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Introduction to Genetics
Ch 9
“Animalcules”- tiny people in
sperm?
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Mendel’s Laws of Heredity
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Hereditary factors do not combine, but are passed intact
(Law of Segregation)
Each member of the parental generation transmits only
half of its hereditary factors to each offspring (with
certain factors "dominant" over others) (Law of
Dominance)
Different offspring of the same parents receive different
sets of hereditary factors (Law of Independent
Assortment)
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Laws of Inheritance
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Gregor Mendel (1823-1884)
Augustinian monk
Mendel was the first person to trace the
characteristics of successive generations of
a living thing (pea plants).
So What REALLY Happens?
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The number of
chromosomes divide
to make gamete
cells.
When does this
occur?
Before sexual
reproduction
Sex Chromosomes
Fertilization
Law of Segregation
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Two members of a gene pair segregate
(separate) from each other during the
formation of gametes.
What is a gene pair?
When, in meiosis does segregation occur?
Anaphase I
Law of Independent Assortment
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Alleles for different traits assort (segregate)
independently of one another.
Genes on different chromosomes behave
independently in gamete production.
Why does this law not apply to a one trait
cross?
When, in meiosis does this occur?
Law of Independent Assortment:
The Work of Gregor Mendel
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A. The branch of biology that studies heredity is
called genetics.
B. Gregor Mendel is considered the father of
genetics who studied in a monastery.
C. He planted hundreds of pea plants to study
their traits.
D. He self-pollinated plants:
1. He brushed the pollen of one plant onto
itself
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2. These parents inherited all the
characteristics of the single plant that bore
them.
E. He also cross pollinated plants.
1. He brushed the pollen of one plant
onto another.
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2. He discovered that purebred tall
plants would only produce tall offspring.
F. He studied many traits.
1. Other characteristics of the pea
plants are:
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Round peas
Yellow peas
Gray peas
Smooth pods
Green pods
Axial
Tall
or
or
or
or
or
or
or
Wrinkled
Green
White
Constricted
Yellow
Terminal
Short
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I. Dominance vs. recessive traits:
1. Dominant traits show through with one
parent’s dominant trait: Ex. Tt = Tall
2. This is known as a hybrid or heterozygous
(two different traits)
T= Dominant for tallness
t= Recessive for shortness
3. An organism will always show the
dominant trait.
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4. Recessive traits show through only
when an organism has two recessive traits.
5. Ex. tt = short
6. TT or tt are considered purebred or
homozygous because both genes from the
mother and the father are the same.
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7. Phenotype: The physical
appearance of the offspring in a punnett
square.
8. Genotype: The genetic makeup
of the offspring.
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J. Mendel’s experiment: F1 generation
(segregation)
He crossed a purebred for tallness with a
purebred for shortness:
TT x tt
Mendel’s Experiment
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The F2 generation would be Tt xTt
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Do the following cross:
Cross a hybrid for tallness with a purebred
for shortness:
Tt x tt
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Do the following cross:
Two parents are hybrid for brown hair.
What are the probabilities of their offspring
having brown or blond hair?
Review
Test Crosses
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SS x SS - true breeding (100%
homozygous dominant.)
ss x ss - true breeding (100% homozygous
recessive.)
How do you determine whether an
individual with the dominant phenotype is
homozygous or heterozygous?
Test Cross
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Used to detect heterozygous condition.
In peas, the allele for tall plant height (T) is
dominant over the allele for short plant height (t)
What cross would you do to determine the
genotype of a tall pea plant?
What results would you expect if the tall plant is
TT?
What results would you expect if the tall plant is
Tt?
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What would the gametes of a plant of
genotype SsYy have?
SY, Sy, sY, and sy
Each gamete will receive either an S or
s allele, and either a Y or y allele.
Following a SsYy x SsYy cross, what
fraction of the offspring are predicted to
have a genotype that is heterozygous for
both characteristics?
Probability
Probability = the # of times an event is expected to
happen / the # of opportunities for an event to happen.
 For example, The probability of rolling a 2 on a dice in
one trial is:
1/6
The rule of multiplication states that the probability of two or
more events happening at the same time is the product of
the independent events.
What is the probability of rolling two 2’s?
(1/6) x (1/6) = 1/36
 How can the principles of probability be used to predict
the outcomes of genetic crosses?
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Ratio:
9:3:3:1
Cross a homozygous running,
heterozygous black mouse with a
waltzing brown mouse
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B=black and b=brown
R=running and r=waltzing
Cross a waltzing, brown mouse
with a waltzing, brown mouse
Cross a heterozygous running, brown
mouse with a heterozygous running,
homozygous black mouse
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Deep down in the Pacific Ocean in the city
of Bikini Bottom, square pants (S) are
dominant to round pants (s), and yellow skin
(Y) is dominant to green skin (y). Sponge
bob is a Yellow skinned, square pants
sponge, whose mother had green skin and
round pants.
A. What is Sponge Bob’s genotype?
B. Sponge Bob wants children and he
mates with a spongey woman with square
pants and green skin. Her father had curved
ears. What is her genotype?
C. What is the phenotypic ratio of the
children that are expected as a result of this
mating?
D. How many of their children will have the
following genotype?
PPYy? Ppyy?
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A man with brown eyes marries a woman with blue eyes. The mans
mother had blue eyes. What is the probability that they will have a
blue-eyed child?
B= brown b= blue
You do not immediately know the genotype of the brown-eyed man
because he could be either Bb or BB. Begin to state the cross by
using a blank for the unknown gene. One of the genes of the browneyed man must be B or he would have blue eyes. His genotype is B_
and the cross is: B_ x bb --> . Fill in any blanks in a problem by
using other information in the problem. The brown-eyed man had a
blue-eyed mother. Her genotype must be bb (If she had even one B
gene she would have brown eyes). The man's mother could give him
only a b gene so his genotype must be Bb. The cross now is: Bb x bb
-->. Do the cross.
Ie. RrYy x RrYy
What is the probability of obtaining an
offspring with the genotype rrYY?
Genetics Quiz