Genetics


Gregor Mendel is
considered the father
of genetics
Published his work in
1866
© 2009 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 4/e
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Seven Characters Mendel Studied
Mendel’s Laws

First Law: Segregation


the two alleles of a trait separate from each other
during the formation of gametes, so that half of the
gametes will carry one copy and half will carry the
other copy
Mendel’s Second Law: Independent
Assortment

genes located on different chromosomes are
inherited independently of one another
Human Genetics

human somatic cell: 46
chromosomes
22 pairs same in both
males/females: autosomes
 1 pair different: sex
chromosomes: females

XX, males XY

Aneuploidy: errors during meiosis,
leads to individuals with
abnormal # chromosomes;






chromosome 21: Down
syndrome
chromosome 22: individuals with
delayed development and
mental impairment
XXX female, taller than average
XXY Klinefelter syndrome, male,
sterile, with female
characteristics and diminished
mental capacity
XO Turner syndrome, female,
sterile with diminished stature
XYY males, fertile and normal
appearance

human heredity: study with family trees or
pedigree: sex-linked/autosomal or
dominant/recessive
Essential Terms in
Genetics

Genotype—the genetic makeup
Phenotype—observable characteristics

Diploid: Two copies of each chromosome, such as somatic cells


Homologous pair, Paternal & Maternal

Haploid: Reproductive cells have only one copy of each chromosome

Genetic Cross: When two individuals are mated

P generation—parent generation
F1 generation—first generation
F2 generation—second generation


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
Alleles: Different versions of
a given gene
 Human
blood groups: A,B, AB,
and O


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Three alleles of one gene:
 IA: A type sugar
 IB: B type sugar
 i: neither A or B
Homozygote—same two
alleles (AA or aa)
Heterozygote—two different
alleles (Aa)
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
dominant traits are capitalized, lower-case
letter is for recessive trait: flower color in
peas

P signifies purple, p signifies white


Gametes: 2 from each father/mother: P or p
Punnett square: shows the possible gametes
and genotypes of potential offspring from a
cross
Huntington’s disease:
progressive
deterioration of
brain cells, affected
individuals only
need 1 copy
(dominant)
 Expression
30’s
in the
Sickle-cell anemia: affected individuals (homozygous
recessive) with defective hemoglobin that produces a
sickle-shape red blood cell which cannot move through
the blood vessels and tends to clot

Heterozygous individuals resistance to malaria infection
Hemophilia: sex-linked (mostly on the X
chromosome) trait, creates a difficulty of
the body to stop any bleeding
Steps to answer properly a genetic problem:
Assign letters to each allele, dominant and
recessive, with the corresponding phenotype
2. Determine the phenotype and genotype of the
parents
3. Write down the possible gametes produced by
the parents,
4. write down your cross (mating), and split the
letters of the genotype for each parent and put
them outside a p-square
5. Determine the genotype and phenotype of the
offspring and estimate their probabilities
6. Enjoy your accomplishment
1.
Extensions of Mendel’s Laws

Many alleles do not show complete
dominance
 Incomplete
dominance
 Codominance
 Pleiotropy
 Epistasis
 Environmental effects
 Polygenic traits
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1. Polygenic Traits
• Trait is determined by
two or more genes
• A continuous
distribution
– Skin color
– Height
© 2009 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 4/e
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2. Pleiotropy
• A single gene influences a
variety of traits:
– Albinism— Absence of melanin
• Skin color
• Vision problems
– characteristic of inherited
disorders…..cystic fibrosis,
sickle-cell anemia
© 2009 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 4/e
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3. Epistasis
• Gene interaction affects the phenotype
• such that one gene contributes to or masks
the expression of the other gene
• Alleles:
– BB or Bb = black
– bb = brown
– cc = white
© 2009 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 4/e
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4. Environmental Effects
• Internal and external
conditions influence
phenotype
– Temperature
– Chemicals
– Nutrition
• arctic foxes: only produce
fur pigment when
temperatures are warm
© 2009 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 4/e
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5. Incomplete Dominance
• Heterozygote
is an intermediate
• Horses
• Snapdragons
© 2009 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 4/e
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6. 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
© 2009 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 4/e
23
– gene ABO blood type in humans: affects the
membrane of red blood cells which act as
recognition markers for cells in the immune
system
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PRACTICE #1
In peas the allele for purple flower (P) is
dominant to the allele for white flower (p).
What gametes will be produced by a plant
that has white flowers?
PRACTICE #2
What kinds of gametes can this genotype
produce? AaBB
PRACTICE #3
Suppose that flower color is inherited by
simple dominance and that purple flowers
are dominant to white flowers.
If a homozygous recessive individual is
crossed with a homozygous dominant
individual,
what is the probability of obtaining a purple
flowered offspring?
Show all 5 steps for full credit.
PRACTICE #4
Pea plants are tall if they have the genotype
TT or Tt, and the are short id they have
genotype tt.
A tall plant is mated with a short plant.
Half the offspring are tall and half short.
What do you know about the tall plant?
Show all 5 steps for full credit
PRACTICE #5
Assume tall (T) is dominant to dwarf (t).
If a homozygous dominant individual is
crossed with a dwarf,
What will the offspring look like?
Show all 5 steps for full credit.
PRACTICE #6
Flower color in snapdragons is an example
of incomplete dominance.
If a pink-flowered plant is crossed with a
pink-flowered plant,
What will the progeny of plants look like?
Show all 5 steps for full credit.
PRACTICE #7
Color in snapdragons is inherited by
incomplete dominance.
Suppose you cross a red (R)flowered
snapdragon with a white flowered
snapdragon (r).
ALL of the offspring have pink flowers.
What can you conclude about the genotypes
of the parents and offspring?
Show all 5 steps for full credit.
PRACTICE #8
Wanda has type A blood, and her husband
has type B blood.
Is it possible for this couple to have a child
with type O blood?
Explain your answer, and show all 5 steps
for full credit.
PRACTICE #9
A man who has type B blood and a woman
who has type A blood have a child with
type O blood,
How is this possible?
Explain your answer, and show all 5 steps
for full credit.
PRACTICE #10
Hemophilia is a sex linked trait.
If a woman is a carrier of hemophilia and
she marries a man who does not have
hemophilia,
what is the probability that her sons inherit
the disease?
Show all 5 steps for full credit.
Practice questions
1. Gregor Mendel studied the garden pea plants because:
A) pea plants are small, easy to grow, grow quickly, and produce lots of flowers and
seeds.
B) he knew about studies with the garden pea that had been done for hundreds of
years, and wanted to continue them, using math - counting and recording
differences.
C) he knew that there were many varieties available with distinctive characteristics.
D) all of the above.
2. Human height shows a continuous variation from the very short to the very
tall. Height is most likely controlled by:
A)
epistatic genes. B) environmental factors. C) sex-linked genes. D) multiple
genes.
3. In the human ABO blood grouping, the four basic blood types are type A,
type B, type AB, and type O. The blood proteins A and B are
_________________
4. Nondisjunction:
A) occurs when homologous chromosomes or sister
chromatids fail to separate during meiosis.
B) may lead to Down syndrome.
C) results in aneuploidy.
D) All of the above.
5. A red carnation and a white carnation
produce offspring that are all pink. The type
of inheritance pattern occurring is:
A. Complete dominance
B. Incomplete dominance
C. Codominance
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6. 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
7. 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
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