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Chapter 11
What is genetics?
The scientific
study of
heredity
Gregor Mendel
Born in 1822 in
Czechoslovakia.
Became a monk at a
monastery in 1843.
Taught biology and
had interests in
statistics.
Also studied at the
University of Vienna
Mendel continued
After returning to the
monastery he
continued to teach
and worked in the
garden.
Between 1856 and
1863 he grew and
tested over 28,000
pea plants
Mendel’s Peas
Easy to grow.
Easily identifiable traits
Can work with large numbers of
samples
Mendel’s experiments
The first thing Mendel did was create a
“pure” generation or true-breeding
generation.
He made sure that certain pea plants
were only able to self pollinate,
eliminating unwanted traits.
He did this by cutting away the stamen,
or male part of each flower
Genes and dominance
Trait : a characteristic
Mendel studied seven of these traits
After Mendel ensured that his truebreeding generation was pure, he then
crossed plants showing contrasting
traits.
He called the offspring the F1
generation or first filial.
What will happen when pure
yellow peas are crossed with
pure green peas?
All of the
offspring were
yellow.
Hybrids = the
offspring of
crosses between
parents with
contrasting traits
What did Mendel
conclude?
Inheritance is determined by factors
passed on from one generation to
another.
Mendel knew nothing about
chromosomes, genes, or DNA. Why?
These terms hadn’t yet been defined.
What were Mendel’s
“factors”
The ‘factors” that Mendel mentioned
were the genes.
Each gene has different forms called
alleles
Mendel’s second principle stated that
some alleles are dominant and some
are recessive.
Mendel’s second cross
He allowed the F1 generation to selfpollinate thus producing the F2
generation.
Did the recessive allele completely
disappear?
What happened when he crossed two
yellow pea hybrid (F1) plants?
Results:
¾ of the peas were yellow, ¼ of
the peas were green.
During the formation of the sex cells or
gametes, the alleles separated or
segregated to different gametes. (pollen
and egg)
Probability
The likelihood of a
particular event
occurring. Chance
Can be expressed
as a fraction or a
percent.
Example: coin flip.
Punnett Square
Developed by
Reginald Punnett.
A diagram used to
show the probability
or chances of a
certain trait being
passed from one
generation to
another.
Reading Punnett
squares
Gametes are placed above and to the
left of the square
Offspring are placed in the square.
Capital letters (Y) represent dominant
alleles.
Lower case letters (y) represent
recessive alleles.
Punnett square example
Homozygous = when an organism
possesses two identical alleles. ex.

YY or yy
Heterozygous = when an organism
possesses different alleles. ex.

Yy
Phenotype vs genotype
Genotype
 The genetic makeup
 Symbolized with
letters
 Tt or TT
Phenotype
Physical
appearance of the
organism
Expression of the
trait
Short, tall, yellow,
smooth, etc.
Probability and
statistics
No one event has a greater chance of
occurring than another.
You cannot predict the precise outcome
of an individual event.
The more trials performed, the closer
the actual results to the expected
outcomes.
Punnett square review:
Mendel’s death
Mendel published his paper on heredity
in 1866.
The scientific community saw little if any
importance in his work.
Mendel died in 1884 with no recognition
for his contributions to genetics.
Some exceptions to
Mendel’s principles:
Some alleles are neither
dominant nor recessive.
Many traits are controlled by
more than one gene (polygenic
traits)
Incomplete dominance
A situation in which neither allele is
dominant.
When both alleles are present a “new”
phenotype appears that is a blend of
each allele.
Alleles will be represented by capital
letters only.
Japanese four-o-clock
flowers
Red flower plant genotype = RR
White flower plant genotype = WW
Pink flower plant genotype = RW
What happens when a
red flower is crossed
with a white flower?
According to
Mendel either
some white and
some red or all
offspring either
red or white.
All are pink
Codominance
When two alleles both appear in the
phenotype.
Usually signified using superscripts.
example: color of hair coat in cattle.
crcr = red hairs
cwcw = white hairs
crcw = roan coat (mixture of both colors)
Roan cattle inheritance
Multiple allele
inheritance
When two or more alleles contribute to
the phenotype.
Human blood types: A,B,O and AB
A and B are codominant to each other.
Both A and B are dominant over O.
Human Blood types:
TYPE A
Allele = IA
Blood cells
have small
antigens on the
surface.
TYPE B
Allele = IB
Cells coated
with type B
antigens
TYPE AB
genotype = IAIB
Blood cells
contain both
types of antigens
Known as
universal recipient
TYPE O
Allele = i
No antigens on
the surface of
the blood cells
Known as
universal donor
6 different genotypes
IAIA
IAIB
IBIB
IBi
IAi
ii
Type A
Type AB
Type B
Type B
Type A
Type O
How common are the
different blood types?
Sample Problem:
A man with type AB blood
marries a woman with type B
blood whose father has type O
blood. What are the chances
that they have a child with type
A blood? Type AB?
Polygenic traits
Traits controlled by two or more
genes.
Examples:
 Human height,
eye and skin
color
Rediscovery of Mendel’s
work
Around the turn of the century (early
1900’s) many scientists “rediscovered”
Mendel’s work
1908 – Garrod
1902 – Sutton
1910 – Morgan
Thomas Hunt Morgan
1866-1945
Born in Kentucky,
professor of Biology
at Columbia U.
Worked with fruit
flies (drosophila)
Nobel Prize in
Medicine (1933)
Why the Fruit Fly?
1. Can work with
large numbers of
flies easily
2. Produce many
offspring
3. Short reproductive
cycle
4. Only four pairs of
chromosomes
Meiosis
A method of cell division similar to
mitosis.
2 main differences:
1. There are two divisions to produce 4
daughter cells
2. The cells produce contain ½ the
chromosomes as the original cell
Chromosome number
All cells of an
organism contain a
specific number of
chromosomes.
Most cells are
diploid (2n) meaning
they have two
copies of each
chromosome
Events of meiosis I
During prophase I, each
chromosome pairs with its
homologous chromosome to
form a tetrad
Crossing-over
Crossing-over: an
exchange of
genetic material
between sister
chromatids
Results in greater
variation
Meiosis II
Neither cell replicates its chromosomes.
Each cell splits (similar to mitosis)
Produces four daughter cells.
Animation
Gametogenesis
Literally
means
“creation of
gametes”
Egg and
sperm
2 types: Spermatogeneis
& Oogenesis
Net result:
Spermatogensis
4 mature sperm
Each sperm has
exactly half the
number of
chromosomes as
the father.
Oogensis
1 mature ova or
egg.
Each egg has
exactly half the
number of
chromosomes as
the mother.
Gene Maps
First developed
by Sturtevant in
1911.
The farther apart
two genes are,
the more likely
they will be
separated in
meiosis.