BIO 1 Chapter 11
Introduction to Genetics
11-1 The Work of Gregor Mendel
 Gardener in Austrian monastery
 Born 1822
 Called the “Father of Genetics”
 Used pea plants because
1. Traits exist as clearly different
forms ex: flower is white or purple
2. Both Male & female parts in
plants, so self-fertilization can
occur or can cross-pollinate
3. Small, grow easily, mature quickly
YouTube - Mendel - From the Garden
to the Genome
Heredity- passing on of
characteristics from parents to
offspring
Genetics- study of heredity or
traits & characteristics
Mendel used “Selfpollinating” peas to
produce future offspring
 Knew that each flower
produces both male
(sperm –found in pollen)
and female (egg)
gametes
 Fertilized flower’s egg
with its own sperm
 Produced what he called “true breeding”
 Offspring would be genetically
identical
Cross Pollination
Trait – specific characteristic that varies from one
individual to the next
 Important that Mendel used traits that were
unambiguous
Monohybrid crosses- cross 1 pair
of contrasting traits
 P generation- parents, first two crossed
 F1 generation- first offspring, PxP
 F2 generation- F1 x F1
Plants that were
products of cross
-pollination are known
as “hybrids.”
Mendel’s Hypothesis
1. For each inherited trait, an individual has 2 copies of the
gene, one from each parent
2. There are alternative versions of genes = alleles
(purple, white)
3. When two different alleles occur together, one of them may
be expressed, while one is covered up
Definitions
Mendel concluded that heredity is dictated by
chemical factors called genes
(about 100 years later before Watson & Crick and the whole DNA
thing)
Genes have alternate forms depending on the
plant
 These alternate versions of the same gene are called alleles
Plant height = gene
Short and tall = alleles
Both alleles are present.
One type (recessive) can only be expressed if the
other (dominant) is not present
Definitions
Dominant- expressed trait (D)
Recessive- not expressed (d)
Homozygous- both letters are same, either
both capital or both lowercase (DD or dd)
Heterozygous- one of each, dominant
allele is expressed (Dd)
Genotype- set of alleles (DD or Dd or dd)
Phenotype- physical appearance (purple or
white flowers)
Laws
Law of Segregation- the 2 alleles separate
when gametes are formed
Law of Independent Assortment- one trait
doesn’t affect the inheritance of another
(purple/white flowers has nothing to do with
smooth/rough pod)
11-2 Probability and Punnett
Squares
Probability – likelihood of an event to take place
 What are the chances for a coin to be flipped
heads three times in a row?
½X½X½=⅛
Probability explains everything in predicting
outcomes of genetic crosses
Rules for combining probabilities
1. Probability that one event out of a set of mutually
exclusive events will occur is the sum of their
probabilities. (Mutually exclusive events cannot occur
together.) (Look for the word “or”.)
Ex.
What is the probability of rolling a 3 or a 6 on a
6-sided dice?
p = 1/6 + 1/6 = 2/6 or 1/3
2. Probability that both of two independent events
will occur is the product of the independent
probabilities of the single events. (Independent
events do not affect each other.)(Look for the word
“and”.)
Ex.
What is the probability that I roll a 1 and a 3
when rolling two 6-sided dice?
p = 1/6 x 1/6 = 1/36
Punnett Squares
 1 pair of traits: The letters represent alleles
< Homozygous recessive
Homozygous dominant
All offspring are
heterozygous
Genotypic
Ratio ?
1.
2.
3.
4.
Phenotypic
Ratio?
7. A heterozygous round seeded plant (Rr) is
crossed with a homozygous round seeded plant
(RR).
What percentage of
the offspring will be homozygous (RR)?
____________
8. A homozygous round seeded plant is crossed with
a homozygous wrinkled seeded plant.
What are the genotypes of the parents?
__________ x __________
What percentage of the offspring will also be
homozygous? ________
9. In pea plants purple flowers are dominant to white
flowers.
If two white flowered plants are cross, what percentage of
their offspring will be white flowered? ______________
10. A white flowered plant is crossed with a plant that is
heterozygous for the trait.
What percentage of the offspring will have purple flowers?
_____________
11. Two plants, both heterozygous for the gene that
controls flower color are crossed.
What percentage of their offspring will have purple flowers?
______________
What percentage will have white flowers? ___________
12. In guinea pigs, the allele for short hair is
dominant.
What genotype would a heterozygous short
haired guinea pig have? _______
What genotype would a purebreeding short
haired guinea pig have? _______
What genotype would a long haired guinea
pig have? ________
13. Show the cross for a pure breeding
short haired guinea pig and a long haired
guinea pig.
What percentage of the offspring will have
short hair? __________
14. Show the cross for two heterozygous
guinea pigs.
What percentage of the offspring will have
short hair? ________
What percentage of the offspring will have
long hair? _______
15. Two short haired guinea pigs are mated
several times. Out of 100
offspring, 25 of them have long hair.
What are the probable genotypes of the
parents? ________ x ___________
Show the cross to prove it!
11-3 Exploring Mendelain Genetics
A. Independent Assortment
1. The Two-Factor Cross: F1
2. The Two-Factor Cross: F2
B. A Summary of Mendel’s Principles
C. Beyond Dominant and Recessive Alleles
1.
2.
3.
4.
Incomplete Dominance
Codominance
Multiple Alleles
Polygenic Traits
D. Applying Mendel’s Principles
E. Genetics and the Environment
Dihybrid Crosses “FOIL” method
 AaBb x AaBb
PXP
F1 generation=
100% RrYy
F1 X F1 = F2 generation
9 round, yellow: 3 round, green: 3 wrinkled yellow: 1 wrinkled, green
Practice
RRBb X Rrbb
Practice
rrBb X RrBB
3. A male rabbit with the genotype GgBb . Determine the gametes
produced by this rabbit (the sperm would have these combinations of
alleles) Hint there are 4 combinations.
4. A female rabbit has the genotype ggBb. Determine the gamets
(eggs) produced by this rabbit.
5. Use the gametes from #4 and #5 to set up the punnett
square below. Put the male's gametes on the top and the
female's gametes down the side. Then fill out the square
and determine what kind of offspring would be produced
from this cross and in what proportion.
Science Starter:
An aquatic arthropod called a Cyclops has antennae that are either smooth or
barbed. The allele for barbs is dominant. In the same organism, resistance to
pesticides is a recessive trait.
A Cyclops that is resistant to pesticides and has smooth antennae is crossed with
one that is heterozygous for both traits. Show the genotypes of the parents and then
complete a punnett square showing the results:
______________ x _______________
_______ have barbed antennae and are not resistant to pesticides
______ have barbed antennae and are resistant to pesticides.
_______ have smooth antennae and are not resistant to pesticides
_______ have smooth antennae and are resistant to pesticides
8. Set up a punnett square for the cross.
9. What are the phenotypic ratios of the
offspring?
Beyond Dominance and
Recessiveness
Not all genes show simple patterns of
dominant and recessive alleles.
Genetics is more complicated.
The majority of genes have more than
2 alleles.
Some alleles are neither dominant
nor recessive, and many traits are
controlled by multiple alleles or
multiple genes.
Incomplete Dominance
 A cross between two 4-o’clock plants:
 Red flowered RR
 White flowered WW
 Which is dominant?
 Incomplete Dominance:
Cases in which one allele is not
completely dominant over another
Incomplete Dominance Examples
 In horses, some of the genes for hair color are incompletely dominant.
Genotypes are as follows: brown horses are BB, white horses are WW
and a WB genotype creates a yellow-tannish colored horse with a white
mane and tail, which is called “palomino”. Show the genetic crosses
between the following horses and record the genotypic and phenotypic
percentages:
 1. Brown x white
 2. Brown X palomino
 3. Palomino X palomino
Codominance
Similar to incomplete
dominance however
both traits are actually
visual instead of blended
 Chickens with
speckled black spots
on white feathers
 Humans have gene
for protein controlling
cholesterol – if
heterozygous two
forms are made
Multiple Alleles
When there are more than two alleles for a
particular gene
 Blood type (also demonstrates some
codominance)
Polygenic traits
Traits controlled by many genes







Height
SLE (Lupus)
Weight
Eye Color
Intelligence
Skin Color
Many forms of behavior
Applying Mendel’s Principles
Good animal to test was fruit fly
 Drosophilia melanogaster
Fast reproductive rate/life cycle and produced
many offspring
Environmental impact on gene
expression
• Environmental factors/conditions may alter gene
expression.
 Example: Soil pH and flower color.
11-4 Meiosis
A. Chromosome Number
B. Phases of Meiosis
1.
2.
Meiosis I
Meiosis II
C. Gamete Formation
D. Comparing Mitosis and Meiosis
Meiosis
Process of turning diploid somatic cells into haploid
gametes
Must reduce chromosome number so when
fertilization takes place we re-establish the regular
chromosome number
 Humans
 Hapolid – sex cells
N = 23
 Diploid – somatic cells 2N = 46
Figure 11-15 Meiosis
Section 11-4
Meiosis I
Go to
Section:
Figure 11-15 Meiosis
Section 11-4
Meiosis I
Go to
Section:
Figure 11-15 Meiosis
Section 11-4
Meiosis I
Go to
Section:
Figure 11-15 Meiosis
Section 11-4
Meiosis I
Go to
Section:
Figure 11-15 Meiosis
Section 11-4
Meiosis I
Go to
Section:
Figure 11-17 Meiosis II
Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two
The chromosomes line up in a The sister chromatids
haploid (N) daughter cells,
similar way to the metaphase separate and move toward
each with half the number of stage of mitosis.
opposite ends of the cell.
chromosomes as the original.
Go to
Section:
Telophase II
Meiosis II results in four
haploid (N) daughter cells.
Figure 11-17 Meiosis II
Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two
The chromosomes line up in a The sister chromatids
haploid (N) daughter cells,
similar way to the metaphase separate and move toward
each with half the number of stage of mitosis.
opposite ends of the cell.
chromosomes as the original.
Go to
Section:
Telophase II
Meiosis II results in four
haploid (N) daughter cells.
Figure 11-17 Meiosis II
Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two
The chromosomes line up in a The sister chromatids
haploid (N) daughter cells,
similar way to the metaphase separate and move toward
each with half the number of stage of mitosis.
opposite ends of the cell.
chromosomes as the original.
Go to
Section:
Telophase II
Meiosis II results in four
haploid (N) daughter cells.
Figure 11-17 Meiosis II
Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two
The chromosomes line up in a The sister chromatids
haploid (N) daughter cells,
similar way to the metaphase separate and move toward
each with half the number of stage of mitosis.
opposite ends of the cell.
chromosomes as the original.
Go to
Section:
Telophase II
Meiosis II results in four
haploid (N) daughter cells.
Figure 11-17 Meiosis II
Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two
The chromosomes line up in a The sister chromatids
haploid (N) daughter cells,
similar way to the metaphase separate and move toward
each with half the number of stage of mitosis.
opposite ends of the cell.
chromosomes as the original.
Go to
Section:
Telophase II
Meiosis II results in four
haploid (N) daughter cells.
Crossing Over
 Parts of chromosomes
“switch places
 A tetrad forms in prophase
1
 4 chromosomes to a tetrad
Gamete Formation
 Figure 11-17 on page 278
 Males
 Animals = sperm
 Flowering plants = pollen
 4 equal sized gametes
 Females
 Animals = egg
 Flowering plants = ovule
 Only 1 large egg results from meiosis, the other 3 cells are called
polar bodies and not involved in reproduction.
Mitosis VS. Meiosis
Mitosis
Purpose
Location in Body
Number of Daughter
Cells
Change in
chromosome number
Number of phases
Number of Cell
Divisions
Difference in DNA
between parent cell
and daughter cell
Meiosis
11-5 Linkage and Gene Maps
Found that some genes were linked to other ones
 Seems to violate the rule of independent
assortment
 Ex: fruit flies – red eyes and miniature wings
Mendel missed this….
 He thought the genes independently assorted
 Actually it was the chromosomes that do!
 Why did he miss it?
 6 of the 7 genes he studied in peas were on
separate chromosomes
 The two that were on the same chromosome
were so far apart they independently assorted
Gene Maps
Do two genes on the
same chromosome mean
they are forever linked?
 No! Crossing over
may put them on
separate
chromosomes
The further apart genes
are the more likely they
could be separated during
meiosis
Low rate of separation
and then recombination
means genes are close to
one another.