Section 11-4:
Meiosis
Chapter 11 – Introduction to
Genetics
Genes are located on chromosomes
in the nucleus of the cell
Mendel’s principles of genetics required two things;
that each organism must inherit a single copy of every gene
from each of its “parents”
and that when the organism forms its own gametes, that the
two sets of genes must be separated so that each gamete
only contains one set of genes
The two sets of chromosomes an organism has are
homologous = each chromosome from the male parent has a
corresponding chromosome from the female parent
Diploid = a cell that contains both
sets of homologous chromosomes
Diploid cells contain two
complete sets of chromosomes
and two complete sets of genes
 this satisfies requirement
number 1
The gametes of sexually reproducing organisms
contain only a single set of chromosomes which
means only one set of genes
Haploid = cells that contain
only one set of chromosomes
 this satisfies requirement
number 2
So how do we get haploid cells from diploid
cells?  that’s where meiosis comes in!
Meiosis = a process of reduction
division in which the number of
chromosomes per cell is cut in half
through the separation of homologous
chromosomes in a diploid cell
Meiosis usually involves 2
sets of divisions; meiosis I
and meiosis II
By the end of meiosis II, the
diploid cells that we started
with produces 4 haploid cells
Meiosis works in a cycle very
much like mitosis, the cells
undergo interphase and then go
through prophase, metaphase,
anaphase, and telophase
Meiosis I
Interphase I - Before meiosis
I begins, each chromosome is
replicated
Prophase I – Each chromosome
pairs up with its homologous
chromosome forming a tetrad
 there are 4 chromatids in a
tetrad
While forming tetrads, the
chromatids exchange portions in a
process called crossing-over
Crossing-over helps to produce
new combinations of alleles
Metaphase I – Spindle fibers
attach to each chromosome
Anaphase I – The fibers pull
the homologous chromosomes
toward opposite ends of the
cell
Telophase I/ Cytokinesis –
Nuclear membranes form – there
are now two separate cells
Unlike mitosis, the two resulting cells DO NOT have a
complete set of chromosomes since the homologous
chromosomes were split up
The two cells have different sets of chromosomes and
alleles
Meiosis II
Interphase does not occur this
time and the chromosomes are
not replicated
Prophase II – Each cell has half
the chromosomes as the original
cell
Metaphase II – Chromosomes
line up at the center of the cell
Anaphase II – The sister
chromatids separate and move
toward opposite ends of the cell
Telophase II/ Cytokinesis –
Each of the 4 daughter cells
receives 2 chromatids – they
are each a haploid cell
Gamete formation in males and
females is a little different
In males, the gametes are called sperm
During the process of meiosis, 4 sperm
are produced from 1 diploid cell
In females, the gametes are called eggs
The cell divisions at the end of meiosis I
and meiosis II are uneven so that 1 cell
gets most of the cytoplasm  this one
cell becomes the egg
The other 3 cells are known as polar
bodies and usually do not participate in
reproduction
Comparing mitosis and meiosis
While the two processes go
through similar cycles, the
end results are very different
Mitosis results in the
production of 2 genetically
identical diploid cells
Mitosis allows an organism’s
body to grow and replace
cells
Meiosis produces 4 genetically
different haploid cells
Meiosis is how sexually
reproducing organisms
produce gametes
Section 11-1:
The Work of Gregor Mendel
Genetics = the scientific study of heredity (how
characteristics are inherited from parents)
Gregor Mendel was important to
understanding biological inheritance
He was an Austrian monk and worked
in the monastery gardens – this is
where he did a lot of the work that
helped him understand inheritance
Mendel carried out his work
with ordinary garden peas
Each plant contained a part that produced
pollen (the male reproductive cells)
Each plant also contained a female part
that produced eggs cells
During sexual reproduction the male and
female cells would join
The fertilization produced a new cell that
would be encased in a seed
Pea plants are normally selfpollinating so that the sperm
and egg cells came from the
same plant
Seeds produced this way inherited
all traits from 1 parent
True-breeding = producing offspring
that are identical to the parent
True breeding plants were the basis for Mendel’s work
because he knew what traits to expect
Using true-breeding plants, Mendel wanted to cross breed
plants (give them 2 parents)
He would use a brush to dust pollen
from one plant to another, giving it
2 different parents and 2 different
traits
In doing his work, Mendel studied
7 different pea plant traits
Trait = a specific characteristic that
varies from individual to individual
Each of the 7 traits studied had 2
contrasting characters; ie. green
seed color and yellow seed color
Mendel crossed plants with the 7
contrasting characters and studied
their offspring
Each original pair of plants was
the P (parental) generation
The offspring are called F1 (first
filial) – filius and filia are Latin
for “son” and “daughter”
The offspring of crosses between
parents with different traits = hybrids
Traits did not blend in hybrids,
instead all of the offspring had the
character of one parent and the other
seemed to disappear
Mendel drew 2 conclusions from this set of experiments
1. Biological inheritance is determined by
factors (genes) that are passed from one
generation to the next
Each trait was controlled by 1
gene with 2 contrasting forms
The different forms of the
gene = alleles
2. The principle of dominance, which
states that some alleles are dominant and
others are recessive
An organism with the
dominant trait will
always show that trait
An organism will only
show the recessive trait if
they DO NOT have the
dominant trait
Mendel still wondered about the
trait controlled by the recessive
allele that seemed to disappear in
F1 – was it completely gone or To answer this question,
still present?
he allowed the F1
generation to self-pollinate
to produce an F2 generation
In observing the F2 generation, Mendel
discovered the trait controlled by the
recessive allele reappeared
So why did the trait disappear in one
generation and reappear in another?
The reason the trait reappeared has to
do with segregation (separation)
Mendel suggested that the alleles
for 2 contrasting characteristics
segregated from each other
during gamete formation
Gamete = sex cells
Ex. Let’s assume that each F1 plant inherited a tall trait from
one parent and a short trait from the other
Due to segregation, F1 forms 2
different gametes, one with the tall
allele and one with the short allele
If a plant in the F2 generation
happened to inherit 2 short alleles,
then the trait would reappear
We represent the dominate allele
with a capital letter, tall = T
We represent the recessive allele
with a lower case letter, short = t
Section 11-2:
Probability and Punnett Squares
When Mendel preformed crosses, he
labeled/counted the offspring and
noticed patterns
He realized that the principles
of probability could be used to
explain the results of genetic
crosses
Probability = the likelihood that
a particular event will occur
Ex. In flipping a coin, there are 2
outcomes; heads up or tails up
The probability of either
outcome is equal; a one in
two chance (½ or 50%)
Each coin flip is independent, it
has a probability of ½ ,  the
outcome of one flip does not
affect the next
An important thing to remember is that
past outcomes DO NOT affect future ones
The way in which alleles
segregate is completely random,
just like flipping a coin
The gene combinations that might result from a genetic
cross can be determined by drawing a diagram known as a
Punnett square
The types of gametes
produced by each F1 parent
are shown along the top and
left sides of the square
The possible gene combinations
for the F2 offspring appear in the
4 boxes that make up the square
The letters represent the different alleles;
T = the tall allele and t= the short allele
Organisms that have 2
identical alleles (TT and
tt) are homozygous
(true-breeding)
Organisms that have 2
different alleles for a trait
(Tt) are heterozygous
(hybrids)
All of the tall plants have
the same phenotype =
physical characteristic
However, they do not
have the same genotype =
genetic makeup
Punnett squares can be used to predict and compare the
genetic variations that will result from a cross
Taking a look at the cross, we
can see that ¼ of the F2 plants
have 2 alleles for tallness (TT)
2/4, or ½ of the F2 plants
have one of each allele (Tt)
And ¼ of the F2 plants have
2 alleles for shortness (tt)
Since the allele for tallness is
dominant, ¾ of the F2 plants
should be tall (true!)
Overall, there are 3 tall plants
for every short plant, so there
is a phenotypic ratio of 3:1
Probabilities are better for
predicting the outcome of a
large number of events,
rather than a particular event
Section 11-3:
Exploring Mendelian Genetics
After showing that alleles segregate during the formation of
gametes, Mendel wondered if one set of alleles would have
an effect on another or if they were independent?
To answer the question, Mendel
performed an experiment to follow
two different genes - this was
known as a two-factor cross
First, Mendel produced plants that only
produced round, yellow peas (genotype RRYY)
and wrinkled, green peas (genotype rryy)
He crossed these plants and found that all of
the F1 offspring produced round, yellow peas
– this just showed that yellow and round are
dominant over green and wrinkled
This Punnett square shows the
cross and that all offspring had
a genotype of RrYy
Next, he did a cross with two individuals from the
F1 generation – this is where he would be able to see
if the genes segregated independently
Mendel found that along with
phenotypes of the parents, he
This meant that the
had new phenotypes; round,
alleles for shape & color
green peas and wrinkled,
segregated independently
yellow peas
– they did not influence
each other
His results were very close to a 9:3:3:1 phenotypic ratio
The principle of independent
assortment states that genes
for different traits can
segregate independently
during the formation of
gametes
There are some exceptions to Mendel’s principles: some
alleles are neither dominant nor recessive, and many traits
are controlled by multiple alleles or multiple genes
Incomplete dominance happens
Ex. In crossing a red
when one allele is not completely
flower (RR) with a white
dominant over another
flower (WW), the F1
generation will consist of
In incomplete dominance, the
pink flowers (RW)
heterozygous phenotype is
somewhere in between the two
homozygous phenotypes
Codominance happens when both
alleles contribute to the phenotype
(similar to incomplete dominance)
In codominance, the heterozygous
phenotype will show both of the
homozygous phenotypes instead
of blending them
Ex. In some chickens, the
allele for black feathers is
codominant with the allele
for white feathers 
heterozygous chickens have
feathers that are speckled
with both black and white
Multiple alleles – many genes
have more than 2 alleles
Individuals don’t end up with
more than 2 alleles (they can
only get one from each parent),
it just means that there are
Ex. Rabbit’s coat color: the
more than 2 possibilities
coat color of a rabbit is
controlled by a gene that has
at least 4 different alleles
Polygenic traits – traits that are
controlled by 2 or more genes
Ex. There is a wide range in
skin color in humans because
more than 4 genes help
control this trait
Ex. Human eye color is
controlled by at least 3
different genes so there is a
wide variety of eye colors
Mendel’s principles don’t only
apply to plants, but they apply
to animals and humans as well
Another important organism
used to study genetics is the
common fruit fly
Fruit flies have many traits to
study, they produce many
offspring, and have short
lifespans