1. Introduction to Genetics: Meiosis
Ch. 11
Ms. Haut

2. Introduction to Heredity
Each organism must inherit a single copy of every gene from each of its “parents.”
Offspring acquire genes from parents by inheriting chromosomes
Inheritance is possible because:
Sperm and ova carrying each parent’s genes are combined in the nucleus of the fertilized egg

3. Actual passing on of genes depends on the behavior of chromosomes
Chromosomes -organizational unit of hereditary material in the nucleus of eukaryotic organisms
Contain hundreds of thousands of genes, each of which is a specific region of the DNA molecule, or locus

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Chromosome Number
Chromosome Number
All organisms have different numbers of chromosomes.
A body cell in an adult fruit fly has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent.
These chromosomes are from a fruit fly. Each of the fruit fly’s body cells has 8 chromosomes.
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Chromosome Number
These two sets of chromosomes are homologous.
Each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent.
Chromosomes are the same size and shape and have the same genes in the same locations
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Chromosome Number
A cell that contains both sets of homologous chromosomes is said to be diploid.
The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N.
For Drosophila, the diploid number is 8, which can be written as 2N=8.
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Chromosome Number
The gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes.
These cells are haploid. Haploid cells are represented by the symbol N.
For Drosophila, the haploid number is 4, which can be written as N=4.
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8. Human Life Cycle
Each human somatic cell (body cell) has 46 chromosomes or 23 matching pairs (diploid)
Karyotype:
male
Sex chromosomes:
determine gender (XX; XY)
Autosomes: non-sex chromosomes

9. Human Life Cycle
Gametes (sex cells) have a single set of 22 autosomes and a single sex chromosome, either X or Y
With 23 chromosomes, they are haploid
Haploid sperm haploid ova zygote (2n)
fertilization 2n n
haploid number: n = 23
diploid number: 2n = 46

10. Fertilization Sperm compete to fertilize the egg Only one will win
Once sperm penetrates the egg, egg creates a barrier to prevent other sperm from entering
Only sperm nucleus is released inside egg
Union of nuclei form zygote (fertilized egg)

11. Chromosomal Inheritance

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Phases of Meiosis
Phases of Meiosis
Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.
Meiosis involves two divisions, meiosis I and meiosis II.
By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.
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Phases of Meiosis
Meiosis I
Meiosis I
Interphase I
During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.
Prophase I
Metaphase I
Anaphase I
Telophase I and Cytokinesis
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Phases of Meiosis
Cells undergo a round of DNA replication, forming duplicate chromosomes.
Interphase I
Interphase I - Cells undergo a round of DNA replication, forming duplicate chromosomes.
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Phases of Meiosis
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
There are 4 chromatids in a tetrad.
MEIOSIS I
Prophase I
MEIOSIS I Prophase I - Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
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Phases of Meiosis
When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over.
Crossing-over produces new combinations of alleles.
Crossing-over occurs during meiosis. (1) Homologous chromosomes form a tetrad. (2) Chromatids cross over one another. (3) The crossed sections of the chromatids are exchanged.
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17. Crossing Over
In Prophase I of Meiosis I, synapsis occurs allowing the crossing over of genetic material between non-sister chromatids
Creates new combinations of genes not seen in either parent

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Phases of Meiosis
Spindle fibers attach to the chromosomes.
MEIOSIS I
Metaphase I
MEIOSIS I Metaphase I - Spindle fibers attach to the chromosomes.
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Phases of Meiosis
The fibers pull the homologous chromosomes toward opposite ends of the cell.
MEIOSIS I
Anaphase I
MEIOSIS I Anaphase I - The fibers pull the homologous chromosomes toward opposite ends of the cell.
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Phases of Meiosis
Nuclear membranes form.
The cell separates into two cells.
The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.
MEIOSIS I
Telophase I and
Cytokinesis
MEIOSIS I Telophase I and Cytokinesis - Nuclear membranes form. The cell separates into two cells.
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Phases of Meiosis
Meiosis II
The two cells produced by meiosis I now enter a second meiotic division.
Unlike meiosis I, neither cell goes through chromosome replication.
Each of the cell’s chromosomes has 2 chromatids.
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Phases of Meiosis
Meiosis II
Meiosis II
During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.
Telophase I and Cytokinesis I
Prophase II
Metaphase II
Anaphase II
Telophase II and Cytokinesis
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Phases of Meiosis
MEIOSIS II
Prophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.
MEIOSIS II Prophase II - Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.
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Phases of Meiosis
MEIOSIS II
Metaphase II
The chromosomes line up in the center of cell.
MEIOSIS II Metaphase II - The chromosomes line up in a similar way to the metaphase state of mitosis.
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Phases of Meiosis
MEIOSIS II
Anaphase II
The sister chromatids separate and move toward opposite ends of the cell.
MEIOSIS II Anaphase II - The sister chromatids separate and move toward opposite ends of the cell.
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Phases of Meiosis
MEIOSIS II
Telophase II and Cytokinesis
Meiosis II results in four haploid (N) daughter cells.
MEIOSIS II Telophase II and Cytokinesis - Meiosis II results in four haploid (N) daughter cells.
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Gametogenesis
Spermatogenesis
In male animals, meiosis results in four equal-sized gametes called sperm.
Sperm cells made in testes (over 100 million each day in adult male)
Takes over 3 weeks to complete
Meiosis produces four genetically different haploid cells. In males, meiosis results in four equal-sized gametes called sperm.
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Gametogenesis
Oogenesis
In many female animals, only one egg results from meiosis. The other three cells, called polar bodies
Egg cells made in ovaries
Meiosis I takes place and process stops before birth (1-2 million cells at this stage; by puberty only 400,000 left)
Meiosis II begins each month once egg cell is released from ovary
Meiosis produces four genetically different haploid cells. In females, only one large egg cell results from meiosis. The other three cells, called polar bodies, usually are not involved in reproduction.
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29. Formation of Polar Body
Occurs because of uneven division of cytoplasm during cytokinesis
Egg cells must contain enough supplies to synthesize all the products needed by the developing embryo until the embryo implants into the uterine wall. 
Implantation does not occur until 7-10 days after fertilization. 

30. Comparing Mitosis and Meiosis
Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells.
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31. Comparing Mitosis and Meiosis
Cells produced by mitosis have the same number of chromosomes and alleles as the original cell.
Mitosis allows an organism to grow and replace cells.
Some organisms reproduce asexually by mitosis.
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32. Comparing Mitosis and Meiosis
Cells produced by meiosis have half the number of chromosomes as the parent cell.
These cells are genetically different from the diploid cell and from each other.
Meiosis is how sexually-reproducing organisms produce gametes.
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33. Meiosis
Quiz
Wiz

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If the body cells of humans contain 46 chromosomes, a single sperm cell should have
46 chromosomes.
23 chromosomes.
92 chromosomes.
between 23 and 46 chromosomes.
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During meiosis, the number of chromosomes per cell is cut in half through the separation of
daughter cells.
homologous chromosomes.
gametes.
chromatids.
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The formation of a tetrad occurs during
anaphase I.
metaphase II.
prophase I.
prophase II.
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Compared to egg cells formed during meiosis, daughter cells formed during mitosis are
genetically different, while eggs are genetically identical.
genetically different, just as egg cells are.
genetically identical, just as egg cells are.
genetically identical, while egg cells are genetically different.
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In many female animals, meiosis results in the production of
only 1 egg.
1 egg and 3 polar bodies.
4 eggs.
1 egg and 2 polar bodies.
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