10.2 Mendel and
Meiosis
10.2 Meiosis
*Organisms have tens of thousands of genes that
determine individual traits. Genes are lined up
on chromosomes, which exist in the nucleus of
a cell. A thousand or more genes are arranged
on a single chromosome!
In the body cells of animals (us!) and most plants,
chromosomes occur in pairs: 1 chromosome
from the male parent and the other
chromosome from the female parent.
Diploid Cell: a cell with two of each kind of
chromosome; (2n)
In the sex cells/gametes of animals (us!) and
most plants, only 1 of each type of chromosome
is present.
- Haploid Cell: a cell with one of each
- kind of chromosome, (n)
Why would a cell need only ½ the # of
chromosomes?
Because two gametes = a new organism!
o Egg (n) + sperm (n) = a baby (2n)
23 chrom. + 23 chrom. = 46 chromosomes
Each species of organism contains a
certain specific, characteristic # of
chromosomes.
The # of chromosomes is NOT related to the
complexity of the organism.
Chromosome #’s of Some Common Organisms
Organism
Body cell (2n) Gamete (n)
Garden pea plant
14
7
Apple
34
17
Human
46
23
Chimpanzee
48
24
Dog
78
39
The paired chromosomes in a diploid (2n)
cell help determine what the organism will
look like, and are called homologous
chromosomes: paired chromosomes
(paired during meiosis) that have genes for the same
traits, arranged in the same order
Ex:
Homologous
Chromosomes:
Karyotype
Because there are different ‘versions’
(alleles) for the same gene, the two
chromosomes in a homologous pair are not
always identical.
This organism is heterozygous
for each of the 3 traits shown
here → (a homologous pair)
[During meiosis, homologous chromosomes are made up of
chromosome pairs for genes with the same corresponding loci.
One homologous chromosome is inherited from the organism's
mother; the other is inherited from the organism's father.]

Think back to Ch. 8 when we talked about
Mitosis: cell division that produces two
identical daughter cells that have the same
# of chromosomes AND carry out the exact
same functions as the parent cell

Cells → tissues → organs → organ systems.

If mitosis were the only means of cell
division, then gametes would also have the
same # of chromosomes as the parent cells.
 If
these gametes were to fuse during
fertilization, the resulting offspring
would have TWICE the number of
chromosomes than normal!
Ex.: egg (46 chrom.) + sperm (46 chrom.) = a
baby with 92 chromosomes!!
*The nucleus would be too crowded!!
Obviously, there must be another type
of cell division…
Meiosis: cell division that produces gametes
containing ½ the # of chromosomes (n) as
the parent’s body cells
Meiosis consists of two separate divisions:
1. Meiosis I begins with one diploid cell (2n)
2. Meiosis II ends with four haploid cells (n)
*Meiosis occurs only in specialized body cells
found in the gonads of each parent:
Gonads: organs that make gametes;
Male gonads = testes; which produce…
- male gametes = sperm
Female gonads = ovaries; which produce…
- female gametes = eggs
- When a sperm (n) fertilizes an egg (n), the
resulting cell, called a zygote, once again
has the diploid (2n) # of chromosomes!
The zygote () can then develop by mitosis
into a multicellular organism.
Sexual reproduction: the pattern of
reproduction that involves the production and
fusion of haploid cells
*Remember the phases of mitosis:
P.M.A.T.? (prophase, metaphase,
anaphase, telophase)
In meiosis, we have the EXACT SAME PHASES as
in mitosis, except they occur twice!
The Phases of Meiosis
Interphase:
The period during which the cell makes a
copy of its chromosomes, in preparation
for cell division
Each chromosome now consists of 2
identical sister chromatids, held together
by a centromere
Meiosis I:
Prophase I:
- chromosomes coil and become
visible
Spindle forms (fibers that pull
chromosomes apart)
Homologous chromosomes come
very close together, matched gene
by gene, to form a tetrad
Crossing over occurs: when nonsister chromatids from homologous
pairs exchange genetic material
with each other
This results in new combinations of
alleles and even more random
mixing of genes which increases
genetic variety!
Tetrad
Metaphase I:
Spindle fibers attach to centromeres
Tetrads are pulled to the middle of the cell
Anaphase I:
Homologous chromosomes are pulled apart,
however, sister chromatids stay together (for
now)
Telophase I:
Spindle breaks down
Cytoplasm divides (2 new cells)
This is where mitosis stops! However, the result is
two diploid (2n) cells, and we need haploid (n)
cells to make gametes, so we must keep going!
Meiosis 1
Meiosis II: (more like mitosis)
Prophase II:
-spindle forms
-spindle fibers attach to sister chromatids
Metaphase II:
-chromosomes (still made up of sister
chromatids) are pulled to the middle of
the cell and line up randomly along the
equator
Anaphase II:
-centromere of each chromosome splits
-sister chromatids separate and move to
opposite ends of the cell
Telophase II:
-spindle breaks down
-nuclei reform
-cytoplasm divides
The result is 4 haploid cells / gametes,
which each contain 1 chromosome
from each homologous pair!
These gametes will then transmit their
genes to the offspring they help
create!
Meiosis II
Mitosis vs. Meiosis
Meiosis Provides for Genetic Variation
*Meiosis (sexual reproduction) →
the shuffling of chromosomes →
genetic variation!
How is it that there are so many
varieties of genotypes possible?
Aren’t we going to run out of
combinations one day and people
will start to look exactly alike?
*Three Processes That Lead to Genetic Variation:*
1. The Independent Assortment of Chromosomes:
During Metaphase I, the chromosomes line up
randomly and it’s only a matter of chance which
gametes will get which chromosomes.
2. Crossing Over:
-This happens during Prophase I, when
homologous chromosomes exchange genetic
material between non-sister chromatids.
-Two to three cross-overs usually occur per
chromosome!
3. Random Fertilization of an Egg by a Sperm:
-It is impossible to determine which egg and
which sperm will fuse to form a zygote.
Here are the statistics:
Egg = (n) = 23 chromosomes
These chromosomes can line up in two
different ways during Metaphase I
223 = 8,388,608 different
kinds of eggs
Sperm = (n) = 23 chromosomes
These chromosomes can line up in two
different ways during Metaphase I
223 = 8,388,608 different kinds of sperm
Fertilization = 223 x 223 = >70 trillion
different zygote possibilities PER
COUPLE, PER CHILD!
Therefore, one couple is capable of
conceiving a number of genetically
different offspring that is far greater than
the number of human beings who have
ever lived!!
*Add to this crossing over, as well as
genetic mutations that could occur, and
there are nearly an endless number of
different possible genetic combinations.
Mistakes in Meiosis
Nondisjunction: the failure of homologous
chromosomes to separate properly during meiosis
Trisomy: a gamete with an extra chromosome is
fertilized
Zygote has extra chromosome
Ex.: Down syndrome (Trisomy 21)
 47 chromosomes instead of 46
Monosomy: a gamete with a missing
chromosome is fertilized
Zygote has one less chromosome
(usually do not survive)
Ex. (above): Turner syndrome (female has
only 1 X chromosome instead of XX)
Polyploidy: an
organism inherits
a complete diploid
(2n) set of
chromosomes
(instead of (n))
from either one or
both gametes
Rare in animals
(usually lethal), but
common in plants →
usually leads to
larger, healthier
plants
X = Female chrom. Normal female = XX
Y = Male chrom.
Normal male = XY
XXY: Klinefelter syndrome:
-Male sex organs (small)
-Sterile
-Feminine characteristics
-Normal intelligence
XYY: No characterized
syndrome
-Taller than average
XXX: you would never know except by doing a
karyotype of your DNA
Monosomy X: Turner syndrome
-Only known monosomy in humans to survive
-Female organs do not mature
-Sterile
-Most have normal intelligence
*Meiosis, however, usually proceeds accurately.