Classical Genetics
Gregor Mendel
The Work of Gregor Mendel
Genetics: the scientific study of heredity,
or how traits are passed from one
generation to the next
Gregor Mendel’s Peas
- Mendel was an Austrian monk who is
credited as the “father of genetics.”
The Work of Gregor Mendel
- Mendel began working with pea plants
that were true-breeding, meaning
that if allowed to self pollinate, they
would only produce offspring identical
to themselves
-ex. Tall plants produce tall plants,
green seeded plants produce green
seeded plants
The Work of Gregor Mendel
Genes and Dominance
- Mendel studied 7 different plant
traits. Traits are characteristics such
as height or flower color.
The Work of Gregor Mendel
Seed
Shape
Round
Wrinkled
Round
Seed
Color
Yellow
Green
Yellow
Seed Coat
Color
Gray
White
Gray
Pod
Shape
Smooth
Constricted
Smooth
Pod Color
Flower
Position
Green
Axial
Yellow
Green
Terminal
Axial
Plant Height
Tall
Short
Tall
The Work of Gregor Mendel
- Mendel called the original plants the P
(parent) generation. The offspring
were the F1 (first filial) generation .
- the offspring of crosses between
parents of different traits are called
hybrids
The Work of Gregor Mendel
Mendel's F2 Generation
P Generation
Tall
Short
F2 Generation
F1 Generation
Tall
Tall
Tall
Tall
Tall
Short
11-1 The Work of Gregor
Mendel
From his experiments, Mendel drew two
conclusions
1) biological inheritance is determined
by factors that are passed from one
generation to the next. Today we call
these factors genes.
- the different forms of a gene are
called alleles
The Work of Gregor Mendel
ex.gene (trait) is eye color
alleles (forms) are blue, brown,
green, etc.
2) the second conclusion is called the
principle of dominance
- the principle of dominance states that
some alleles are dominant and others
are recessive
The Work of Gregor Mendel
Segregation
- Mendel saw that some forms of traits
would disappear during the F1
generation, but reappear during the F2
generation
- he explained this by showing that the
alleles segregated during the
formation of gametes, or sex cells.
The Work of Gregor Mendel
Section Quiz
Gametes are also known as
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genes.
sex cells.
alleles.
hybrids.
Section Quiz
The offspring of crosses between parents with
different traits are called
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alleles.
hybrids.
gametes.
dominant.
Section Quiz
In a cross of a true-breeding tall pea plant with a
true-breeding short pea plant, the F1 generation
consists of
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all short plants.
all tall plants.
half tall plants and half short plants.
all plants of intermediate height.
Section Quiz
If a particular form of a trait is always present
when the allele controlling it is present, then
the allele must be
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mixed.
recessive.
hybrid.
dominant.
Probability and Punnett
Squares
Genes and Probability
- the likelihood that an event will occur is
called probability
ex. a coin landing on heads has a
probability of 50%
- the principle of probability can be used
to predict the outcome of genetic crosses
Probability and Punnett
Squares
Punnett squares
- the gene combinations that might
result from a genetic cross can be
determined by drawing a diagram
known as a Punnett square
Probability and Punnett
Squares
Probability and Punnett
Squares
Organisms that have two identical alleles
for a particular trait are called
homozygous.
ex. TT
Organisms that have two different alleles
for a particular trait are called
heterozygous
ex. Tt
Probability and Punnett
Squares
Probability and Punnett
Squares
The physical characteristics of an
organism are called its phenotype
ex. tall plant
The genetic makeup of an organism is its
genotype
ex. TT or Tt
Section Quiz
Probability can be used to predict
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average outcome of many events.
precise outcome of any event.
how many offspring a cross will produce.
which organisms will mate with each other.
Section Quiz
Compared to 4 flips of a coin, 400 flips of the coin
is
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more likely to produce about 50% heads and 50%
tails.
less likely to produce about 50% heads and 50%
tails.
guaranteed to produce exactly 50% heads and 50%
tails.
equally likely to produce about 50% heads and 50%
tails.
Section Quiz
Organisms that have two different alleles for a
particular trait are said to be
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hybrid.
heterozygous.
homozygous.
recessive.
Section Quiz
Two F1 plants that are homozygous for shortness
are crossed. What percentage of the offspring
will be tall?
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100%
50%
0%
25%
Exploring Mendelian
Genetics
Independent assortment
- the principle of independent
assortment states that genes for
different traits can segregate
independently during the formation of
gametes
ex. Seed color does not depend on seed
shape
Exploring Mendelian
Genetics
Exploring Mendelian
Genetics
Beyond dominant and recessive alleles
- some alleles are neither dominant nor
recessive , and many traits are
controlled by multiple alleles or multiple
genes
Exploring Mendelian
Genetics
Incomplete dominance or
Codominance
- the heterozygous phenotype is
somewhere between the two
homozygous phenotypes
ex. red flower + white flower = pink
Punnett square animation
Exploring Mendelian
Genetics
Exploring Mendelian
Genetics
Multiple alleles
- multiple alleles contribute to the phenotype
ex. Blood type (i, IA, IB)
Type O = i i
Type A = IA IA or IA i
Type B = IB IB or IB i
Type AB = IA IB
Exploring Mendelian
Genetics
Exploring Mendelian
Genetics
Polygenic traits
- two or more genes control the
outcome of a particular trait
ex. Skin color in humans is controlled
by the combinations of 4 different
genes
Exploring Mendelian
Genetics
Section Quiz
In four o'clock flowers, the alleles for red flowers
and white flowers show incomplete dominance.
Heterozygous four o'clock plants have
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pink flowers.
white flowers.
half white flowers and half red flowers.
red flowers.
Section Quiz
Mendel's principles apply to
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pea plants only.
fruit flies only.
all organisms.
only plants and animals.
Human Heredity
Human Chromosomes
A typical human body cell contains 46
chromosomes
- 44 autosomes and 2 sex
chromosomes
Chromosomes are arranged in a
karyotype, a picture of the
chromosomes
Human Heredity
Human Heredity
Males and females differ in the sex
chromosomes they contain
- males are XY
- females are XX
All egg cells carry a single X
chromosome. However, half of all
sperm cells carry a X chromosome
while half carry a Y chromosome.
Human Heredity
Human Heredity
Human Traits
Scientists use a pedigree to help study
how traits are passed from one
generation to the next.
Pedigree: a chart which shows the
relationships within a family
Pedigree animation
Human Heredity
Circle =
Female
Square =
Male
Parental line
Marriage line
carrier of
the trait
shaded = express the
trait
not shaded =
does not
express the
trait
Human Heredity
Since most human traits are polygenic
(many genes), they can not be traced
to a single parent
Many genes have been discovered
through the study of genetic disorders
- they can be dominant or recessive
Human Heredity
In both cystic fibrosis and sickle cell
disease, a small change in the DNA of
a single gene affects the structure of a
protein, causing a serious genetic
disorder
Human Heredity
Cystic Fibrosis
- caused by a recessive allele on
chromosome 7
- causes digestive and respiratory
problems
- only half of people with CF live into
their 20s
Human Heredity
Human Heredity
Sickle Cell Disease
- causes hemoglobin to be less soluble
- changes the shape of red blood cells;
become bent into the sickle-shape
Human Heredity
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Malaria and the Sickle Cell Allele
Regions where malaria is
common
Regions where the sickle
cell allele is common
Section Quiz
A chromosome that is not a sex chromosome is
know as a(an)
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autosome.
karyotype.
pedigree.
chromatid.
Section Quiz
An individual with a blood type phenotype of O
can receive blood from an individual with the
phenotype
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O.
A.
AB.
B.
Human Chromosomes
Human genes and chromosomes
- on each chromosome there is the
information for several hundred genes
ex. Gene for ALS is on 22nd
chromosome
Human Chromosomes
Sex-linked genes: genes located on the
X or Y chromosome
ex. colorblindness, hemophilia,
muscular dystrophy
Males have only 1 X chromosome and
cannot be heterozygous for the trait.
Females can be carriers
(heterozygous).
Human Chromosomes
Father
(normal vision)
Colorblind
Normal
vision
Male
Female
Daughter
(normal vision)
Son
(normal vision)
Daughter
(carrier)
Son
(colorblind)
Mother
(carrier)
Human Chromosomes
Chromosomal disorders
The most common error occurs when
homologous chromosomes fail to
separate during meiosis. This is called
nondisjunction.
Human Chromosomes
Homologous
chromosomes
fail to separate
Meiosis I:
Nondisjunction
Meiosis II
Human Chromosomes
Human Chromosomes
If nondisjunction occurs, abnormal
numbers of chromosomes may find
their way into gametes, and a disorder
of chromosome numbers may result.
- Down syndrome results from a
nondisjunction of the 21st chromosome
- Turner’s syndrome (XO) and
Klinefelter’s syndrome (XXY)