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11.4.  Homologous chromosomes: pairs of chromosomes with genes for the same traits, but they can have different information about those traits.

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Presentation on theme: "11.4.  Homologous chromosomes: pairs of chromosomes with genes for the same traits, but they can have different information about those traits."— Presentation transcript:

1 11.4

2  Homologous chromosomes: pairs of chromosomes with genes for the same traits, but they can have different information about those traits

3  Humans have 46 chromosomes or 23 pairs of homologous chromosomes.  Somatic (body) cells contain 46 chromosomes.  46 is the diploid number for humans. -Diploid means that a cell has both homologs for each chromosome  Somatic cells reproduce by mitosis

4  ½ the chromosomes in a cell come from Mom, ½ the chromosomes in a cell come from Dad

5

6  Gametes(eggs and sperm) are called haploid because they contain one set of chromosomes.  Eggs also called Ovum and sperm contain 23 chromosomes. and sperm are produced by meiosis- a process that reduces chromosomes by

7  A type of cell division where ONE diploid cell (46 chromosomes) become FOUR haploid cells(23 chromosomes each).  The cells produced by meiosis are gametes (sex cells) 46 23

8  Plants  Fungi  Animals (including you) ◦ Males start meiosis at puberty. ◦ Females start meiosis in utero.

9  In your gonads ◦ Females- the ovary ◦ Males-the testicles

10  Genetic Variation ◦ Mixes genes from Mom and Dad to create a unique baby the chromosome number by  Ensures that each cell produced has one copy from each homologous pair of chromosomes  23 in sperm + 23 in egg unite (fertilization) leads to a 46 zygote(baby)

11  In two stages called Meiosis I and Meiosis II.  Each stage contains 4 distinct phases prophase metaphase anaphase teleophase telophase is followed by the division of cytoplasm known as cytokinesis.

12  Homologous chromosomes join to form a tetrad -chromosomes in the tetrad cross over and exchange genes

13 over produces new genetic - It is another way to increase genetic diversity

14  During Metaphase 1, the tetrads ( groups of 4 homologous chromosomes ) line up down the center of the cell - they assort independently -Chromosomes from each parent line up independently of each other

15 Lesson Overview Lesson OverviewMeiosis During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell..

16 Lesson Overview Lesson OverviewMeiosis During telophase I, a nuclear membrane forms around each cluster of chromosomes. Cytokinesis follows telophase I, forming two new cells.

17 Lesson Overview Lesson The two cells produced by meiosis I now enter a second meiotic Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II.

18 Lesson Overview Lesson OverviewMeiosis As the cells enter prophase II, their chromosomes—each consisting of two chromatids—become visible. The chromosomes do not pair to form tetrads, because the homologous pairs were already separated during meiosis I.

19 Lesson Overview Lesson OverviewMeiosis During metaphase of meiosis II, chromosomes line up in the middle of each cell.

20 Lesson Overview Lesson OverviewMeiosis As the cell enters anaphase, the paired chromatids separate.

21 Lesson Overview Lesson OverviewMeiosis These four daughter cells now contain the haploid number (N)—just two chromosomes each.

22 assortment increases genetic - There are 23 pairs of homologous chromosomes in humans - independent assortment ensures that there are 2 23 possible combinations 2 23 = 8,388,608 combinations - When two gametes join to form the zygote in fertilization the possible number of combinations is 2 23 X 2 23 = 64 trillion

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25  Crossing over

26  23 chromosomes

27  Mitosis creates 2 daughter cells with same amount of chromosomes  Meiosis creates 4 daughter cells with half amount of chromosomes  Meiosis goes through 2 cycles  Mitosis occurs in somatic cells, Meiosis in sex cells


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