nurture (genetics vs. environment)
• Norms of reaction are broadest for polygenic
characters such as skin color which are
usually referred to as multifactorial (both
genetic and environmental factors influence
phenotype).
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• For example, hydrangea flowers of the same
genotype range from blue-violet to pink,
depending on soil acidity
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Environmental Genetic Interaction
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VI. MENDELIAN INHERITANCE IN
HUMANS
A. Difficult to study because:
• generation time
• number of offspring
• can not control breeding experiments
• different genetic backgrounds
B. Human Pedigrees (Family Tree)
• Shows inheritance pattern of a particular phenotypic
character
• Used to show Mendelian inheritance in humans
• Helps to understand the past and predict the future
• Important in genetic counseling for disabling or lethal
disorders
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a
n
Brackett
Grimmizon
Clint
Austin
Effie Elizabeth
Lindsey
Austin
101
83
Robert
Carn
Woosley
Callie Bee
McElw ain
Woosley
Lynn
Rickard
James
Potter
Thomas
Sally
Rickard
My Family Pedigree
Darrell
Austin
Harrell
Austin
Charles
Austin
Guy
Orther Gladys Aubrey
Woosley Woosley Woosley
Walt Charles Frank
Lee
Joe Grundy
Rickard Rickard Rickard Rickard Rickard Rickard
Carrico
8
72
72
76
Billy
John
Thomas
Charlie
Louis
Thomas
Grace Louise
Thomas
Totton
78
76
87
63
Martha Leona
Woosley
Austin
RD
Rickard
Marjorie Loraine
Thomas Rickard
Michael
Shane
Austin
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Bryan
Keith
Austin
6
Stephen Luke
Michael Andrew
Austin Austin
80
Jean Evon
Thomas
Coffman
Wayne
Leon
Austin
50
Andrea
Lynn Jones
Austin
25
Benjamin
Thomas
Austin
Jessica
Marie
Hale
Bryan
Adam
Austin
Kimberly
Sutherlin
Austin
23
20
Ashley Adam
Mischelle Hardin
Austin
Alyssa
Kendall
Austin
Michael
Moore
4
Hadley
Mischelle
Hardin
57
Roger
Wayne
Austin
54
Janet Louise Susan Kay Franklin
Rickard
Rickard Dale
Austin
Young Young
22
20
Kelly
Jan
Austin
Laura
Beth
Austin
Jackie Kaye Brandon
Knight Young Shane
Underw ood Young
12
Austin
Michael
Moore
Anna Loraine
Brackett
Thomas
8
Ely Gavin
Brandon Lee
Young Young
Patty
David
Rickard Rickard
Christina
Michelle
Rickard
Tonya
David
Andrew
Rickard
Pedigree of Queen Victoria
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56
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C. Autosomal Recessive Disorders
1. Recessively inherited disorders show up only in
individuals homozygous for the allele
2. Carriers are heterozygous individuals who carry the
recessive allele but are phenotypically normal (i.e.,
pigmented)
3. Albinism is a recessive condition characterized by a lack
of pigmentation in skin and hair
4. If a recessive allele that causes a disease is rare, then the
chance of two carriers meeting and mating is low
5. Consanguineous matings (i.e., matings between close
relatives) increase the chance of mating between two
carriers of the same rare allele
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Autosomal Recessive Disorders
• Usually involves a malfunctional protein or no protein
production at all
• Consanguinity
-genetic relationship resulting from matings between
closely related people
-often produces homozygous recessive offspring
-not always harmful
• Heterozygotes can be normal
• Examples:
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1. Cystic Fibrosis
• Most common lethal genetic disease in US
• Common in Caucasians
• Caused by lack of or defective membrane protein that pumps Cl- out of
cells
• striking one out of every 2,500 people of European descent
• Increased secretions of mucus from pancreas and lungs
• No cure
• Symptoms include mucus buildup in some internal organs and
abnormal absorption of nutrients in the small intestine
• Treat with diet and antibiotics
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2. Tay-Sachs
• Most common in individuals of Central European Jewish descent
• Enzyme does not function properly; can not metabolize gangliosides
(lipid)
• Lipids accumulate in brain
• Lethal gene
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3. Sickle-cell Anemia
• Most common in African-Americans
• Caused by single amino acid substitution in hemoglobin
(valine for glutamic acid)
• Causes abnormally shaped rbc
• Heterozygous individuals are said to have sickle-cell trait
• Heterozygous condition is malaria resistant
4. Albinism
• Decreased or lack of production of melanin
5. Glactosemia
• Lack an enzyme to break down galactose
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ALBINISM
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6. PKU
• Phenylketanuria
• Can not metabolize amino acid
phenylalanine
• Can result in mental retardation
7. Sickle- Cell Anemia
• Most common in African-Americans
• Caused by single amino acid substitution in
hemoglobin (valine for glutamic acid)
• Causes abnormally shaped rbc
• Heterozygous individuals are said to have sicklecell trait
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Autosomal Dominant Disorders
1. Achondroplasia
• Type of dwarfism
• Lethal in homozygous state
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Achondroplasia
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2. Huntington’s Disease
•Degeneration of nervous system
•Late-acting lethal dominant
•Onset about age 40
3. Manic-Depressive Disease
4. Polydactyly
•More than 5 digits on hand and/or feet
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E. Genetic Screening and Counseling
1. Carrier Recognition
•Tests are available to detect carriers of TaySachs, cystic fibrosis, and sickle-cell trait.
2. Fetal Testing
a. Amniocentesis
•involves removal of amniotic fluid
•can analyze fluid for chemical content
•culture cells for karyotyping
•done between 14-16 weeks
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Amniocentesis
Sampling
Chorionic Villus
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b. Chorionic Villi Sampling
• fetal tissue suctioned from chorionic villi of placenta
• results within 24 hours
• done at 8-10 weeks
c. Ultrasound
• sound waves used to study fetal structure
d. Fetoscopy
• involves insertion of fiber-optic scope into uterus
e. Newborn Screening
• PKU screening
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F. Multifactorial Disorders
• Disease that have both genetic and
environmental influences
• Ex: heart disease, diabetes, cancer, alcoholism,
some forms of mental illness
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You should now be able to:
1. Define the following terms: true breeding, hybridization,
monohybrid cross, P generation, F1 generation, F2
generation
2. Distinguish between the following pairs of terms:
dominant and recessive; heterozygous and homozygous;
genotype and phenotype
3. Use a Punnett square to predict the results of a cross and
to state the phenotypic and genotypic ratios of the F2
generation
4. Explain how phenotypic expression in the heterozygote
differs with complete dominance, incomplete dominance,
and codominance
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4. Define and give examples of pleiotropy and epistasis
5. Explain why lethal dominant genes are much rarer than
lethal recessive genes
6. Explain how carrier recognition, fetal testing, and
newborn screening can be used in genetic screening
and counseling
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Monohybrid Cross
• A brown dog is homozygous for the gene
that controls coat color. The brown dog is
mated with an albino (all white) dog. The
dogs have many puppies. All of the
puppies have brown coat color.
• Draw a punnett square for this cross and give
the expected genotypic and phenotypic
outcomes.
• What are the dominant and recessive alleles?
Provide symbols for both alleles.
• What would be the results if these offspring
mated with an albino dog? A homozygous
brown dog? A heterozygote?
Dihybrid Cross
• About 70% of Americans perceive a bitter taste from the
chemical phenylthiocarbamide (PTC). The ability to taste this
chemical results from a dominant allele (T) and not being able to
taste PTC is the result of having two recessive alleles (t).
Albinism is also a single locus trait with normal pigment being
dominant (A) and the lack of pigment being recessive (a). A
normally pigmented woman who is heterozygous for PTC
tasting, has a father who is homozygous for both albinism and
PTC tasting. She marries a heterozygous, normally pigmented
man who is a taster but who has a mother that does not taste
PTC
• Give the phenotypic and genotypic ratios of the offspring.
• A blue-eyed, left-handed woman marries a brown-eyed, right
handed man who is heterozygous for both traits. Blue eyes and
left-handedness are recessive.
• Give the phenotypic and genotypic ratios of the offspring.
Exit Slip
• If a man with type AB blood marries a
woman with type O blood, what
blood types would you expect in
their children. (Popcorn)
Homework:
• Read chapter 14, sections 3