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CHAPTER 10.1
MEIOSIS
Chromosomes & Chromosome Number
Meiosis
 The Human body has 46 chromosomes
 The reproductive cells (Egg and Sperm) have 23
chromosomes
 So each parent contributes 23 chromosomes to = the 46
chromosomes we need
 The process of reducing the number of chromosomes from
46 to 23 is called Meiosis
 Homologous chromosomes: are a pair of
chromosomes.
 The 46 chromosomes create 23 pairs of
homologous chromosomes:
 1. same length
 2. same centromere position
 3. Carry genes that control
Inherited traits
 Homologous chromosomes: are a pair of
chromosomes.
 The 46 chromosomes create 23 pairs of
homologous chromosomes:
 1. same length
 2. same centromere position
 3. Carry genes that control
Inherited traits
 An organism produces gametes to maintain
the same number of chromosomes from
generation to generation
 Human gametes contain 23 chromosomes.
 Gametes are sperm or egg.
 A cell that contains 2n (46) chromosomes is
called a diploid cell
23 Chromosomes
-haploid (n)
46 Chromosomes
-diploid (2n)
Meiosis
Reduces the chromosome number
by half through the separation of
homologous chromosomes
Involves 2 consecutive cell
divisions:
1. Meiosis I
& 2. Meiosis II
Meiosis 1
 INTERPHASE
 Chromosomes replicate.
PROPHASE 1
 Homologous chromosomes pair up
 Each chromosome consists of
two chromatids
 The nuclear envelope breaks down
 Spindle forms
Metaphase 1
 Chromosome centromeres
attach to spindle fibers.
 Homologous chromosomes
line up at the equator.
Anaphase 1
Homologous chromosomes separate
and move to opposite poles of the cell
Telophase 1
 The spindle breaks down
 The cell divides
Meiosis II
Prophase II
 A second set of phases begin
as the spindle apparatus forms
and the chromosomes condense.
 Metaphase II
 A haploid number of chromosomes
line up at the equator
Anaphase II
The sister chromatids are pulled apart
at the centromere by spindle
fibers and move toward the opposite
poles of the cell
 Telophase II
 The chromosomes reach the poles,
and the nuclear membane
and nuclei reform
 Cytokinesis: results in four
haploid cells, each with n number
of chromosomes.
Meiosis Results in:
 Produces 4 haploid daughter cells that are NOT identical
 Skim Read: pg. 277-282
 Homologous chromosomes: are a pair
of chromosomes.
 The 46 chromosomes create 23 pairs of
homologous chromosomes:
1. same length
2. same centromere position
3. Carry genes that control
Inherited traits
Meiosis Provides Variation
 Depending on how the chromosomes line
up at the equator, four gametes with four
different combinations of chromosomes can
result.
 Genetic variation also is produced during
crossing over and during fertilization, when
gametes randomly combine.
 In Prophase I: Crossing over produces
exchange of genetic information.
Crossing over—chromosomal segments are
exchanged between a pair of homologous
chromosomes.
Genetic Variation increases a species
ability to survive and adapt
 Meiosis = 4 Different Gametes
 Crossing over increases the Genetic
Variation of the gametes
 Sexual Reproduction: Two different sets of
genes are creating a new individual. One
from MOM and the other from DAD
 Unlike asexual reproduction, where the
organism inherits all of its chromosomes
from a single parent (genetically identical
to parent.)
How Genetics Began:
 Gregor Mendel, and Austrian monk and plant breeder
 He studied how traits are passed on in pea plants
 Flowering plants are often able to self- fertilize and
cross-pollination.
 Cross- pollination is reproduction between 2 plants. For
pea plants Mendel had to do this by hand.
 The passing of traits to the next generation is called
inheritance, or heredity.
 Mendel started the branch of science we call Geneticsthe science of heredity
In Mendel’s Experiments:
 He studied seven different traits. Pea plants often
form offspring with a specific trait such as yellow
seeds.
 He cross-pollinated pea plants with different traits to
see what the outcome would be…….
 The seven traits he looked at are







Seed or pea color
Flower color
Seed pod color
Seed shape or texture
Seed pod shape
Stem length
Flower position
Mendel’s Experiments with True –
breeding pea plants
The parent generation is
known as the P generation.
The offspring of this P
cross are called the F1
generation.
The offspring of the F1
generation are called the F2
generation.
True- breeding pea plants
that are cross- pollinated
Mendel concluded from his
experiments:
 That there must be two forms of a each trait in
the pea plants. Such as yellow seed color and
green seed color.
 Those two forms are controlled by alleles.
 Alleles are an alternative form of a single gene
passed from generation to generation
 Alleles are found on chromosomes
•
Alleles
One of the alleles is dominate over the
other.
 Dominate traits mask recessive traits
 What is the
Dominate trait in
this cross?
 The dominate trait is yellow seed color.
 In the F1 generation the only seed color you see
is yellow. Showing you that it is dominate over
the green seed color.
 Dominate traits are written using a Capitol Letter
 Recessive traits are written using a lower case
letter
The dominate yellow seed would be represented by Y
The recessive green seed would be represented by a y
Alleles on chromosomes can have:
. the same alleles for a particular trait is called
homozygous.
• OR
 2. two different alleles for a particular trait is called
heterozygous.
 1
The Alleles on the chromosome determine the:
 Genotype: what an organisms allele types
are (the genes) ex.YY ,Yy ,yy
 Phenotype: what an organism looks like
Mendel’s Law of Segregation
 Two alleles for each trait separate during meiosis.
 During fertilization, two alleles for that trait unite.
• Heterozygous organisms are called
hybrids.Yy
Monohybrid Cross
 A cross that involves hybrids for a single
trait is called a monohybrid cross.
 In this example the
Single trait is seed color.
DIHYBRID CROSS
 Predicting the inheritance of two or more traits in
the same cross.
 EXAMPLE: Looking at seed color and
 Seed Color- Yellow
 Seed type
Y
Round R
 YyRr (genotype)
Yellow seed
& round (phenotype)
seed type.
green
y
wrinkled r
X
yyRR (genotype)
Green seed
& round (phenotype)
Law of Independent Assortment
 Random distribution of alleles occurs
during gamete formation
 Genes on separate chromosomes sort
independently during meiosis
 Each allele combination is equally likely
to occur.
Punnett Squares – Monohybrid
 Are used to predict the possible offspring of
a cross between two known genotypes
Punnett Square—Dihybrid Cross
 Four types of alleles from the male gametes and four
types of alleles from the female gametes can be
produced.
The resulting phenotypic
ratio is 9:3:3:1.
Probability
 Each time you perform a cross you have the same
probability of each outcome occurring.
 Like flipping a coin-
You have a 50% chance of
getting heads or tails
each time you flip a coin.
What you flipped before
also does not matter.
CHAPTER 10.3 Gene Linkage
 The linkage of genes on a chromosome results in an exception to
Mendel’s law of independent assortment because linked genes
usually do not segregate independently.
 Polyploidy is the occurrence of one or more
extra
sets of all
chromosomes
in an organism
A triploid organism,
For instance, would be
designated 3n, which means that it has
three complete sets of chromosomes