Meiosis and Mendel
Section 6.1- Chromosomes
 You have many different types of cells in your body, but
there are only 2 general types.
 The differences in these two is the amount of
chromosomes in them.
 Somatic Cells
 Gamete Cells
Section 6.1- Chromosomes
 Somatic Cells
 Are also called body cells
 These are the cell that make up
your body tissues and organs

Everything from eyeballs to your
heart
 Your body cells are not passed on
to offspring
 Somatic cells are DIPLOID!
Skin Cells
Section 6.1- Chromosomes
 Gamete Cells
 These are sex cells
 The sex cells are eggs in females and sperm in males
 The two cells come together to make the 1st cell of a new
human being.
 Gamete cells are HAPLOID!
Section 6.1- Chromosomes
 All organisms have a different number of
chromosomes per cell.
 The number of chromosomes; however,
is not related to the complexity of the
organism.
 For example, yeast have 32 chromosomes
and a fern plant has 1200 chromosomes.
 Humans have 46 chromosomes that
come in 23 pairs.
 The diploid number for humans is 46
and the haploid number is 23.
Section 6.1- Chromosomes
 So, in each cell in your body
(except your sex cells) you have
46 chromosomes.
 These 46 chromosomes come
in 23 sets, and for each set one
chromosome came from your
mother and one came from
your father.
 Together, each pair of
chromosomes is known as a
homologous pair.
Section 6.1- Chromosomes
 homologous chromosomes are two chromosomes
that have the same general shape and structure.
 One of these came from the mother and the other from
the father.
 Most importantly, these homologous chromosomes
have copies of the same genes on them.
 These copies may be different copies, but they are
copies of the same gene.
Section 6.1- Chromosomes
 Of the 23 pairs of chromosomes we have, the first 22
pairs are known as autosomes.
 These are chromosomes that contain genes for
characteristics not directly related to the sex of an
organism.
 The chromosomes in the first 22 pairs code for
everything except sex.
Section 6.1- Chromosomes
 The 23rd pair of chromosomes are called your sex





chromosomes.
This pair controls what sex that organism is going to
be.
The two possible sex chromosomes are “X” and “Y”.
If an organism has “XX” pair, that organism is a female.
If an organism has “XY” pair, that organism is a male.
If you are a male- your 23rd set of chromosomes are not
homologous because they are not identical.
Section 6.1- Chromosomes
 Males determine the sex
of an offspring because
they are the only one that
can give a “Y”
chromosome.
 When sexual
reproduction occurs, two
gametes come together
resulting in a genetic
mixture of both parents.
 This is called
fertilization.
Section 6.1- Chromosomes
 This 1st cell of the new organism must have the correct
number of chromosomes (46 in humans).
 This is why all sex cells are in the haploid condition.
 This means that these cells only have one copy of the
homologous pair.
 If these cells were diploid, (which is the full number of
chromosomes), when they came together during
fertilization there would be double the number of
chromosomes needed (92 in humans).

This would cause mass mutations and spontaneous abortion
of the cell.
Section 6.1- Chromosomes
 The correct number of chromosomes must be
maintained for the organism to survive.
 Also remember that having too many chromosomes is
just as harmful as not enough.



Down Syndrome- caused by the presence of an extra
chromosome 21
Patau Syndrome- caused by the presence of an extra
chromosome 13
Turner’s Syndrome- children only have one X chromosome
and no Y chromosome, so they have only 45 chromosomes.
Section 6.1- Chromosomes
 How do organisms maintain chromosome numbers?
 How do sex cells have a different number of
chromosomes than all other cells?
Section 6.1- Meiosis
 Meiosis is similar to mitosis in that it is a form of
nuclear division.
 However, unlike mitosis, meiosis reduces the number
of chromosomes to haploid numbers.
 Sex cells undergo cellular division called MEIOSIS
Section 6.1- Meiosis
 Mitosis and Meiosis both divide the nucleus but there
are differences!
MITOSIS:
•Produces identical cells
•Results in diploid cells
•Takes place all through life
MEIOSIS:
•Produces unalike cells
•Results in haploid cells
•Occurs only at certain times
 Meiosis’s most important feature is that it reduces the
number of chromosomes in half so when a sperm and
egg come together, the diploid number is restored.
Section 6.2- Process of Meiosis
 One round of meiosis yields 4 unidenitcal sex cells
from one diploid cell.
 The chromosomes get reduced because meiosis goes
through division twice.
 Meiosis’s phases are the same in name as mitosis;
however, there are two rounds of division without any
replication between meiosis I and meiosis II.
Section 6.2- Process of Meiosis
 Meiosis I:
 Prophase I
 Metaphase I
 Anaphase I
 Telophase I
 Meiosis II:
 Prophase II
 Metaphase II
 Anaphase II
 Telophase II
Section 6.2- Process of Meiosis
 In Meiosis I, paired homologous chromosomes split
and in meiosis II the duplicated chromosomes split
so you end up with 4 unidentical cells in the haploid
condition.
 The initial cell has 4 chromosomes and the 4 cells in
telophase II have 2 chromosomes.
Section 6.2- Process of Meiosis
 Haploid cells are the end result
of meiosis
 However, these cells are not yet
ready to fertilize and must go
through more changes that
“mature” the cell.
 In sperm production, all sperm
cells formed from meiosis are
functional and ready to
fertilize.
 In egg production, only 1 out of
every 4 cells become an egg
because as meiosis occurs the
cytoplasm divide unevenly.
Section 6.3- Mendelian Genetics
 When we talk about differences among organisms, we
talk about their traits.
 TRAITS are characteristics that are inherited.
 GENETICS is the study of how these traits get passed
on from parent to offspring
Section 6.3- Mendelian Genetics
 Initial studies in genetics
started in the mid 1800’s
by an Austrian monk
named GREGOR
MENDEL.
 Most scientist believed at
this time that offspring
were a blend of their
parents.
 For examples, if one
parent was tall and one
was short, then the
offspring would be
medium in size.
Section 6.3- Mendelian Genetics
 Mendel didn’t believe this theory because there were
too many traits that remained “undiluted”

meaning they were not blends.
 So, Mendel started doing thousands of test crosses
breeding plants.
 He was a mathematician by trade so he analyzed all of
these results.
 Mendel primarily used the pea plant because
 It reproduced quickly
 Its traits were easy to see
Section 6.3- Mendelian Genetics
 As Mendel started his studies, he took over the monastery
garden.
 He made sure to use only purebred pea plants to start with
 This way he knew what the traits were for all his starting pea
plants.
 Purebred means both copies of a gene are the same
 We represent genes with letters.

Purebred:
 TT
 Tt
 Rr
 RR
Section 6.3- Mendelian Genetics
 Remember Mendel chose pea plants because they
reproduce quickly.
 He could also control how they mated.
 He allowed certain plants to self-pollinate over and
over again until he was sure he had purebred plants.
 It was vital that Mendel start his experiments with
purebreds so he knew what he had at the beginning.
Section 6.3- Mendelian Genetics
 Mendel than removed the male part of the plant of
his purebreds
 he then could control what plant pollinated what
plant.
 In doing this Mendel knew any variation in pea plant
offspring was a result of his experiment and not any
other random crossing.
Section 6.3- Mendelian Genetics
 Mendel chose 7 specific
traits to follow:
 Pea Shape
 Pea Color
 Pod Shape
 Pod Color
 Plant Height
 Flower Color
 Flower Position
Section 6.3- Mendelian Genetics
 All of the traits
Mendel looked at
were simple “EitherOr” traits.
 Plant height= tall
or short
 Pod shape=
smooth or
constricted
Section 6.3- Mendelian Genetics
 In genetics, when you mate two organisms, this is
called a cross.
 The 1st plants Mendel started with is the P1
generation or parental.
 The offspring of the P1 is called F1 (filial).
 Offspring of F1 is the F2 and so on….
Section 6.3- Mendelian Genetics
 Mendel took all 7 of the traits he indentified and
crossed the two purebred conditions for each trait.
 Round x Wrinkled
 Yellow x Green
 Smooth x Constricted
 Green x Yellow
 Tall x Short
 Purple x White
 Axial x Terminal
Section 6.3- Mendelian Genetics
 What Mendel found in the (F1) was that all the
offspring looked like one of the two parents and the
trait of the other parent seemed to disappear.
 Tall x Short = ALL TALL
 Purple x White = ALL PURPLE
Section 6.3- Mendelian Genetics
 Mendel then allowed his F1
plants to self- pollinate and
what he discovered was
that most of the offspring
(F2) looked like the
parents, but a small
percentage of them looked
like the P1 parent whose
characteristic disappeared
in the F1.
 There was about ¾ of the
offspring that looked like
F1 and ¼ that looked like P1
parent that disappeared.
Section 6.3- Mendelian Genetics
 Summary of what Mendel has done to this point:
P1 Cross:
Tall Plant x Short Plant
F1:
All tall plants
F1 plants are allowed to
self-fertilize
F2:
¾ Tall Plants
¼ Short Plants
Section 6.3- Mendelian Genetics
 From these initial crosses, Mendel came up with 3
important conclusions:
 Traits are inherited separately as discrete units
 Organisms inherit 2 copies of a gene, one from each
parent
 Organisms donate only one copy of each gene in their
gametes, so genes separate upon gamete formation
Section 6.3- Mendelian Genetics
 The last two points make up the LAW OF
SEGREGATION
 Genes of traits don’t stick together
 We will learn why this is in the following sections when
we start doing crosses!