1. Meiosis Sexual reproduction occurs only in eukaryotes. During formation of gametes, the number of chromosomes is reduced by half, and returned to the full amount when the two gametes fuse during fertilization. Sexual reproduction occurs only in eukaryotes. During formation of gametes, the number of chromosomes is reduced by half, and returned to the full amount when the two gametes fuse during fertilization.

2. Let’s Define Some Terms Gametes: Haploid reproductive cells (ovum and sperm) “Ploidy” is a term referring to the number of sets of chromosomes. Haploid and diploid are terms referring to the number of sets of chromosomes in a cell. Gametes: Haploid reproductive cells (ovum and sperm) “Ploidy” is a term referring to the number of sets of chromosomes. Haploid and diploid are terms referring to the number of sets of chromosomes in a cell.

3. More Terms Di ploid: Organisms are those with two (di) sets of chromosomes. Human beings (except for their gametes), most animals and many plants are diploid. We abbreviate diploid as 2n. Di ploid: Organisms are those with two (di) sets of chromosomes. Human beings (except for their gametes), most animals and many plants are diploid. We abbreviate diploid as 2n.

4. More Terms Haploid: organisms/cells have only one set of chromosomes, abbreviated as n. Organisms with more than two sets of chromosomes are termed polyploid. Haploid: organisms/cells have only one set of chromosomes, abbreviated as n. Organisms with more than two sets of chromosomes are termed polyploid.

5. Polyploidy Polyploidy occurs in some animals, such as goldfish, salmon, and salamanders, but is especially common among ferns and flowering plants, including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat. Polyploidy occurs in some animals, such as goldfish, salmon, and salamanders, but is especially common among ferns and flowering plants, including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat.

6. Trisomy 21(Down Syndrome) Down syndrome is caused by an extra chromosome 21 (trisomy 21).Down syndrome People with Down syndrome are usually mentally retarded and have a host of physical defects, including heart disorders.heart Other individuals, called Down syndrome mosaics, have a mixture of normal cells and cells with three copies of chromosome 21, resulting in a mild form of the disorder. Down syndrome is caused by an extra chromosome 21 (trisomy 21).Down syndrome People with Down syndrome are usually mentally retarded and have a host of physical defects, including heart disorders.heart Other individuals, called Down syndrome mosaics, have a mixture of normal cells and cells with three copies of chromosome 21, resulting in a mild form of the disorder.

7. Even more terms Ch romosomes that carry the same genes are termed homologous chromosomes. Genes: Specific segments of DNA that control cell structure and function; the functional units of inheritance. Alleles: Alternate forms of a gene. Ch romosomes that carry the same genes are termed homologous chromosomes. Genes: Specific segments of DNA that control cell structure and function; the functional units of inheritance. Alleles: Alternate forms of a gene.

8. Alleles Th e alleles on homologous chromosomes may differ, as in the case of heterozygous individuals. Organisms (normally) receive one set of homologous chromosomes from each parent. Heterozygous: Having two different alleles (one dominant, one recessive) of a gene pair Homozygous: Having identical alleles for a given gene. Th e alleles on homologous chromosomes may differ, as in the case of heterozygous individuals. Organisms (normally) receive one set of homologous chromosomes from each parent. Heterozygous: Having two different alleles (one dominant, one recessive) of a gene pair Homozygous: Having identical alleles for a given gene.

9. Meiosis Meiosis: is a special type of nuclear division, which segregates one copy of each homologous chromosome into each new "gamete".

10. Compared to Mitosis Mitosis maintains the cell's original ploidy level (for example, one diploid 2n cell producing two diploid 2n cells; one haploid n cell producing two haploid n cells; etc.). Meiosis, on the other hand, reduces the number of sets of chromosomes by half, so that when gametic recombination (fertilization) occurs the ploidy of the parents will be reestablished. Mitosis maintains the cell's original ploidy level (for example, one diploid 2n cell producing two diploid 2n cells; one haploid n cell producing two haploid n cells; etc.). Meiosis, on the other hand, reduces the number of sets of chromosomes by half, so that when gametic recombination (fertilization) occurs the ploidy of the parents will be reestablished.

11. Compared with Mitosis M ost cells in the human body are produced by mitosis. These are the somatic cells. Cells that become gametes are referred to as germ cells (sex cells). M ost cells in the human body are produced by mitosis. These are the somatic cells. Cells that become gametes are referred to as germ cells (sex cells).

12. Meiosis The vast majority of cell divisions in the human body are mitotic, with meiosis being restricted to the gonads. Meiosis comprises two successive nuclear divisions with only one round of DNA replication. Four stages can be described for each nuclear division. The vast majority of cell divisions in the human body are mitotic, with meiosis being restricted to the gonads. Meiosis comprises two successive nuclear divisions with only one round of DNA replication. Four stages can be described for each nuclear division.

13. Meiosis I nterphase: Before meiosis begins, genetic material is duplicated.

14. Meiosis I P rophase 1: Duplicated chromatin condenses. Each chromosome consists of two, closely associated sister chromatids. Crossing-over can occur during the latter part of this stage. P rophase 1: Duplicated chromatin condenses. Each chromosome consists of two, closely associated sister chromatids. Crossing-over can occur during the latter part of this stage.

15. Meiosis I M etaphase 1: Homologous chromosomes align at the metaphase plate.

16. Meiosis I A naphase 1: Homologous pairs separate with sister chromatids remaining together.

17. Meiosis I T elophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

18. Meiosis II P rophase 2: DNA does not replicate.

19. Meiosis II M etaphase 2: Chromosomes align at the metaphase plate.

20. Meiosis II A naphase 2: Centromeres divide and sister chromatids migrate separately to each pole.

21. Meiosis II T elophase 2: Cell division is complete. Four haploid daughter cells are obtained.

22. Meiosis O ne parent cell produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different.

23. Meiosis in females

24. Meiosis in Males