Download Chapter 11 Introduction to Genetics 2015

The Work of Gregor Mendel
Essential Questions

Where does an organism get
its unique characteristics?
How are different forms of a
gene distributed to offspring?
Who was Gregor Mendel?

Monk born in 1822 in Czech
Republic.
At age 21, entered a monastery
 Performed scientific experiments
in the garden.
Discovered the principles of
heredity(genetics), the passing
of traits from parents to
offspring.
Mendel’s Model System


Pea Plant – Pisum sativum
Capable of





Self-fertilization (pollination)
Cross-fertilization
(pollination)
Reproduces quickly
Yields high # of offspring
Studied distinctive
phenotypes




Flower color
Stem length
Pod shape of color
Seed shape or color
Self vs. Cross Pollination

Self vs. Cross Fertilization
Pea plant
Cross
Pollination
Process
Stigma
Carpel
Pollen
Anthers
a. This flower has been sectioned to show the
location of its anthers (male) and of the carpel
with its attached stigma (female). Pollen grains
form in the anthers. Egg cells develop,
fertilization takes place, and seeds mature inside
the carpel.
b. Pollen from one plant is brushed onto the
stigma of a second plant. The anthers have been
cut from the second plant so that it cannot selffertilize.
c. The cross-fertilized plant produces seeds,
which may be scored for seed traits, such as
smooth or wrinkled shape, or may be grown into
plants for scoring of adult traits, such as flower
color.
d. The adult pea plant (F1 generation)
Fig. 14-2a
TECHNIQUE
1
2
Anther (male parts)
Parental
generation
(P)
Stigma(female)
3
4
RESULTS
5
First
Filial
generation
offspring
(F1)
Unraveling the Mystery

Characteristics
 Mendel studied only one pea
trait at a time. (Monohybrid
crosses)
 Trait: Characteristic that has
different forms in a
population. (Ex. Seed color)
Mendel cross pollinated to
learn more about how traits
are inherited.
Unraveling the Mystery

Mendel used
plants that were
true breeding
(homozygous) for
each trait he was
studying.
DO NOW:
What did Mendel do to guarantee the
plants he wanted to cross pollinate did not
self pollinate first?
Objectives:
1. Describe Mendel’s studies and conclusions about
inheritance.
2. Explain the relationship between traits and
heredity.
3. Differentiate between dominant and recessive
traits.
4. Explain how genes and alleles are related to
genotype and phenotype.
Mendel’s First Experiments

Studied seven different characteristics.
Noticed one trait was always present in the 1st
generation, and the other trait seemed to disappear.
Dominant trait: Trait that appeared
Recessive trait: Trait that seemed to fade into the
background.
Mendel’s Second Experiments

Mendel allowed the 1stgeneration (F1) plants to selfpollinate (heterozygous cross).
Recessive trait reappeared in
2nd generation plants (F2).
Hybrids: Offspring of crosses
b/w parents with different
traits.
Monohybrid cross

One allele different: Flower color
 P=Purple (dominant)
p=white (recessive)
Parental generation PP x pp
Possible parental gametes:
 ♀= P
 ♂=p
Mendel’s Second Experiments

Results of the F2 generation:
 Recessive trait did not
show up as often as the
dominant trait.
Mendel figured out the ratio
of dominant traits to
recessive traits
Mendel’s Second Experiments

 In all cases
the ratio was
about 3:1
dominant :
recessive.
Mendel’s Summary

Genes: Characteristics determined by factors that
are passed from the parental generation.
Alleles: 2 different forms of a gene.
 Example:
Trait = Seed shape
Allele 1 = Round seed; Allele 2 = Wrinkled
seed
Gene = Round seed is inherited because it is a
dominant allele (masks recessive allele)
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers
Explaining the F1 Cross: Segregation
• Dominant alleles mask
recessive allele in the F1
generation but reappear
in the F2 generation.
• Conclusion: Alleles
separate during the
formation of sex cells
(gametes) and recombine
during fertilization
(zygote)
Explanation of Segregation
• F1 plant
• Inherited an allele for
tallness from its tall
parent and an allele
for shortness from its
short parent
• Result: Tt
• All plants tall
Explanation of Segregation
• When each parent, or
F1 adult, produces
gametes (sperm or
egg cell), the alleles for
each gene segregate
from one another, so
that each gamete
carries only one allele
for each gene.
Explanation of Segregation
• Each F1 plant
produced two kinds
of gametes:
• Those with the allele
for tallness (T) and
those with the allele
for shortness (t).
Explanation of Segregation
• The F2 plant inherits one
of each gamete in
different combinations
• Combination possibilities:
• TT = Tall
• Tt = Tall
• tt = Short
Explaining Mendel’s Results
(Summary)

Proposed theory of Particulate inheritance
Hereditary determinants:
Do not blend together
Do not become modified through use
Maintain their integrity
Exist in distinct discrete units (genes)
Each individual has two versions (alleles) of
each gene
Terms
Genetics

 Scientific study of hereditary
Gene
 Hereditary determinant
Alleles
 Two different versions of the same genes
Genotypes
 All of the alleles found in an individual
Phenotype
 The physical traits which are observed
DO NOW:
If you flip a coin what are the chances it will
land on heads? Tails? Suppose that you
flipped the coin and got heads. What are the
chances that you will get heads again?
Objectives:
1. Explain how geneticists use the principles of
probability to make Punnett squares.
2. Describe the principle of independent
assortment.
3. Solve genetic word problems using a Punnett
square.
Applying Mendel’s
Principles
Essential Questions

How can we use probability to
predict traits?
How do alleles segregate when
more than one gene is involved?
What did Mendel contribute to our
understanding of genetics?
Phenotype vs. Genotype

Phenotype : Organism’s
appearance (purple flowers).
 Genes affect the phenotype.
Genotype: Combination of
inherited alleles together.
 Homozygous: Genotype that
has 2 dominant OR recessive
alleles (PP or pp).
 Heterozygous: Genotype that
has one recessive and one
dominant (Pp).
What are the Chances?

Gene: 2 Alleles
Probability:
Mathematical chance
that something will
happen.
Determines phenotypic
ratios in offspring.
Calculating Probability

What are the Chances?

Genotype Probability
 Each offspring of a Pp X Pp cross has a
50% chance of receiving either a P allele
or p allelle from the parent.
 So, the probability of inheriting two p
alleles is 1/2 X 1/2, which equals 1/4,
or 25%.
Probability in Mendel’s Crosses

 Probability of PP zygote = ½ × ½ = ¼
 Probability of pp zygote = ½ × ½ = ¼
Punnett Squares organize all the possible genotype
combinations of offspring from particular parents.
What are the genotypes? Phenotypes?

1. What is the genotype of the offspring
represented in the upper left-hand box of
the Punnett square?

A Homozygous
dominant
B Homozygous
recessive
C Heterozygous
2. What is the genotype of the offspring
represented in the lower right-hand box
of the Punnett square?

A RR
B Rr
C rr
D rrr
3. What is the ratio of Rr (round seeds) to
rr (wrinkled seeds) in the offspring?

A 1:3
B 2:2
C 3:1
D 4:0
Two Factor Crosses

Law of Independent
Assortment: Alleles of genes
that govern two different
characters segregate
independently during
formation of gametes.
RR, Rr= Round; rr=wrinkled
YY, Yy = Yellow; yy= green
Cross: Rr Yy x Rr Yy
Gametes (pollen)

Gametes
(eggs)
Phenotypic ratio: 9 round yellow : 3 round green :
3 wrinkled yellow : 1 wrinkled green
Monohybrid vs. Dihybrid

Test cross

T
T
TT x tt
t
t
Tt
Tt
Tt
Tt
Tt x tt
t
t
T
t
Tt
tt
Tt
tt
A Summary of Mendel’s
Principles

Early 1900s, geneticist
Thomas Hunt Morgan used
fruit flies for genetic
experiments.
Ideal because it produces
plenty of offspring.
Conclusions: Mendel’s
principles applied to most
traits in most organisms.
TWO-FACTOR CROSS
In pea plants, green pods (G) are dominant
over yellow pods (g), and smooth pods (N) are
dominant over constricted pods (n). A plant
heterozygous for both traits (GgNn) is crossed
with a plant that has yellow constricted pods
(ggnn). What are the probable genotypic and
phenotypic ratios for this cross?

GgNn x
ggnn
GN
Gn
gN
gn
gn
GgNn
Ggnn
ggNn
ggnn
gn
GgNn
Ggnn
ggNn
ggnn
gn
GgNn
Ggnn
ggNn
ggnn
gn
GgNn
Ggnn
ggNn
ggnn

Solution!

Genotypic ratio
4 GgNn : 4 Ggnn : 4ggNn : 4 ggnn
= 4:4:4:4 = 1:1:1:1
Phenotypic ratio
4 green smooth : 4 green constricted :
4 yellow smooth : 4 yellow constricted
= 4:4:4:4=1:1:1:1
DO NOW:
Construct a Punnett square that
demonstrates the cross between 2 pea
plants that are heterozygous for purple
color and seed shape (round seeds are
dominant over wrinkled seeds).
Objectives:
1. Describe other patterns of inheritance that are
exceptions to Mendelian genetics.
2. Explain the relationship between genes and the
environment.
Chapter 11.3:
Essential Questions

What are some exceptions
to Mendel’s principles?
Does the environment have
a role in how genes
determine traits?
Other Patterns of
Inheritance

Mendel worked with a
genetically simple system
Most traits are controlled by
a single gene
Each gene has only 2 alleles,
1 of which is completely
dominant to the other
There are many exceptions to
simple Mendelian genetics.
Incomplete Dominance

 Heterozygote has an
intermediate phenotype
between that of either
homozygote (one trait is
not completely dominant
over another).
 When a red and white
snapdragon flower breed
it makes offspring that are
all pink.
 Example in humans:
Wavy hair
Incomplete dominance
X
true-breeding
true-breeding
P red flowers
white flowers

F1
100% pink flowers
100%
generation
(hybrids)
self-pollinate
F2
generation
25%
red
50%
pink
2005-2006
25%
white
1:2:1
Incomplete Dominance in
Human Traits

Normally, red blood cells are round and
disk-shaped.
With sickle cell anemia the red blood cells are
sickle-shaped.
Incomplete Dominance in
Human Traits
 Sickle-cell disease

 Homozygote recessive has sickle-cell disease
 Heterozygote has milder sickle-cell trait
Codominance

Codominance: Organisms have two
different alleles of a gene and both
phenotypes show at the same time.
Codominance

Both alleles for the trait
are dominant, resulting
in offspring with both
phenotypes.
Example:
 Crossing a homozygous
white cow with a
homozygous red cow
produces a roan cow
with a coat of red AND
white spots.
Codominance in
Humans

ABO blood groups
3 alleles
 IA, IB, i
 Both IA & IB are dominant
to i allele
 IA & IB alleles are codominant to each other
Determines which
antigens are produced on
the red blood cells.
Human ABO Blood Group
 Immune system produces antibodies against antigens
not found on its own red blood cells

Blood donation

2005-2006
Human Blood Types

Greatest percentage in U.S?
Total positive RH factor?
Negative?
Percentage that can be
used for the most
transfusions? The least?
Could a person with O+
have 2 parents that are O-?
Could that person have a
daughter with AB+ blood?
Multiple Alleles

This term describes
traits for which
there are more than
2 alleles.
Ex: A gene for
animal fur color
has 4 different
alleles
Polygenic Traits

Polygenic Traits:
Inherited traits that
are determined by
more than one gene.
Examples:
Eye color in humans
have at least 3
different genes with
multiple alleles.
Human Height
Polygenic inheritance

Phenotypes on a continuum:
Environment will also
influence polygenic traits.
Human traits
 skin color
 height
 weight
 eye color
 intelligence
 behaviors
Environmental Factors

 Genes aren’t the only influences on traits.
 Genes provide a plan but that plan also depends on
the environment.
 Ex: Height and skin tone
Environmental factors

In certain reptiles,
sex is determined by
temperature.
During development
in the egg, higher
temperatures favor
the production of
males.
Nature vs.
Nurture
Phenotype is
controlled by both
environment &
genes
Color
of Hydrangea flowers is
2005-2006
influenced by soil pH

Human skin
color is
influenced by
both genetics &
environmental
conditions
Coat color in
rabbits is
sometimes
influenced by
temperature
also
Coat color in arctic
fox influenced by
heat sensitive alleles
Objectives:
1. Contrast the number of chromosomes in body
cells and in gametes.
2. Summarize the events of meiosis.
3. Contrast meiosis and mitosis.
4. Describe how alleles from different genes can
be inherited together.
5. Explain how chromosomes determine sex.
Chapter 11.4:
Essential Questions

How many sets of genes are found in
most adult organisms?
What events occur during each phase
of meiosis?
How is meiosis different from mitosis?
How can 2 alleles from different genes
be inherited together?
Asexual Reproduction
Asexual reproduction

 Only one parent cell is needed.
 Structures inside the cell are copied, and then
the parent cell divides, making two exact
copies
This type of cell reproduction is called
mitosis.
Most of the cells in your body (body cells)
reproduce this way.
Cell Cycle

Interphase
 G1
 S
 G2
Mitosis




Prophase
Metaphase
Anaphase
Telophase
Mitosis

Sexual Reproduction

In sexual
reproduction, two
parent cells (gametes)
join together to form
offspring that are
different from both
parents.
Sexual Reproduction

Genes and Chromosomes
 Walter Sutton studied
meiosis in sperm cells in
grasshoppers.
Using his observations
and his knowledge of
Mendel’s work, Sutton
proposed that:
 Genes are located on
chromosomes.
Chromosomes

Chromosomes: Structure
consisting of DNA that transmit
genetic information to each
subsequent generation
Homologous chromosomes:
Chromosomes that carry the
same sets of genes.
One chromosome from the
father and one from the mother.
Homologous Chromosomes

Sexual Reproduction

Body Cells
 Human body cells have 46 chromosomes in 23 pairs.
 Human body cells are referred to as somatic cells.
• Examples: Liver cells, heart cells
Gametes (Sex cells)
 Gametes have only one of the chromosomes from
the pair (total of 23 chromosomes per cell)
 Gametes are made during meiosis.
• Examples: Sperm and Egg
Meiosis
 that produces sex cells
Meiosis is a copying process
(sperm and egg) with half the usual number of
chromosomes.
Why? When the sperm (23 chromosomes) combines
with the egg (23 chromosomes) it produces a cell
with 46 chromosomes.
Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote
Humans have 23 Sets of Homologous Chromosomes
Each Homologous set is made up of 2 Homologues.
Homologue

Homologue
Autosomes
(The Autosomes code for most of the offspring’s traits)
In Humans the
“Autosomes”
are sets 1 - 22

Sex Chromosomes
The Sex Chromosomes code for the sex of the offspring.
** If the offspring has two “X” chromosomes it will be a female.
** If the offspring has one “X” chromosome and one “Y”
chromosome it will be a male.
In Humans the “Sex
Chromosomes” are
the 23rd set
XX chromosome - female
XY chromosome - male
Sex Chromosomes
“Sex Chromosomes”
…….the 23rd set
This person has 2 “X”
chromosomes… and is
a female.
23
Steps of Meiosis

Meiosis consists of 2
distinct divisions:
Meiosis 1 & Meiosis 2
The result at the end will
be 4 haploid cells with
half the number of
chromosomes in the
body cells.
Meiosis
Meiosis: Prophase 1

Maternal + paternal
chromosomes separate &
replicate.
Duplicated homologous
chromosomes pair with one
another.
Prophase 1: Tetrads

When homologous chromosomes pair up they
form a structure called a tetrad, which contains
4 chromatids (chromosomes that have been
duplicated).
Prophase 1: Crossing
Over
undergo
As tetrads form, they
crossing-over.
Genes from one chromatid are
exchanged with genes from a
chromatid of the homologous
chromosome it matches with.
Crossing-over produces
new combinations of alleles.
Meiosis Metaphase I

Homologous pairs
align at the metaphase
plate (middle)
Nuclear membrane
dissolves and the
homologous
chromosomes attach
to spindle fibers
Meiosis Anaphase I

Spindle fibers
pull each
homologous
chromosome
pair toward
opposite ends of
the cell
Meiosis: Metaphase 1 and
Anaphase 1

Metaphase 1: Homologous pairs align independently
at the metaphase plate.
Anaphase 1: Homologous chromosomes separate and
move toward the poles.
Meiosis: Telophase 1
and Cytokinesis

Cell divides into
daughter cells.
Pairs of homologous
chromosomes are
separated randomly.
Sets are shuffled and
sorted to 2 separate
daughter cells.
Meiosis 2: Prophase 2

Cells have one homologous
pair from each tetrad.
(Tetrads were separated
during Meiosis 1)
Meiosis 2: Metaphase 2

Chromosome
pairs align at the
metaphase plate.
Spindle fibers
attach to the
centromeres of the
chromosomes.
Meiosis 2: Anaphase 2

Sister
chromatids
separate and
become
daughter
chromosomes.
Meiosis 2: Telophase 2
and Cytokinesis

Spindle disappears, nuclei
form, and cytokinesis takes
place.
With the formation of 4
cells, meiosis is over.
Each gamete cell carries
half the number of
chromosomes of body cells.
Meiosis


Comparing Meiosis and
Mitosis

 Mitosis
Meiosis
 When the duplicated
chromosomes separate each
daughter cell receives one
complete set of chromosomes.
 Does not change the
chromosome number of the
original cell.
 Results in the production of
two genetically identical
diploid cells.
 Asexual reproduction
 One major division
 Homologous chromosome
pairs line up and then
move to separate daughter
cells.
 Reduces the chromosome
number by half.
 Produces 4 genetically
different haploid cells.
 Sexual reproduction
 Two major divisions
Meiosis vs. Mitosis

Objectives
Contrast the number of chromosomes in body
cells and in gametes.
1. Summarize the events of meiosis.
2. Contrast meiosis and mitosis.
3. Describe how alleles from different genes can
be inherited together.
4. Explain how chromosomes determine sex.
Gene Linkage

Thomas Hunt Morgan’s
research on fruit flies led him to
the principle of gene linkage.
After identifying more than 50
Drosophila (fruit fly) genes,
Morgan discovered that many
of them appeared to be
“linked” together in ways that
seemed to violate the principle
of independent assortment.
Gene Linkage

 Linkage is defined genetically as the
failure of two genes to assort
independently.
 Linkage occurs when two genes are
close to each other on the same
chromosome.
 However, just because 2 genes are on
the same chromosome doesn’t mean
they are linked.
 Genes far apart on the same
chromosome assort independently
during crossing over: they are not
linked.
Gene Linkage

Gene Linkage

 Morgan’s findings led to two
remarkable conclusions:
 First, each chromosome is
actually a group of linked genes.
 Second, it is the chromosomes
that assort independently, not
individual genes.
 Alleles of different genes tend to
be inherited together when those
genes are located on the same
chromosome.
Gene Mapping

 In 1911, Columbia University student
Alfred Sturtevant wondered if the
frequency of crossing-over between genes
during meiosis might be a clue to the genes’
locations.
 Sturtevant reasoned that the farther apart
two genes were on a chromosome, the
more likely it would be that a crossover
event would occur between them.
 If two genes are close together, then
crossovers between them should be rare. If
two genes are far apart, then crossovers
between them should be more common.

Gene Mapping

 By this reasoning, he could use
the frequency of crossing-over
between genes to determine their
distances from each other.
 Sturtevant gathered lab data and
presented a gene map showing
the relative locations of each
known gene on one of the
Drosophila chromosomes.
 Sturtevant’s method has been
used to construct gene maps ever
since this discovery.