Download Meiosis

What we know so far…
• DNA is the instructions for your cells
• DNA is like a cookbook – the recipes are
GENES
• Each GENE provides instructions for
making a specific protein
• Specific proteins have specific shapes –
mutations in the instructions (DNA) can
result in changes in the protein.
Blonde hair
Blue eyes
Hitchhiker’s thumb
detattached / attached earlobes
widow’s peak
How are these traits
passed from one
generation to the next?
MEIOSIS!!
A VERY SPECIAL TYPE OF CELL
DIVISION.
Words you need to know:
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Mitosis
Diploid
Haploid
Homologous
Chromosome
• Meiosis
• Sperm
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Egg
Sexual Reproduction
Crossing Over
Genetic
Recombination
• Nondisjunction
• Allele
You have 46 Chromosomes
• Each of your body cells has 46
chromosomes, or 23 pairs.
• Each pair is made of HOMOLOGOUS
CHROMOSOMES
Homo = same
These chromosomes contain the same genes
in the same order
You have 46 Chromosomes
• Half of your chromosomes (23) came from
your mom
• Half of your chromosomes (23) came from
your dad
Each parent gave you one chromosome from
a homologous pair.
Homologous Pairs
gene
From Mom
From Dad
From Mom
From Dad
MITOSIS Review
• When your BODY cells (autosomal) divide,
they make exact copies
• Results = two cells with the same type and
number of chromosomes as the parent
cell.
• Many animals reproduce ASEXUALLY –
through mitosis
ex: flatworms, sponges, jellyfish
This is a problem for most other
animals.
If humans were created through mitosis:
1st generation
46 from mom + 46 from dad = 92 chromosomes for baby
then
2nd generation
92 from mom + 92 from dad = 184 chromosomes for baby
And so on, and so on, and so on…
MEIOSIS
• A second type of cell division that occurs
in the SEX CELLS (gametes: sperm and
egg)
• Division creates four cells with half the
number of chromosomes as the parent
cell.
Haploid vs Diploid
• Body cells (autosomal cells) are DIPLOID
• DI = 2; Each cell has 2 copies of each
chromosome
• These are considered “2n”
– “n” is the chromosome number
• Sex cells are HAPLOID
• Each sex cell has HALF the normal
number of chromosomes
•These are considered “n”
Sperm and Egg
• Sperm are the sex cells in males
• Eggs are the sex cells in females
Both are produced through MEIOSIS
Any reproduction that involves SPERM
and EGGS is considered SEXUAL
REPRODUCTION.
Steps of MEIOSIS
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Prophase I
Metaphase I
Anaphase I Division 1
Telophase I
Prophase II
Division 2
Metaphase II
Anaphase II
Telophase II
• Looks a lot like
Mitosis, huh?
• Meiosis has 2
DIVISIONS
How does this result
in unique
individuals?
Crossing Over
• Prophase 1: Homologous Chromosomes
line up next to each other
– CROSSING OVER
Homologous
chromosomes exchange
sections =
Genetic Recombination
Crossing Over
• Crossing over creates new genetic
combinations
• This results in new and unique individuals
This is why you look similar to, but not
identical to, your brothers and sisters!
Mendel’s Laws of Heredity
Gregor Mendel worked with pea plants in
the mid – nineteenth century
When he crossed tall pea plants and short
pea plants, all offspring were tall.
These were F1 generation
Mendel’s Laws of Heredity
When he crossed
the F1 generation,
the offspring were
mixed Tall and
Short.
hmmmmmmmmmm.
These were the F2
generation
Mendel’s Laws of Heredity
1. The rule of unit factors
each organism has two “factors” that
control the traits
Now, we call those factors GENES.
A homologous pair can have two variations
of the same genes.
Mendel’s Laws of Heredity
Identical ALLELES
Different ALLELES
TALL GENE
TALL GENE
TALL GENE
SHORT GENE
mom dad
mom
dad
Mendel’s Laws of Heredity
2. The Rule of Dominance
Alleles (variations of a gene) can be
DOMINANT or RECESSIVE
Dominant genes contain instructions for a
functional protein
Recessive genes contain instructions for a
non-functioning protein
Mendel’s Laws of Heredity
For Example:
Cystic Fibrosis is caused by a non-functional
protein (recessive gene)
To have Cystic Fibrosis you must have 2
recessive alleles – one from each parent.
This is called HOMOZYGOUS recessive.
HOMO = same
Mendel’s Laws of Heredity
It is possible to have one dominant allele (a
functional protein) and one recessive allele
(non-functioning protein).
This is called HETEROZYGOUS.
HETERO = different
These individuals are carriers.
Mendel’s Laws of Heredity
3. The Law of Segregation
During meiosis, homologous chromosomes
separate and each sex cell receives one of
the homologues.
Long chromosomes
are HOMOLOGUES
Short chromosomes
are HOMOLOGUES
Errors in MEIOSIS
• Nondisjunction – homologous
chromosomes do not separate during
meiosis; results in gametes with too many
chromosomes
EX. Down Syndrome
Individuals with an unusual number of
chromosomes are POLYPLOID.
Polyploidy
• Some chromosome abnormalities do not
cause disease, however abnormal
numbers of chromosomes can be lethal or
give rise to genetic disorders.
– Cri-du-chat
– Down Syndrome
– Edwards Syndrome
– Patau Syndrome
Punnett Squares
• Allow you to make predictions of offspring
GENOTYPE and PHENOTYPE
Genotype = the actual genes an individual
has
Phenotype = the physical appearance of an
individual
Punnett Squares
• Genotype can be HOMOZYGOUS or
HETEROZYGOUS
Homozygous = both alleles (copies of the
gene) are the same – both genes for eye
color are “blue”
Heterozygous = both alleles are different –
one gene for “blue,” one gene for “brown”
Punnett Squares
• Genotypes are written using letters. Each
letter represents one allele
All genotypes will have 2 letters
• Dominant genes (functional protein) are
CAPITALIZED
• Recessive genes (non-functional protein)
are lowercase.
Punnett Squares
Example: Brown eyes are dominant, blue eyes are
recessive.
Genotype of an individual with brown eyes:
BB (homozygous) or Bb (heterozygous)
Genotype of an individual with blue eyes:
bb (only homozygous)
To have a recessive trait, you need to have both
recessive alleles – one from mom, one from dad.
Recessive traits are always HOMOZYGOUS.
Punnett Squares
• Phenotypes are written as descriptions;
blonde hair, brown hair, brown eyes, blue
eyes, etc.
Punnett Squares
Example: Black fur is dominant to white fur
in rabbits.
What are the possible genotypes for a black
rabbit? BB, Bb
What are the possible genotypes for a white
rabbit?
bb
Cross 2 white rabbits
b
b
b
b
bb
bb
bb
bb
Punnett Squares
• Monohybrid cross: only work with ONE
gene/trait
• Determine what gametes are formed
– Homologous pairs are split up, one goes to
each gamete
– Gametes from male and female are combined