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Mendel and Heredity
What does segregation imply?
This happens with your chromosomes
We have 2 copies for each chromosome
but can only give 1 copy to the
gametes
So the 2 copies you have separate or
segregate when they move to the
gametes
Mendel and Heredity
Mendel’s conclusions:
1. Traits are inherited as units and are not
blended or diluted over generations
2. Organisms inherit 2 copies of each
gene, one from each parent
3. Organisms donate only 1 copy of
each gene in their gametes.
Traits, Genes, and Alleles
These genes have a locus, specific
place, or address on the DNA.
Alleles are the different forms of a
gene that occurs at that locus
You have 2 alleles for each gene
(you got one from each parent)
They are located on homologous
chromosomes
Process of Meiosis
OBJECTIVE: Understand how diploid cells
undergo 2 cell divisions to produce haploid cells
•Meiosis produces 4 haploid cells from a
diploid cell
•This is broken into Meiosis I and Meiosis II
•Each round has 4 phases (similar to mitosis)
•Its important to pay attention to how
chromosome numbers are reduced
•Remember what homologous chromosomes
and sister chromatids are…
Process of Meiosis
•Homologous: separate chromosomes, 1 from
mom and 1 from dad (code for same trait)
•Sister: duplicated chromosomes attached at
the centromere (copies / identical)
•Meiosis I
1) Interphase: G1, S, G2
2) Prophase 1: nuclear membrane breaks down,
centrioles move to opposite sides of cell,
spindle fibers form. Chromosomes condense.
Homologous chromosomes pair up
Process of Meiosis
3) Metaphase 1: Chromosome pairs are
randomly lined up at the center of the cell
Meaning which chromosomes get passed is
random (I’ll explain)
Total of 8,388,608 possible combinations of
chromosomes (not including mutations) for
parent’s gametes
4) Anaphase 1: Paired homologous
chromosomes separate and move to opposite
sides of the cell (sister chromatids stay together)
Process of Meiosis
5a) Telophase 1: nuclear envelope reforms and
spindles break down
5b) Cytokinesis 1: produces 2 diploid cells
MEIOSIS II
No interphase!
All of these
steps are taking
place in 2 cells
1) Prophase 2: nuclear envelope breaks down,
centrioles move to opposite sides of cell, and
spindle fibers form
Process of Meiosis
2) Metaphase 2: spindle fibers align the 23
chromosomes at the center of the cell
(sister chromatids still attached)
3) Anaphase 2: sister chromatids are pulled
apart and move to opposite sides of cell
4a) Telophase 2: nuclear envelopes form around
each set of chromosomes (on opposite sides)
and spindle break down.
4b) Cytokinesis 2: produces 4 haploid cells
Process of Meiosis
Key Differences between Meiosis and Mitosis
# of divisions
# of cells produced
These haploid cells will become gametes
following Gametogenesis – process that
completes gamete formation
Process of Meiosis
Males
Produces 4 viable
haploid gametes
- sperm
-smaller than egg
-must swim to egg
How?
Whiplike flagellum
Females
Produces only 1
viable haploid
gametes
-egg
-uneven divisions of
cytoplasm produces 3
polar bodies. Why?
Must nourish the
fertilized egg and be
easier to find
Meiosis and Genetic Variation
OBJECTIVE: Understand causes of genetic
diversity (ex. crossing-over)
Sexual Reproduction gives us unique
combinations of genes
-due to (1) independent assortment
(2) Random gamete fertilization
Remember the possible number of
chromosomal combinations? 8,388,608
So, you take that number for each parent and
multiply them together
Meiosis and Genetic Variation
That gives us over 70 trillion possible
combinations of chromosomes
This is what makes us all genetically unique!!
Greater variation occurs during a process
called crossing over
This is where homologous chromosomes
exchange segments during Prophase I
Figure 6.20 (pg 190)
Sometimes occurring many times on the same
chromosomes
Meiosis and Genetic Variation
Recombination – refers to the mixing of
parental alleles
How come it appears that some traits are
inherited with other traits?
The reasoning for some of these trait is due to
their genetic linkage – tendency for genes to
be inherited together
Locus plays a role in this linkage. The closer
genes are on the same chromosome the more
likely they are to be “linked”